Chapter 1

General Introduction

1.1 Rationale

Following the world food crisis in 2008 and with the projected global population increasing towards 9 billion by 2050, food costs have remained high and volatile in the world to date (The Food and Agricultural Organization (FAO); February 2011). In light of the global climatic changes, unpredictable weather patterns have greatly affected crop production at every level over the world. As such, renewed initiatives by the World Bank have been put into place aimed at enhancing overall food security. This has placed further demands on fresh water supplies as concerted efforts get underway to improve crop yields. Poor wetting soils represent one of the key issues limiting the efficient use of potentially available fresh water in agriculture.

Agriculture accounts for ~90% of the world’s consumption of fresh water with cereals accounting for ~27 %. Despite specific genotypes having been engineered to favour early emergence of shoots and foliage and the production of high quality grain, water losses from the soil surface during the growing season still remains a significant limitation to productivity. Water deficits have been shown to stress plant physiological stages leading to embryo abortion or a pre-mature ‘end of the grain filling’ which results in a severely damaged crop particularly with cereals [1-3]. Wheat production, for example, has fallen short of a theoretical yield level such as 20 kg ha-1 mm-1 (commonly known as the water limited potential yield) when compared to its conceptual maximum [4]. The impact of non- and poorly-wetting (hydrophobic) soil particles is typified by the 5.75 million hectares of Australian agricultural soils which yield ~ 40% less than their water limited potential each year. Taking into account the Australian natural soil ecology as well as the potential effects of climate change, moisture conservation represents a priority that requires new or improved strategies in its management.

1.2 Aims and objectives of the research

Surfactants have been employed to ameliorate non- and poorly-wetting soils in the turf and horticultural industries as well as being trialled in broad acre crops. Here,

1 this study investigates their behaviour in the context of broad-acre cereal producing soils with particular emphasis on the interactions between the molecular character of the surfactant and the soil particle surface. The main aim of this study was to investigate the role that surfactants and their additives (hydrotropes) have on the surfactant-amended wetting of model and in-field hydrophobic soils. The outcomes are aimed at identifying an optimal surface active functionality to manage water transport in these non- and poorly-wetting soils.

In meeting this aim the following specific objectives are addressed:

1. To extract, identify and quantify the components of soil organic matter (SOM) found in non-wetting soils from two regions in Western Australia using GC-MS. 2. To define the nature of interactions of specific SOM components at the surface– air interface of model hydrophobic surfaces using AFM since they directly contribute to soil hydrophobicity and water repellence. 3. To evaluate the effectiveness of non-ionic surfactants and their blends in controlling interfacial phenomena such as wetting, spreading and adhesion in aqueous systems. 4. To evaluate the impact of the functionality and molecular motifs of the selected hydrotropic additives on the wetting behaviour of surfactants on these soils, in terms of the specific mechanisms e.g., adsorption, capillary imbibition and gravitation infiltration.

1.3 Thesis outline

Chapter 2 provides a literature review of the current state of soil water repellency based on available publications. Here the causes and consequences of this phenomenon are discussed together with a description of the mechanisms underpinning surfactant-based amelioration of soil water repellency.

Chapter 3 provides the materials and methods, outlining the experimental methods and techniques used for analysis in this study with model hydrophobic surfaces and the two soil systems from Western Australia having low—moderate and severe hydrophobicity as a central point of reference.

2

Chapter 4 details the hydrophobic surfaces studied namely; (i) model hydrophobic surfaces comprising of planar and particulate packed beds and (ii) Western Australian non-wetting soils obtained from South Stirling and Dandaragan areas. The chapter investigates the nature, molecular distribution and specific interactions of SOM components extracted from Western Australian soils.

Chapter 5 provides the solution properties and wetting behaviour of two non-ionic surfactants poly (ethylene oxide) poly (propylene oxide) block copolymer and an alcohol ethoxylate and their blends during soil wetting and water transport which are investigated with respect to the following underlying mechanisms; (i) adsorption, (ii) capillarity and (iii) gravitational infiltration.

Chapter 6 details the impact hydrotropic agents had on the solution properties of the non-ionic surfactants detailed in Chapter 5. Specifically, additives include (i) a biopolymer polysaccharide-based hydrotropic surfactant; alkyl polyglucoside and (ii) solvo surfactants; hexyl glycol ether, butyl glycol ether and butyl diglycol ether.

3

General Introduction

Setting the context of the research Chapter 1

Literature review Soil Water Repellency Surfactants Materials and - Single surfactant Methods Chapter 2 systems Chapter 3 -Surfactant blends Chapter 5

Surfactant- enhanced amelioration of soil water repellency

Soil Systems Surfactant - 2 Western Additives Australian soils - Hydrotropes - Model soils - Solvo-surfactants Chapter 4 Chapter 6 Conclusion and Future Work

Chapter 7

Figure 1.1 Flow chart outlining the structure of the thesis

4

Chapter 2

Literature Review

2.1 Introduction

Soil water repellency (SWR) or soil hydrophobicity is the resistance towards wetting over time as a result of the reduced affinity of the soil surface towards water spreading. Soils that exhibit a contact angle of 72° or greater at the soil water interface are considered water repellent [5] which results in reduced and in some cases totally impeded water infiltration through the soil bed. The occurrence of soil water repellency is global, seasonal and can be widespread in a variety of sandy, loamy and clayey type of soils [6-8]. This surface phenomenon is temporal, varies spatially and is highest during long dry periods and less severe when soils are moist. This review aims to address the causes, effects and management of SWR particularly as they pertain to the use of surfactants and surfactant blends.

2.2 Factors affecting soil water repellency 2.2.1 Abiotic factors The degree of severity of SWR is a function of several factors. The soil texture, particle size and clay content are known to influence the degree of hydrophobicity in soils. Soils with large particle size, coarse sandy soil textures with smaller surface area per unit volume and soils with less than 10% clay for example, are more prone to increased hydrophobicity. Sandy soils or soils with low clay content experience poor infiltration since these surfaces, when coated with organic matter, prevent water from spreading in a continuous film but rather tend to form individual droplets (Figure 2.1). This behaviour is typical of surfaces with low surface tension (free energy) where the cohesive forces in water exceed the adhesive forces between water and the surface under investigation. The initial soil moisture and temperature variations also contribute to the severity of SWR where they have been shown to dictate water transport and distribution at the wetting front during infiltration [9]. During dry seasons when the moisture content in the soil is low, polar hydrophobic materials with an amphiphilic nature, orient themselves in such a way that their hydrophobic groups face outward to form reverse

5

Figure 2.1 Water droplets resisting infiltration into soil due to extreme water repellency (hypodermic needle for scale). From Doerr et al. 2000 [7].

micelle-like arrangements reminiscent of vesicles and thus enhance SWR [10]. Fires have also been reported as another cause of soil water repellency where the high temperatures were postulated as responsible for strengthening the bonds between organic compounds and the soil particles and also redistributing or concentrating the hydrophobic materials [7, 11]. The main cause of SWR however, has been attributed to the presence of soil organic matter (SOM) coatings themselves found on soil particles (given that soil minerals are hydrophilic in nature and have surface energies greater than 72.75 mN/m) [12]. Soil organic matter therefore plays a pivotal role throughout these studies and will be discussed in detail below.

2.2.2 Soil organic matter

Soil organic matter (SOM) plays an integral role in the various processes that occur in terrestrial ecosystems. It is the second largest reservoir (after the oceans) that regulate the carbon cycle on earth [13], an excellent source of N, P, and S required for plant growth and plays a key role in soil structure. Despite its utility, SOM can have devastating effects on soil water transport and distribution. The origin of SOM is diverse making its composition highly complex and heterogeneous.

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Origin of SOM Vegetation: One of the major contributors of SOM is living or decomposing vegetation. Evergreen trees (such as eucalyptus and pines), shrubs particularly those that are rich in resins, waxes or aromatic oils and certain grasses have been frequently associated with SWR [14-18]. Root exudates from plants have also been known to contribute to the hydrophobicity in soils. A variety of these rhizodeposits from different parts of the root system have been shown to also have an effect on the activity of the microbial communities as well as the wetting front. [19, 20].

Soil fungi and micro-organisms: A range of soil fungi and micro-organisms have also been associated with SWR. [7, 21, 22]. Fungi, such as basidiomycete, associated with the dry patches observed in turf grass all over the world cause the fairy ring disease, which have been shown to be associated with soil water repellency [23]. Microbial activity and community structures are significant in providing plants with nutrients such as nitrogen and phosphorous, and they also play a major role in regulating the dynamics of SOM decomposition. Although there is conflicting information in the literature regarding the contribution of certain microorganisms found in non-wetting soils [24], it is has been considered that during the decomposition process newer polar waxes are produced that contribute to the existing pool of hydrophobic materials. In the work of Franco et al., for example, polar wax extracts obtained from fungi and bacteria (Actinomycetes) showed similar chemical composition to the wax extracts from eucalyptus and lipin trees commonly known to contribute to soil hydrophobicity [25].

SOM composition: Irrespective of its origin, soil organic matter is considered to cause hydrophobicity by either forming coatings on soil particles or existing as interstitial particulates between the soil particles. A number of organic compounds have been associated with soil water repellence and are commonly divided into two categories, aliphatic hydrocarbons and polar amphiphilic compounds with fatty acids (FA) and fatty acid esters being most commonly cited [25-28]. It has been demonstrated by Ma’Shum et al., that long chain fatty acids (FAs) with aliphatic groups between C16 – C32 are capable of inducing hydrophobicity when applied to hydrophilic soils. Although the induced levels of hydrophobicity did not match those of native non-wetting soils, it was concluded that other existing interactions involving this class of compounds and other

7

SOM components may play a role in establishing the levels of hydrophobicity [29]. In a separate study, the increased persistence of SWR was attributed to high pH conditions including complex formation between fatty acid moieties (carboxylates) and cations such as Ca2+, which favourably orientates hydrophobic moieties outwards on a surface thus increasing hydrophobicity[26]. Interestingly, fatty acids and their derivatives have also been found in significant proportions in wetting soils despite their close relationship with SWR. Another group of compounds considered to be responsible for SWR are the sterols. Plant sterols, such as sistosterol and stigmasterol, as well as the fungi–derived sterol, ergosterol, together with small amounts of cholesterol, have been found in polar extracts of non-wetting soils. When in a mixture with FAs, sterols have been shown to exhibit changes in phase behaviour at specific temperatures. Cholesterol, in an equimolar mixture with palmitic acid for example, is capable of existing in both the crystalline (80%) and liquid or gel-like (20%) phase in equilibrium at a specific temperature [30]. It is thought that the planar molecular sterol structure enables them to penetrate into the ordered bilayer aggregates formed by FAs significantly changing the ordered orientation found within the lipid acyl chains in a bilayer formed through these intermolecular interactions [31]. Examples of other classes of compounds identified in non-wetting soils include alkanes, long chain fatty alcohols and phytones [25, 28, 29, 32-34]. Despite the extensive work that has been undertaken to identify the specific compounds responsible for soil hydrophobicity, clear gaps relating the precise composition of compounds to their mechanistic action on a given surface remains ill-defined when determining their causal effect in soil water repellency. Nonetheless, it has been postulated that (i) very small amounts of hydrophobic material are able to induce severe levels of hydrophobicity over significant volumes of soil, (ii) organic compounds do not form uniform monolayers on surfaces, rather they absorb as islands of globules, (iii) polar molecules found in abundance in non-wetting soils may also play a significant role in SWR [18, 25, 26, 29, 32]. A better understanding of the constituents of SOM, together with their distribution and interactions (between each other and with the soil surface under specific conditions of temperature and pH) would be critical in designing effective surfactant regimes to overcome SWR and thus improve wetting and water

8 transport in these non-wetting soils. A summary of factors affecting soil water repellency is illustrated in Figure 2.2.

Figure 2.2 A summary of the factors that influence soil water repellency. Modified from Doerr et al., 2000 [7]

2.3 Effects of soil water repellency

During the last two decades a significant research effort has been directed towards non-wetting soils as well as the repercussions of SWR on plant growth and food production [35, 36]. System level studies on to the wetting behaviour of non-wetting and poorly wetting soils indicate adverse hydro-geomorphological [7, 37-41] effects such as;

9

i. reduced infiltration rates leading to surface run-off and the likelihood of enhanced erosion ii. preferential or fingered flow through the soil matrix which can result in uneven wetting patterns and distribution of soil moisture, or the accelerated transport of nutrients and contaminants into groundwater iii. the vulnerability to wind erosion during drier periods iv. the enhanced leaching of plant nutrients and agrochemicals Since the repellency of water is a relative property, the degree of water repellency and the effect a particular soil has on crop production needs to be considered more specifically rather than in such general terms.

2.4 Determination of soil water repellency

The two most commonly used methods in assessing water repellence, including bulk soils, are the Water Droplet Penetration Time (WDPT) and Molarity of Ethanol Droplet Test (MEDT) [42-44]. Here, the WDPT involves placing a given volume of water droplet on a soil surface and recording the time for complete penetration. The level of repellency of a soil is identified using the following classes; Class 0, wettable (WDPT < 5 s); Class 1, slightly water repellent (5-60 s); Class 2, strongly water repellent ( 60-600 s); class 3, severely water repellent (600-3600 s); class 4 (1-3 h), Class 5 (3-6 h) and Class 6 (> 6 h) all considered extremely water repellent (> 3600 s). Importantly, the WDPT estimates the initial water repellency and persistence, and is therefore more qualitative than quantitative [45].

The Molarity of Ethanol Droplet test, on the other hand, consists of placing drops of increasing ethanol concentrations (0 - 36%) on the soil until one drop penetrates within a time range of 5 to 10 s [44]. Like the WDPT, a number of classes are used to distinguish the degree of repellency: Class 1, very hydrophilic (< 3%); Class 2, hydrophilic (3%); Class 3, slightly hydrophobic (5%); Class 4, moderately hydrophobic (8.5%); Class 5, strongly hydrophobic (13%); Class 6, very strongly hydrophobic (24%) and Class 7, extremely hydrophobic (36%) [46]. The ethanol test is not a direct measure of water repellency, but rather of the liquid surface tension needed to produce a spontaneous advancing contact angle. The advantage of this method is that it is a simple procedure able to be carried out in the field [7], although it does not relate directly to the

10 ability of a surfactant to produce a contact angle approaching zero on a particular SOM surface.

The sessile drop method can also be used to measure the degree of hydrophobicity of a soil material when a drop of water is applied to soil particles adhered to, say, to adhesive tape, smoothed to form a one grain layer in order to reduce the effect of porosity on wettability. The contact angle at the soil-water interface is then measured using a microscopic image fitted with a goniometer scale, when the effects of droplet evaporation is avoided [47]. In the three methods above, variation in drop volumes make it difficult to compare and interpret these respective values. In addition, these techniques only provide information on the wetting behaviour of the top surface of the soil rather than addressing the extent of capillary water infiltration and kinetics which is a significant parameter in water transport.

A more informative approach would be to couple such empirical techniques with information derived from capillary infiltration measurements in packed soil beds which has been traditionally based on the Washburn expression, which provides better definition of the effect of wetting and permeability in tortuous porous systems [48-52]. Here, such infiltration techniques are normalized against a completely wetting liquid in a similarly packed particulate bed thereby removing geometrical effects such as the porous tortuosity. With this normalization, the initial slope of the capillary imbibition curve then provides a value for the advancing water contact angle of the soil particles.

2.5 Surfactant amelioration of water repellent soils

Wetting being an entirely physical process with no chemical reaction, is governed by the relative free energies of the phases in contact (solid, liquid, air/vapour) and specifically by either interfaces at the three-phase line (TPL) of contact. These respective free energies are represented by their surface or interfacial tension (γ). During the wetting process, a water droplet in contact with a solid substrate may cause (i) complete wetting (contact angle θ = 0o), (ii) partial wetting (θ < 90o) or (iii) non- wetting (θ > 90o). In the absence of surfactants, a water droplet in contact with a “hydrophobic” surface has 90o < θ ≤ 150o. Contact angles above 150o represent the super- hydrophobic state where the contact angle has an additional component due to 11 surface roughness [53, 54]. Wettability (the ability of a liquid to spread over a surface) on a solid planar surface (Figure 2.3) is quantified by the contact angle, θ, which is expressed as a function of the balanced interfacial tensions in the Young equation [55]:

cos θ = (γSV – γSL)/γ 2.1

where; γSV, γSL and γ are the solid-vapour, solid-liquid and liquid-vapour interfacial surface tensions respectively [56].

Since the process of spreading of a liquid on a surface represents a replacement of the solid-vapour (air) interfaces by a solid-liquid interface, it also represents a competition between the forces in the liquid drop maintaining the droplet shape (forces of liquid cohesion) and the forces between the liquid and the solid surface (forces of adhesion). This difference in forces is generally expressed in the terms of the work of adhesion (Wa) and cohesion (Wc). When equal (Wa = Wc), spreading is unlimited and θ o = 0 and γSV > γSL + γ. As above, when the free energy change is negative and wetting is spontaneous, the quantity S = Wa - Wc is commonly termed the spreading coefficient which is related to the respective interfacial and surface tension and hence cos θ.

Across a great many technologies and applications, the ability to effectively wet porous materials is a critical parameter in overall performance. Surfactants, which manipulate both γSV and γSL, are widely recognized as one way to lower contact angle and favour wetting and spreading. Aqueous surfactant solutions modify the wetting characteristics of hydrophobic surfaces via adsorption of surfactant molecules at interfaces. During the adsorption processes, spreading on a hydrophobic surface is enhanced by the transfer of surfactant molecules to the moving three-phase contact line. The adsorbed surfactant molecules reduce the free energy of water at these interfaces causing both spreading and imbibition within porous substrates which can result in partial or complete wetting.

12

A (i) (ii)

air air

water θ θ

Solid surface Solid surface

B γ air

TPL

γSG θ γSL

Solid surface

Figure 2.3 Wetting behaviour of a water droplet on a solid surface at finite contact angles: A (i) and (ii) exhibiting complete partial wetting (θ ≤ 90o) and non-wetting (θ > 90o) behaviour respectively; B depicts all three interfacial tensions; solid-liquid (γSL), liquid-gas (γ) and solid-gas (γSG) on the three-phase contact line (TPL), adapted from [57]

The decrease in contact angle with the addition of surfactant and its adsorption at interfaces has been demonstrated by El Ghzaoui et al.,[58] and Zhang et al., [59] on flat hydrophilic surfaces and modelled by a combination of the Gibbs adsorption isotherm and Young’s equation. Modes of adsorption of non-ionic surfactants at the solid-liquid and liquid-vapour interfaces have been extensively studied [60]. Starov et al., [57, 61, 62] extended this to hydrophobic smooth and porous membrane substrates and modelled this assuming that surfactant molecules adsorb ahead of the TPL decreasing the solid surface tension γSG. Such an assumption is not required to sufficiently model the impact of amphiphilic surfactants on the wetting dynamics of rough and porous substrates [53]. Non-ionic surfactants have been shown to have similar maximum adsorption densities irrespective of the degree of hydrophobicity [63]. Scales et al., [63] have also shown that these surfactants are initially un-associated on the surface when at low concentrations but as concentration is increased, lateral self-association increases, which 13 is paralleled by a corresponding decrease in contact angle. Adsorption from non-ionic surfactant solutions well above the CMC yields an aggregated (multilayer) surface produced by hydrophobic interactions between the surfactant molecules.

During capillary imbibition in packed bed columns, the surfactant concentration at the advancing wetting front has a significant effect on the advancing contact angle θa and the liquid-air interfacial tension. By neglecting the effects of slip, gravity and inertial forces and in the absence of external pressure gradients, the contact angle in a packed bed column can be measured using the Washburn equation for capillary imbibition:

dh r cos  dt 4h 2.2 which when integrated becomes 푟훾푐표푠휃 ℎ2 = 푡 2.3 2휂 where h is the height of liquid front at any given time t, r is the effective capillary (pore) radius, lv, surface tension and , viscosity of liquid. Here the porous bed is modelled as a series of parallel capillaries having an effective equivalent capillary radius r; and the equation represents an imbalance of the Laplace capillary pressure and the hydrostatic (gravity) pressure. The value of r can then be obtained by using a liquid that completely wets the particulate packing i.e., cos = 1. Subsequently from measurements with other liquids for the same packing density, values of cos and hence effective wettability of the latter liquids may be determined [48, 64, 65].

As noted, surfactant-amended amelioration of water repellent soils is one of the techniques able to alleviate the impact of SWR and improve water transport and moisture retention in hydrophobic soils [66-69]. Key to their operational mechanisms is the ability of surfactants to change surface properties by lowering the interfacial tension at the liquid-air and liquid-surface interfaces. As seen here, this may be made possible by the adsorption of surfactant molecules on the hydrophobic SOM coatings found on soil surfaces thus changing the wetting dynamics [57, 61]. Spatial differences in surface tension create gradients in capillary pressure that can also induce flow and favour water transport in hydrophobic soils e.g. wicking. Thereby, surfactants can increase water

14 infiltration rates and lower water run-off at the soil surface [70-73]. Among the different types of surfactants available, non-ionic surfactants have been found most suitable as potential soil wetting agents since they significantly lower the surface tension of water even at low concentrations. Non-ionic surfactants also interact favourably with SOM- coated surfaces since they have no electrical charge and have also been shown to have lower adsorption on surfaces giving greater mobility at the wetting front. In addition, non-ionic surfactants have been found to be less phytotoxic as compared to anionic or cationic surfactants.

As in many technological processes, utilization of surfactants requires their concentrations to be above the critical micellar concentration (CMC) where the aggregation of the surfactant molecules in solution occurs. Hence the kinetics of the advancement of a water wetting front is dependent upon properties of the micellar surfactant solution, such as their diffusion rate to the TPL and their disintegration rate to single molecules in solution (micelle relaxation rates) as adsorption at the liquid-solid interfaces occurs. Non-ionic surfactant micelles have relatively longer micelle relaxation times (seconds to minutes) compared to ionic surfactants due to the absence of electrostatic repulsion within the micelles [74]. Hence the micelle relaxation time has been found to be proportional to the dynamic contact angle through its influence in replenishing surfactant monomers adsorbed at the moving interfaces. Although the rate of water wetting in a soil bed does not require consideration of the dynamic contact angle, the mechanistic aspects continue to be relevant.

Due to the molecular complexity of the SOM coatings on the soil surfaces and limited understanding of the interactions of the surfactant molecules at the surfaces including adsorption/desorption on the SOM coated surfaces, several synthetic surfactants have been extensively trialled, yet they have not provided ideal solutions to moisture control and soil ecology. Different strategies regarding the use of surfactants as ameliorative agents are therefore timely. One technique adopted in modifying surfactant behaviour is the use of hydrotropes, which has been shown to affect surfactant properties such as CMC, cloud point, solubilization capabilities amongst others. The addition of hydrotropes to surfactants and surfactant blends remains an area of little mechanistic study in non-wetting soil (NWS) systems. In the following

15 discussion, the mechanisms of hydrotropy as well as the interactions of hydrotropes with surfactants in aqueous solutions are addressed.

2.6 Hydrotropic agents

The enhanced solubility of hydrophobic compounds in water by the addition of water-soluble organic compounds was first termed ‘Hydrotropy’ by Neuberg in 1916 [75]. Unlike molecular solubility, hydrotropy is a collective molecular phenomenon where the hydrotropic molecules create a microenvironment of lower polarity [75, 76], through self-assembly, enhancing solubilization of hydrophobic compounds. This phenomenon is only possible if the structural architecture of the solubilizing molecule has both hydrophilic and hydrophobic functionalities. Hydrotropic agents are a general category of compounds that enhance hydrophobic solubility and may or may not be surfactant-like in structure and mechanism. This has been extended to include additives such as ‘solubility enhancers’ such as solvo-surfactants and co-solvents. Hydrotropes therefore, are predominantly small amphiphiles which can have some similarity to ideal surfactants, consisting of these hydrophilic and hydrophobic moieties, importantly (with the latter being too short usually < C8, to induce formation of micelles and operate at the high concentrations to solubilize hydrophobic molecules in aqueous systems. One typical characteristic of these amphiphiles is their low molecular weight. To date a vast array of different classes of hydrotropic materials exist and can be generally classified as ionic or non-ionic (Figure 2.5). Examples of non-ionic hydrotropic agents include alcohols such as ethanol, resorcinol, amides (e.g urea), amino-acids such as proline and short-chain glycol ethers. A particular group of non-ionic hydrotropes made from renewable materials such as the polysaccharides (and polyols) commonly known as alkyl polyglycoside (APG) have recently gained much attention due to their safe ecological and toxicological properties[77-79].

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A. Ionic hydrotropic agents - + O SO3 Na O - + O Na - + O Na

H3C n

O + - NH Cl 3 - + HO O Na

HO OH O

OH

Benzene derivatives

O B. Non-ionic hydrotropic agents O

H2N OH R-OH NH2 N H Short chain alcohols Amides (R = C2- C3) e.g urea and proline

OH O HO O R OH HO OR HO OH

Alkyl poly(glycosides) Glycerol monoethers (APG) (R = alkyl or Aryl)

CH3 HO HO O R n O R n

Short chain glycol ethers (n ≥ 1)

Figure 2.5 Molecular structures of typical hydrotropes

17

2.6.1 Solution properties of hydrotropes

Self- association in water: In the same way as surfactants, true hydrotropes are capable of forming aggregates at high concentrations which have been shown to significantly enhance the solubility of hydrophobic compounds [80, 81]. Various applications such as drug solubilization and separation and extraction of organic compounds rely on the self- aggregating nature of the hydrotropes. The mode of action during this process has however been found to be remarkably different from surfactant micellization. The hydrotrope concentration that marks the onset of the formation of these aggregates is known as the minimum aggregation concentration (MAC) and is dependent on several factors such as temperature, pH, the presence of salts and solvents. Although the detailed mechanism of formation of hydrotropic aggregates remain elusive, there is evidence to suggest that these self-assemblies comprise loose dynamic fluctuating clusters reminiscent of micelles, which can be observed in hydrotropic solution above the MAC [82-84].

The changes that take place at the MAC of hydrotropes have been analysed using a number of techniques. X-ray scattering (SAXS) [85] and small angle neutron scattering (SANS) [86] experiments, for example, suggest that some associations, particularly those found among aromatic hydrotropes, may be caused by end-to-end assemblies or overlapping or stacking of adjacent layers of the aromatic hydrophobic rings. A second mechanism of aggregation known as mixed-aggregation is well described by Srinivas et al., [85] who suggested that hydrotropic action in aqueous solutions may be caused by mixed aggregation between the hydrotrope and the solubilizate. Using X-ray crystal structure analysis, Srinivas et al. showed that hydrophobic molecules pack in a fashion that create alternating hydrophobic and hydrophilic regions where the solubilizate interacts with the hydrophobic part in a cooperative manner thus stabilizing the aggregates. Several explanations have been suggested for the mode of action during mixed aggregation (i.e. hydrotrope in the presence of a solubilizate). First, it is considered that hydrotropes have the capacity to change the structure of solvents thus acting as structure makers or breakers [82, 87]. A second possibility is that a complex may be formed between the hydrotrope and the solubilizate [80, 88] and finally and most widely adopted, is the mechanism of self- assembly forming aggregates similar to micelles above their CAC with the aggregate

18 molecule surrounding the solubilizate [89-91]. Although techniques, in particular: SANS, SAXS including light scattering, have been used to determine the onset of aggregation; these methods are not definitive since the hydrotropic aggregates are usually small and, for the most part, poly-dispersed. Other techniques including viscosity measurements, conductivity and 13C-NMR have also been used to determine and investigate the changes that take place at the MAC. One of the simplest ways to determine the onset of aggregation is through the measurements of surface tension with increasing hydrotrope concentration [91]. Similar to surfactants, increases in hydrotrope concentration decreases the surface tension to a point where it begins to form a plateau. This point usually corresponds to the MAC although it has been found not to be as sharp as the CMC found with surfactants. In this thesis, the interactions of surfactant and hydrotrope plays an important role in the behaviour of the aggregates state (CMC/MAC) and hence molecular availability at the TPL.

Hydrotropic solvo-surfactants: Solvo-surfactants are amphiphilic molecules that exhibit properties of both solvents and surfactants. To date, the commonly used amphiphilic solvents are those derived from ethylene and propylene glycol although a number of “green” alternatives have been offered including those based on alkyl polyglycosides and glycerol ethers. Some of the properties exhibited by this group of molecules include but not limited to, solubilization of hydrophobic molecules, volatility, reduction of interfacial tension, self-aggregation in water and formation of emulsions. Studies have shown that both the hydrophobic moiety and the head group of a solvo-surfactant influence the surface tension and the MAC values [92].

Hydrotropes as co-solvents: The solubility of an organic electrolyte in water has been given by Neera et al., [93] in terms of its melting point by the general solubility equation[93];

log Sw = 0.5 – logKow – 0.01(MP - 25) 2.4 where Sw is the aqueous solubility of the solute; Kow is its octanol-water partition coefficient and MP the melting point (oC). The aqueous solubility of a hydrophobic substance may be increased by changing its activity. In the pharmaceutical industry, for

19 example, the most common way to increase the activity parameter (Sw) in equation 2.4 involves modification of the solvent system for the drug. This has been achieved by the use of co-solvents and additives during drug development. Co-solvents such as ethanol, polyethylene glycol (PEG) and glycerine are commonly used for this purpose. The efficiency of a co-solvent as a solubility enhancer is dependent on its capacity to weaken the self-association in water environments by generally reducing its polarity. This enhancement has been found to depend on its hydrophobic interactions. The polar functionality then removes the entropically unfavourable interaction with the water structure at the interface. Such additives, particularly those with hydrotropic characteristics, have been found to be useful co-solvents in several applications [94-98]. The addition of a hydrotropic co-solvent e.g. propanol in formulation of hydrogels for example, has also been shown to strengthen the gel network and have the additional advantage of solubilizing hydrophobic materials [95, 99].

2.7 Interaction of hydrotropes with surfactants and polymers

Mixtures comprising of hydrotropic agents and surfactants exhibit different solution properties which may be superior to those of the individual components. Such mixtures form mixed micellar aggregates in solution where the presence of the hydrotropes may be expected to influence the structure and stability (relaxation times τ1 and τ2) of surfactants above their CMC and thereby influence the kinetics of surfactant molecules reporting to the TPL during advancement of the wetting front in a soil matrix. This synergistic effect, as exemplified by the addition of APG hydrotropes to non-ionic surfactant systems, not only requires less surfactant for a given application but has been shown to improve infiltration and water transport into some hydrophobic soils. [100- 102].

Hydrotropes also influence the efficiency of surfactant solubilization in aqueous systems. This was demonstrated in a study that examined the flushing properties that different surfactants had on tetrachloroethane, a common ground water contaminant when ethanol as the hydrotrope was added [103]. It was determined that the rate- limiting micellar solubilization was enhanced as incremental additions of ethanol (at

20

2.5%, 5% and 10% wt.) to a 4% Tween 80 (polyoxyethylene (20) sorbitan monooleate) solution were made.

Hydrotropes are also capable of impacting on systems which form micro- emulsions and liquid crystals where they alter the phase behaviour of such systems. The hydrotropic effects of different APG hydrotropes (with alkyl lengths of C4 and C8, d.p = 1 and 1.9 respectively for example) on liquid-crystalline phases (in a three component system comprising of water, SDS and n-pentanol) was determined to be both structure and concentration dependent [78]. In the absence of a hydrotrope, the system is comprised of four distinct phase regions consisting of: aqueous micellar solutions, inverse micellar solutions, a bicontinuous region, joining these two micellar regions, and a region of different types of lamellar liquid crystal phase. The C4 APG hydrotrope gradually extended the phase regions towards higher surfactant concentrations with a significant extension within the bicontinuous phase as well as minimizing the liquid crystalline region while the C8 APG had a diminishing effect on the bicontinuous phase and maintained the liquid crystalline region.

Variations in temperature also affect the solubility of surfactants in water; remembering that (Australian wheat soils can have temperatures that rise to ≥ 35 oC) [104]. Increases in temperature may cause the surfactant to precipitate from solution at particular threshold temperature making the aqueous solution turbid and cloudy. This occurs when the polar head group in surfactants become dehydrated at higher temperatures and a phase separation takes place followed by aggregation. This sharp transition commonly referred to as the cloud point (CP) takes place very rapidly and can be readily monitored visually since this transmission is a direct response of the stability of surfactant hydration, i.e. an entropically driven process. Hydrotropes have the capacity to directly manipulate surfactant CP (stability) and in some cases prevent it. Several studies have been conducted to determine the influence of hydrotropes on aqueous surfactants and polymer solutions. In a study to determine the influence of four different hydrotropes, sodium salicylate (Na S), urea (U), resorcinol (Rc), pyrogallol (Pg) and proline (Pr) on aqueous solutions of methyl cellulose and triton-X 100, Roy and Moulik [105] determined that the cloud points of aqueous solutions were dependant on the concentration of hydrotropes. Here Pr, Rc and Pg significantly decreased the cloud point of the solutions whereas NaS and U increased it. It was suggested that the

21 interaction between the hydrotropes (Pr, Rc and Pg) and the solutions decreased their solubility by causing them to lose water from the hydrated polar groups reducing the cloud point. On the other hand, since urea is a water structure breaker, it disrupts the water structure and hydrophobic interactions favouring molecules residing in the aqueous environment leading to an increase in the cloud point, a situation that may diminish the effectiveness of agricultural surfactants.

Aqueous solutions of ethylene oxide-propylene oxide block copolymers are capable of forming micelles and display similar phase behaviour to other non-ionic surfactants. Any increase in temperature will decrease the interaction of EO and PO units with water leading to a separation of the two phases; water and polymer-rich phases. It has been suggested that at higher temperatures, the EO unit adopts to a less polar conformation which favours polymer-polymer interactions rather than solvation. The cloud point behaviour of two ethylene oxide-propylene oxide triblock copolymers,

(EO)2.5(PO)31(EO)2.5 and EO)13(PO)30(EO)13, in solution was determined in the presence of hydrotropes such as urea, dimethyl urea, nicotineamide and sodium-p-toluene sulfonate alongside other additives [106]. The cloud point of both polymers increased with respect to all the added hydrotropes with sodium-p-toluene sulfonate having the highest rate and urea the least. The study also revealed that the cloud point of block copolymers were dependent on several factors. Those hydrotropes that were better at self-association increased the CP more efficiently, by arranging themselves in an efficient manner around the EO units. Urea and dimethyl urea which are considered not to form aggregates prevented the dehydration of the EO units at higher temperatures. Finally, the CP also varied with the number of EO units in the polymer, where the CP drastically increased in the polymer with higher EO numbers.

22

Chapter 3

Materials and Methods

3.1 Introduction This chapter details the materials and methods used to investigate the influence of hydrotropic additives on the efficiency of two non-ionic surfactants in their amelioration of NWS from Western Australia. More specifically, it outlines the protocols used to study the mechanisms involved during water transport and distribution in hydrophobic soils and model hydrophobic surfaces.

3.2 Non-ionic surfactants and hydrotropic additives

The two non-ionic surfactants used throughout this study are (i) an ethylene oxide propylene oxide block copolymer (BASF); EOxPOyEOz, average MW ~ 2450 g/mol, and (ii) an alcohol ethoxylate (BASF), average molecular weight ~ 600 g/mol (Figure 3.1).

a.

(EO = 20%, PO = 1750g/mol)

b.

(R = iso-C H , x = 8) 13 27

Figure 3.1 Non-ionic surfactants; (a) block copolymer; EOxPOyEOz and (b) alcohol ethoxylate

23

The following hydrotropic additives were used in formulating blends with surfactants at a weight ratio of 1:1; a biopolymer sugar-based hydrotropic surfactant: Alkyl polyglucoside (APG) with alkyl carbon C8 – C10 (average MW ~520, PG8107, BASF), Solvo surfactants; butyl glycol ether; ethylene glycol monobutyl ether (MW = 118.17 g/mol), butyl diglycol ether; diethylene glycol monobutyl ether (MW = 162.23 g/mol)) and hexyl glycol ether; (MW = 146.2 g/mol), all from Sigma Aldrich (Figure 3.2). Hydrotropic blends were maintained at a compositional ratio Surfactant : Hydrotrope of 1:1 by weight which allowed the role of components to be identified as a function of soil type, although optimized component ratios may depend on each soil. All surfactants and hydrotropes used are biodegradable and conform to the Australian APVMA guidelines.

OH 1. HO O APG (where R= C8-C10) HO OR OH

2. HO Butyl glycol ether (C4E1) OC4H9

HO 3. OC 4H9 O Butyl diglycol ether (C4E2)

4. HO Hexyl glycol ether (C6E1) OC6H13

Figure 3.2 Molecular structures of hydrotropic additives

3.3 Soil samples from Western Australia

Low-moderate and severely non-wetting soil samples were collected from Dandaragan and South Stirling areas of Western Australia and their characteristics are given in Table 3.1. Samples were taken from the top 5 cm of the soil profile, air dried, sieved to ≤ 450µm and stored at room temperature in Schott bottles.

24

Property Dandaragan South Stirling

Surface area (m2/g) 1.5 0.79 Clay content (% w/w) < 6.0 2.0 MED (mol L-1) 0.4 4.0 pH (H2O) 7.60 6.06

% Moisture (at 105o C) 0.834 0.288

3+ *Al 0.05 0.054

2+ *Ca 5.71 1.70

2+ *Mg 0.61 0.21

+ *K 0.20 0.07

+ *Na 0.17 0.03 *Cu 0.52 0.58 *Fe 17.77 9.59 *Mn 5.69 2.89 *Zn 1.30 1.14

Table 3.1 Properties of W.A. soils, 0 - 5cm *Exchangeable cations (meq/100g). Cu, Fe Mn and Zn were extracted using Diethylenetriaminepentaacetic acid DTPA (mg/Kg), source: University of Western Australia

3.4 Extraction and characterization of W.A. soil organic matter (SOM)

3.4.1 Soxhlet extraction of SOM

The soil samples (30.0 g) were soxhlet-extracted for 24 h with 150 ml of solvent mixture isopropanol (IPA)/NH3 (7:3 v/v) following the procedure reported by Llewellyn et al., [107]. This solvent mixture was shown to extract the greatest amount of SOM and increasing wettability in non-wetting soils. Prior to refluxing, samples in cellulose thimbles were pre-wetted with 20 ml of the solvent mixture for about 15 min to

25 minimize the loss of NH3 during the extraction process. Soils were extracted twice to ensure maximum extraction of SOM from the soil samples. The extracts were then pooled, filtered using glass fibre filters, concentrated using a rotary evaporator and finally freeze dried. The dried extract was further fractionated by dissolving it initially in 20 ml of chloroform, and the residue re-suspended in warm methanol to obtain the non-polar and polar fractions respectively. The fractions were filtered and dried under

N2 and their dry weights taken up in 1.0 ml of solvent and further analysed using gas chromatography-mass spectrometry (GC-MS).

3.4.2 GC-MS analysis of SOM

Each fraction was further diluted in a 1:2 ratio using the respective solvents above and analyzed using GC-MS. Methanol and chloroform soluble samples (1 µL) were injected splitless (injection port set at 275° C) and then separated using a ZB-5- semi volatile (5%-phenyl-methylpolysiloxane) capillary column (30m, 0.25 mm i.d, 0.5 µL diameter film) with Helium (44.7 cm/sec) as the carrier gas and detected using a flame ionization detector (FID) at 300° C. An isothermal oven temperature of 60o C was used for the first 2 min then ramped to 300° C at 20°/min and finally held at 300° C for 13 min.

Electron impact (EI) spectra were obtained using a gas chromatograph (GC- 2010 Plus, Shimadzu, Colombia, MD) directly coupled to a GC-MS QP2010 Ultra quadruple mass selective detector scanned from 40-1000 Daltons. Palmitic acid, stigmasterol and C8 – C40 alkanes (all from Sigma Aldrich) were used as external standards. Compounds were identified based on their retention times, standards and MS interpretation of fragmentation patterns based on search programs (NIST and NIST/EPA/NIH mass spectral libraries). Quantification of the identified compounds, calculated relative to palmitic acid, were based on comparison of the integrated peak areas to those of the standard in the total ion current (TIC) and converted to the compounds mass using an external calibration curve.

26

3.5 Preparation and characterization of model surfaces

Models of hydrophobic soil were prepared and their wetting behaviour compared to the soils from Western Australia. Hydrophobically modified quartz substrates (ProSciTech Pty Ltd., Thuringowa, Australia) were used as planar surfaces and corresponding particulate models were prepared using crushed mineral α-quartz (density 2.65, Osis Aus Pty Ltd. Perth) with the particle size ranging from 50 to 180 µm.

3.5.1 Model hydrophobic surfaces

Planar surfaces: Quartz slides (1 cm x 2 cm) were cleaned prior to being modified. Briefly, the slides were sonicated in acetone and ethanol for 30 minutes then further cleaned by repeatedly immersing them in 5M HNO3 followed by 5M KOH and finally

-1 rinsing with Milli-Q water (resistance > 18.2 M cm ) before drying under N2. The clean quartz slides were then modified in order to induce hydrophobicity using two SOM components, palmitic acid; PA (> 99%, Sigma-Aldrich, UK) and stigmasterol; ST (> 95%, Sigma-Aldrich, UK) together with their equimolar blend. Palmitic acid and stigmasterol represent two classes of compounds, namely the fatty acids and sterols, found in abundance in the W.A. soils which have previously been closely linked to SWR (Section 2.2.2). 20 µL of sample solution containing 20 mg of either PA or ST in 50 ml of chloroform was slip-cast on the cleaned substrates and annealed at their corresponding melting temperatures of 105 C and 160 C respectively for 30 min to form uniform films. An equimolar mixture of PA and ST (20 mg PA + 32 mg ST in 50 ml of chloroform) was also used (annealing temperature, 160o C). Clean quartz substrates were also treated with 2 M solution of trimethylchlorosilane (TMCS) in double-distilled cyclohexane for 30 mins, and annealed at 125 C after removing excess silane from the surfaces by washing with double-distilled cyclohexane to form homogeneous hydrophobic surfaces.

Particulate surfaces: In order to prepare hydrophobic particles, as-received quartz particles were repeatedly cleaned with HNO3:HCl mixture (2:1 vol. ratio) for 2 h at 80

27

C until there was no further coloration in the acid mixture during the cleaning procedure. The particles were then thoroughly rinsed with Milli-Q water until they were free of excess acid and dried at 125C in a muffle furnace. The particles were then further cleaned with hot 30 % aqueous solution of KOH for 1 min and rinsed with Milli- Q water to remove excess KOH, dried at 125 C [108]. Methanol solutions containing 5.0 mg palmitic acid, 8.0 mg stigmasterol or their equimolar blend (2.5 mg PA + 4.0 mg ST) were each mixed with freshly cleaned quartz particulates (100 g) and air dried initially to drive off the solvent followed by annealing at the corresponding melting temperature of the organic compound for two hours. This procedure yielded hydrophobic quartz particulates with uniform organic coatings. Cleaned particles were also treated with a 2 M solution of trimethylchlorosilane; TMCS (> 99%) in double- distilled cyclohexane for 30 min. as described above to yield silane coated hydrophobic particulate models. These hydrophobic particles were mixed in various weight ratios with the cleaned untreated particles to create bed compositions with various levels of hydrophobicity.

3.5.2 Topographical and friction force analysis of hydrophobic surfaces

Atomic force microscopy (AFM) (Innova, Veeco/Bruker, Santa Barbara, CA) was used to visualize the surface morphology (at nano-scale level) of the modified hydrophobic planar quartz surfaces. The corresponding behaviour of PA, ST and PA+ST on silanated quartz surfaces was not examined since they would form continuous thin films of the hydrophobic silane groups. High resolution topographical imaging was performed in the tapping mode with a phosphorus-doped silicon probe/tip (MPP - 31120-10, Veeco/Bruker) having a radius curvature of 8 nm which was operated at room temperature and 40% humidity. The probe had a spring constant of 0.9 N/m and a resonance frequency of ∼20 kHz for scanning surfaces in the tapping mode. Individual line profiles of the hydrophobic surfaces were analysed using the software Nano-scope Analysis (v1.4, Veeco/Bruker) [109].

AFM-friction force imaging of the organic coatings on planar surfaces was carried out using hydrophilic Si3N4 tips supported by 100 µm triangular cantilevers with a spring constant of 0.58 N/m in a contact mode. Here, the tips provided a nucleus for

28 condensation of water vapour which forms a meniscus between the tip (less than 10nm) and the modified surface. The capillary forces were larger where this occurred increasing the adhesion between the tip and hydrophilic surface domains allowing differentiation between hydrophobic and hydrophilic regions at the nano-scale [110]. AFM, as a non-destructive surface imaging technique benefits from its high three dimensional spatial resolution during the measurement of surface morphology and topology at the nanoscale. Unlike other classical microscopic techniques where imaging is achieved via an incident beam, imaging in AFM is performed by sensing the force between a shaped probe (usually silicon or silicon nitride) and the surface of the sample. During analysis, the nano-scale silicon probe is in close proximity with the surface, and is scanned horizontally over the surface. Here, the interaction between the probe and the surface is measured by the deflection of the cantilever tip which is detected by a scanned laser beam (Figure 3.3). A photodiode detector then measures the deflection which is readily converted into force with the probe spring constant. When operated in tapping mode, the tip oscillates vertically at constant amplitude and lightly probes the surface during scanning exerting low lateral forces. When in contact with the surface, the oscillating amplitude changes as a response to the surface morphology. [111].

Figure 3.3 The fundamental design of a typical Atomic Force Microscope (adapted from Dufrêne, [112]

29

3.6 Soil wetting processes and their dynamics

The impact of surfactants and surfactant blends in the wetting behaviour of these surfaces was evaluated from measurements of contact angle (θ) and the kinetics in two transport regimes; capillary imbibition based on capillary rise, and gravitational infiltration. These experiments were performed in clean open borosilicate glass columns secured at the bottom with a hydrophilic nylon membrane, 11 µm pore size (NY11, Merck Millipore, Billerica, MA) to retain the soil particles.

3.6.1 Surface tension measurements (and CMC/CAC)

The surface tension of aqueous surfactants, hydrotropes and surfactant blends were measured using a bubble pressure tensiometer (SITA t60) at 20 ± 2o C. Surface tension measurements were then used to determine the CMC of surfactant and surfactant blend solutions. All solutions were prepared using Milli-Q water and were of varying concentrations (0 - 4 g/L w/v). The solutions were stirred for 2 h and left to stand overnight to establish equilibria. The tensiometer was calibrated using Milli-Q water prior to making any measurements. During the surface tension measurements air is introduced through a glass bulb containing a capillary (radius rk) that is immersed in the test solution and the air pressure causes the formation of an air bubble (radius r, life time t ≤ 60 s) at the tip of the capillary. The internal bubble pressure (which is inversely proportional to the size of the bubble) increases over the bubble’s life time. The maximum internal pressure Pmax reached at the point when the diminishing gas bubble breaks away from the capillary tip, (i.e. when r = rk) is converted to surface tension using Laplace’s equation (equation 3.1)

Pmax = 2 γ/rk 3.1

where rk = capillary radius, γ = surface tension (liquid vapour interfacial tension). The CMC of each system was determined from plots of these surface tension measurements as a function of surfactant concentration.

30

3.6.2 Surfactant adsorption

As previously discussed (Section 2.5), aqueous surfactant solutions modify the wetting characteristics of hydrophobic surfaces via adsorption of surfactant molecules at the three interfaces. Adsorption of surfactants in soil is dependent on both the nature and concentration of surfactants and the soil properties. The adsorption of surfactants and surfactant blends on the two W.A. soils were conducted using a batch equilibrium technique carried out in duplicate. A series of surfactant solutions with concentrations below and above the CMC ranging from 0.001 g/L to 4 g/L were prepared for these batch experiments. Surfactant solutions (20.0 ml), were added to 2 g of soil in Erlenmeyer flasks secured with Teflon caps. Sealed flasks were shaken in a thermostatted chamber (22 ± 2o C) at 170 rpm for 24 h and allowed to settle for a further 24 h to achieve sorption equilibria. The supernatants were then centrifuged at 8,000 rpm for 30 min, filtered using a 0.45 µm Teflon filter and the surface tension of filtrates measured at 20 ± 2o C. The amount of surfactant adsorbed on the soil at pre-CMC surfactant concentrations was determined using surface tension measurements and calculated using the following expression:

Q ads = (Ci – Ce)(Vsoln/W soil) 3.2

where; Q ads, = amount of surfactant adsorbed per gram of soil (mg/g)

Ci = initial concentration in solution (g/L)

Ce = equilibrium concentration in solution (g/L)

Vsoln = volume of aqueous solution (L)

Wsoil = weight of soil (g)

2 Q ads in mg/g was further converted to mg/m by dividing with appropriate surface area of the soil substrate (Table 3.1) and the values used to generate the respective adsorption isotherms.

The amount of adsorbed surfactants from solutions at concentrations beyond the

CMC was determined using an ammonium ferrothiocyanate (NH4FeSCN) assay prepared by dissolving 30.4 g NH4SCN and 27.03 g FeCl3·6H2O in Milli-Q water made

31 up to 1 L. To determine the surfactants equilibrium concentrations, 3 ml of NH4FeSCN was added to 0.8 ml -1.0 ml of the surfactant solutions before and after adsorption followed by 4.5 ml of chloroform and the mixture was vortexed thoroughly for 2 min and allowed to separate into two phases. After pipetting out the lower CHCl3 layer, absorption measurements were taken using a 1 cm glass cuvette in a CARY 100 spectrophotometer at 510 nm. The amount of adsorbed surfactant or surfactant blend per 2 unit mass of soil (Qads) was calculated and converted to mg/m , to give the adsorption isotherms for equilibrium concentrations greater than the surfactants CMC.

3.6.3 Capillary imbibition in packed bed columns

The rate of capillary rise into packed beds was measured using glass columns (8.0 cm tall, i.d = 0.99 - 1.0 cm) packed with soil to a height of 5 cm according to the modified method of Siebold et al. 1997. Air-dried soil particles (< 450 µm) were packed systematically to controlled packing density of ~1.40 - 1.52 g/cm3 by introducing soil particles into the glass column in steps of 1 g to a the bed height of ~5 cm and the column tapped 5 times on a solid surface from a height of 6 - 7 cm at each step [48]. The packing procedure was maintained throughout all experiments to obtain a constant porosity in the range ~ 43.4 - 44.3% and reproducible packing densities (~1.40 - 1.52 g/cm3) were maintained. The packed soil columns were then placed into a petri dish to which the test solution was dispensed and the rising wetting fronts heights (h) of these solutions recorded over time (t) until the solution reached the top of the packed bed. Individual surfactants and surfactant blends (1:1, w/w ratio) at a concentration of 4.0 g/L were used with water as the control. All experiments were performed in duplicate.

3.6.4 Contact angle measurements

Two techniques were used to determine contact angles between the surfactant solutions and surfaces studied. For the planar surfaces, the sessile drop method was used to measure static water contact angles at ambient conditions (20° C, 40 % relative humidity) using a contact angle goniometer equipped with a nanodispenser (model FTA1000c, First Ten Ångstroms, Inc., Portsmouth, VA). A 1.0 μL droplet of the test solution was dispensed on a hydrophobized quartz surface and the contact angles of 10 32 replicates were measured by recording 50 images over 2 s with a Pelco model PCHM 575-4 camera, measuring contact angles after the droplet had rested on the surface for a period of 2 s. The contact angle measurements were obtained from the images by using the FTA Windows Model 4 software.

The capillary rise method in packed bed columns was used to determine the contact angle measurements of the advancing wetting fronts of the surfactant solutions on the hydrophobically modified model particles and the W.A. soil samples. As noted, this indirect method of calculating contact angle is based on Washburn equation (Equation 2.2) for a porous media [64, 65]. To determine the advancing contact angle

(θadv), of the respective beds, particles and soil were packed as described in Section

3.6.3 and cyclohexane, a completely wetting liquid (cos θ = 1, surface tension lv = 25.3 mN/m at 25o C and viscosity  = 0.99 mPa s), was first used to determine the value of the effective capillary (pore) radius r in the Washburn equation from the corresponding h2 vs t plot. Subsequently, in a similar manner and with r known, the contact angle θ, and hence effective wettability of the surfactants were determined from the initial stages of the h2 vs t plots. In this region, h2 varies linearly with t.

3.6.5 Gravitation infiltration of water following treatment with surfactants

Air-dried soils were packed as in a Section 3.6.3 into glass column (1.0 cm ID and ~ 14 cm tall) to a height of ~ 10 cm. Water infiltration was measured after treatment with surfactant (or surfactant blends) at a concentration of 4 g/L which conform to agricultural practice. Prior to surfactant application, 30 µL of a 4 g/L surfactant solution was pre-mixed with 300 μL Milli-Q water, and applied to the soil surface in two replicates maintaining a constant pressure head of 0.5 cm and ensuring even coverage of the top soil. A total of 330 μL, of surfactant solution, equivalent to a penetration depth of about 2 cm (modelling a sowing depth of ~ 2.5 cm) which corresponds to an application rate of rate of 20 L ha-1 which is an equivalent rate to agricultural practice. Water was used as the control in these studies. The soils were then oven dried at 40o C for ~ 12h to return them to their original weights and were left to equilibrate to room temperature overnight before application of water to the top of the soil column while maintaining a constant head ponding of 5 mm. This procedure was adopted in order to

33 model on a laboratory small scale agricultural practice where, in the field, the treated soil surface is left to dry prior to subsequent rainfall. Water penetration depths were recorded over time and used to determine the initial infiltration rates from a plot of penetrated depth vs time.

34

Chapter 4

Impact of Soil Organic Matter on Water Transport in Soil Systems

4.1 Overview

In this chapter, the nature and molecular distribution of soil organic matter extracted from surface soil material obtained from the two locations in Western Australia are detailed. These soils were selected since they exhibit different degrees of water repellency. As noted previously, soil samples from South Stirling were found to be severely hydrophobic while those from Dandaragan were of lower hydrophobicity as determined by the Molarity of Ethanol Droplet (MED) test. Additionally, the impact of specific SOM components at the solid surface–air interface was examined using model hydrophobic surfaces (planar surfaces and particulate packed beds) prepared with the models palmitic acid and stigmasterol (and their equimolar blend) and compared to corresponding hydrophobized silane treated surfaces. Surface morphological analysis was then performed using AFM, and shows the important role of surface structure on wettability. The final section of this chapter addresses the respective wetting behaviour of these model surfaces.

4.2 Introduction

Soil organic matter, although present in soils in relatively low quantities (2-50 mg/g) [113], is considered the principal cause of water repellence in soils. Several studies suggest that hydrophobic materials from the SOM may either be free, form thin coatings on mineral and aggregate surfaces, or be incorporated as separate interstitial matter into microporous soil structures or, covalently bound within the soils structure [7, 18, 25, 28]. SOM may also be found chelated to certain polyvalent metal ions such as Cu 2+, Mn 2+, Zn 2+, Al3+ and Fe3+ in the soil. Quantities of SOM have also been physically associated with soil mineralogy in particular clay minerals which may form a significant portion of the total soil composition. SOM adsorbed on the soil surface may also provide soils with a high cation exchange capacity (CEC) which emanates from the highly functionalized nature of soil organic matter which is capable of providing a plethora of binding sites with different affinities for interaction with metal ions found in soil [114].

35

The composition of soil organic matter has clearly been shown to contain a wide range of non-polar organic lipids as well as amphiphilic molecules with various chemical structures. Although amphiphilic compounds are capable of associating with water via hydrogen bonds, a number of studies have suggested that amphiphilic molecules play a key role in SWR [25, 28, 34]. As such, the focus of this study is centralized on two amphiphilic groups of compounds commonly identified in both wetting and non-wetting soils namely, fatty acids and sterols although in varying proportions. Fatty acids are prevalent in a variety of soils world-wide [18, 25, 28, 29,

115]. de Blas et al., [18] for example, found that fatty acids, particularly C16 and C18 made up a significant proportion of bound lipids from a variety of severely hydrophobic forest soils. Palmitic acid, one of the commonly identified fatty acids, is a 16-carbon fatty acid consisting of a saturated linear hydrocarbon chain which provides the molecule with its hydrophobic character, and a polar functional head group (Figure 4.1i.). The hydrophilic head group is capable of forming hydrogen bonds with water molecules with no appreciable change in entropy while the hydrophobic moiety which is incapable of forming such hydrogen bonds with water, orient themselves with their hydrophobic tails ordered away from the water surface.

As noted, another group of amphiphilic molecules identified in soil organic matter that has been associated with soil water repellency are the sterols. Among the sterols, cholesterol, stigmasterol and ergosterol (Figure 4.1 ii-iv) have commonly been identified in varying proportions in non-wetting as well as wetting soils. Several sterol analogues (with different structural variations) have also been detected in such soils where the structural variations have been shown to influence the phase behaviour in mixtures consisting of sterols and other lipids[116, 117]. Stottrup et al., [117] for example, was able to demonstrate that lipid monolayers containing sterols with different molecular conformations produced two immiscible liquid phases in phospholipid vesicles that are dependent on the sterols concentration. They further showed that whereas an alkyl chain length ≥ 5 is required to produce co-existing liquid domains in the phospholipid monolayer, the position of the double bond in the sterols ring system and the moity at position 3 (head group) also influenced the miscibility behaviour. In a

36

i. O

OH H3C

26 ii. CH3

21 23 25 CH3 H3C 27 18 24 CH 20 22 12 3 11 17 19 C H CH 13 1 3 H D 16 9 14 2 A 10 H 8 H 15 B 7 HO 3 5 4 6 CH3

iii. CH3 H3C

CH3 CH3 H CH3 H

H H

HO

CH3

CH3 iv. H3C

CH3 CH3 H CH3

H H

HO

Figure 4.1 Chemical structures of palmitic acid (i), cholesterol (ii); showing the ring systems A-D and carbon numbering, stigmasterol (iii) and ergosterol (iv)

37 different study, the interaction between the various conformations of the sterol ring systems and phospholipids in the lipid rafts of biological membranes was investigated [116]. In this study, the phase behaviour of a mixture of cholesterol and the membrane liquid dipalmitoylphosphatidylcholine (DPPC) was investigated. It was demonstrated that the polar head group of cholesterol resides in the phospholipid bilayer of biological membranes while the hydrophobic moiety lies parallel to the lipid acyl chains thus anchoring this molecule within the structure. The interaction between cholesterol and the hydrocarbon chains in phospholipids for example, limits the mobility of these lipids within the membrane. On the other hand, high concentrations of cholesterol within the membrane results in fluidity between the liquid-crystalline and crystalline phase. Whereas sterols with different chemical configurations provide different properties in these microdomains such as elasticity, permeability and even mobility, little is known about the impact and interactions the different structural conformations have on such properties [118].

Stigmasterol is a typical plant sterol similar to cholesterol (Figure 4.1 ii), which has an equatorial C3-OH group in ring A, a C5-C6 double bond (in ring B) and methyl groups at position C10 and C13 (Figure 4.1 iii). The polar head group (3β-OH) of this amphiphilic molecule is small while the hydrophobic moiety consists of a rigid tetracyclic ring and a side chain having an isooctyl backbone which is large. Although stigmasterol is similar in many respects to cholesterol, some differences exist. The main structural difference between these two molecules is in their alkyl side chain at the C17 position of the cyclopentane ring, D. Stigmasterol has an additional ethyl group at C24 thus making the acyl chain more bulky with a relatively larger cross-sectional area which may interfere with packing including with other molecules. Stigmasterol also has a double bond at C22-C23 in the trans configuration, which causes the side chain to be rigid. Ergosterol, another soil sterol (Figure 4.1 iv), on the other hand, has a double bond at C22-C23, similar to stigmasterol, but a methyl group at C24 and an additional double bond at the C7-C8 in the B ring. This structural differences in the alkyl side chain has been shown to influence the phase behaviour of mixtures involving palmitic acid and a variety of sterols where it is considered that the bulky side chain favours phase separation enhancing the formation of solid palmitic acid [119]. Phase separation is considered to occur as a result of the reduced intermolecular van der Waals interactions

38 of the less tightly packed molecules, emanating from the minimized hydrophobic matching between the sterol and fatty acid.

Several analogues of stigmasterol have previously been identified in soil organic matter with different levels of hydrophobicity [25, 120]. These analogues differ in their structural conformations with variations such as the number of functional groups, their position and orientation (α or β) in the ring system and/or alkyl side chain. The heterogeneous composition and distribution of soil organic matter therefore suggest interactions across a range of various components within it, which can be informed by probing interactions between selected molecular types at the micro- and nano-scale. To date, techniques that are able to extract and identify the entire composition of SOM components and further establish the interactions of the existing components are still lacking and would be useful in furthering the knowledge of their contribution towards SWR.

A number of techniques including those that analyse solid soil samples such as pyrolysis-GC/MS, FTIR in DRIFT mode and solid state 13C cross-polarization magic angle spinning NMR (13C CP-MAS NMR) have been used to investigate components of SOM [25, 114, 121, 122]. Despite the main advantage held by these methods, namely being able to probe intact soil samples, techniques such as DRIFT-FTIR and 13C CP- MAS NMR mostly provide information regarding the various functionalities rather than the specific compounds found in soil organic matter. Pyrolysis-GC MS on the other hand shows selectivity towards more volatile compounds such as the N-containing compounds and carbohydrates but also suffers from the risk of modifying the original organic compounds via thermal secondary reactions.

A commonly used technique in analysing the specific components of SOM involves the soxhlet extraction of the various compounds using a variety of solvents followed by separation and analysis using GC-MS [25, 28, 123, 124]. In order to determine the most effective solvent for extracting soil organic matter from a variety of soils, Doerr et al., [124] for example, used solvents with varying polarity

(dichloromethane; DCM, hexane, toluene and isopropanol (IPA) and IPA/NH3 (7:3, v/v) to extract SOM from a hydrophilic and a strongly hydrophobic soil. As well as being able to extract the most organic material, the alkaline polar solvent system, IPA:NH3 (7:3, v/v), also rendered the soils more wettable compared to the other solvents hence

39 proving to be the most efficient in extracting organic compounds from hydrophobic soils. The other organic solvents with low polarity such as DCM, chloroform, hexane and toluene were shown to have a counter effect rendering the soil more hydrophobic since they are capable of affecting the conformation of organic compounds in the extracted soils. This study further confirmed that re-deposition of the solvent extracts on acid-washed quartz sand induced hydrophobicity (although to a lower extent compared to the soils from which they were extracted). More interestingly, the levels of induced hydrophobicity as shown by Doerr et al., were higher at high temperatures (105o C compared to 20o C) confirming the role temperature plays in the interactions between SOM and soil mineral particles and in soil water repellence overall. Furthermore, extracts from wettable soils were also able to induce a certain degree of hydrophobicity on acid washed sand supporting the commonly held suggestion that a small fraction of SOM distributed in a suitable orientation at the soil-air interface can favour the exposure of water repellent non-polar moieties leading to SWR.

The complex topology of SOM film coatings at the soil particle-air interface of water repellent soil have been probed at the micro- and nano-scale level using scanning electron microscopy (SEM) and atomic force microscopy (AFM) [32, 40, 125-127]. In a recent study, soil organic extracts obtained from an isopropanol/ammonia soxhlet extraction were re-deposited to the extracted soil particles and the impact of the solvent system on the physicochemical properties of the soil particles was examined using AFM in tapping mode [32]. It was demonstrated that the solvents reduced the organic layer film on the soil particles as shown by the exposure of large islands of bare underlying mineral surface. Additionally, topographical and phase image analysis of model hydrophobic surfaces formed by using cholesterol, stearic acid and octadecane (typical SOM components with varying degrees of polarity and molecular structure) were also performed. In each case, and as is commonly observed, the conferred hydrophobicity, as measured by the WDPT and contact angle, was found to be lower than the original hydrophobic soils although it increased with concentration in the case of stearic acid. Despite the numerous compounds currently identified, knowledge of how these compounds interact and/or adsorb on soil particle surfaces have remained elusive, making it difficult to identify the precise mechanisms responsible for hydrophobicity. As such, investigations focusing on the specificity and orientations of SOM film components responsible for soil water repellence on soil surfaces are still limited. A 40 better understanding of the constituents of SOM, their distribution and interactions (between each other and with the soil surface) would be significant in designing effective remediation regimes to overcome the limitations of SWR and thus improve wetting and water transport in water repellent soils.

4.3 Results and discussion

4.3.1 W.A. soil properties

The properties of these two W.A. soils are given in Table 3.1 where the Molarity of an Ethanol Droplet (MED) test [44] was used to determine the degree of water repellency of the two soils: very severe hydrophobicity (South Stirling) and lower hydrophobic (Dandaragan) soil. The exchangeable cation content of Dandaragan soil was higher than those of South Stirling with an exception of Al3+. Both Fe and Al ions are known to have strong affinity for soil organic matter [114] and the large surface area together with the high content of total organic carbon (37.95 mg/g of soil) available in Dandaragan soil compared to South Stirling (18.72 mg/g soil) favour this relationship (Table 4.1). Similar results have previously been reported [28] where relatively large amounts of Al and Fe were detected in a sandy loam soil with large specific surface areas which also contained a high total organic carbon content compared to two sandy soils studied which had relatively higher levels of hydrophobicity.

4.3.2 Extraction of SOM from W.A soils

The total organic carbon (TOC) and the amount of solvent extract from < 2 mm samples from South Stirling and Dandaragan are given in Table 4.1. Here, the values are expressed on a surface area basis since SOM is considered to exist predominantly as coatings on the soil surface. The organic matter extracted using IPA/NH3 (7:3 v/v) solvent system varied between soils with almost twice the amount of soil organic matter extracted from Dandaragan compared to South Stirling soil (surface area 1.5 m2/g and 0.79 m2/g respectively). Following solvent extraction, the soil residues were rendered more wettable (as measured by the advancing contact angles calculated from the Washburn equation were > 90.0o before, and 77.2 ± 0.2o after extraction for South 41

Stirling soil and 85.5 ± 0.1o before and 78.4 ± 1.5o after extraction for Dandaragan soil). These results are consistent with the notion that this solvent system is effective in extracting water repellent compounds [123, 124]. Further fractionation of the dried

IPA/NH3 extract revealed that the extracted SOM was more soluble in chloroform (non- polar solvent) than methanol fraction (polar solvent).

Table 4.1 SOM quantities extracted from South Stirling and Dandaragan soils (surface areas 0.79 m2/g and 1.5 m2/g respectively) using isopropanol: ammonia (7:3, v/v) solvent mixture. The total organic matter (TOC) was obtained by heating the soils at 600o C for 3 h.

Soil Type South Stirling Dandaragan Soil Soil TOC (mg/m2) 23.16 25.30

Solvent extracted SOM (mg/m2) 4.039 2.379

2 CH3OH soluble fraction (mg/m ) 1.179 0.7053

2 CHCl3 soluble fraction (mg/m ) 2.699 1.644

4.3.3 GC-MS analysis of SOM

The GC-MS chromatograms of the major compounds obtained from the methanol and chloroform fractions of South Stirling and Dandaragan soils extracts are given in Appendix Figure A1.1. In general, the relative abundance of the identified compounds varied between fractions and soil. The identified compounds from each fraction were quantified (Table 4.2) and classified into their respective homologous series (Figure 4.3). The high temperatures used during the GC-MS analysis enabled the identification of a variety of compounds whose molecular composition included esters, alcohols, fatty acids, amines, alkanes, aromatic/ring compounds, aldehydes, ketones, carbohydrates, amides and sterols. These compounds are typical signatures of the

42

Table 4.2 Yields of compounds of SOM detected using GC-MS from the soxhlet extracts and their quantification expressed as mg/m2 of soil

South Stirling soil Dandaragan soil

Compounds CH3OH fraction CHCl3 fraction Total SOM CH3OH fraction CHCl3 fraction Total SOM (mg/m2) (mg/m2) (mg/m2) (mg/m2) (mg/m2) (mg/m2)

1. Amines 0 0 0 0.0027 0 0.0027 2. Ketones 0.0017 0.0035 0.0051 0.0024 0.0000 0.0024 3. Alkanes 0.0038 0.0424 0.0462 0.0043 0.0159 0.0201 4. Aromatics/ring compounds 0.0079 0.0041 0.0120 0.0089 0.0191 0.0280 5. Aldehydes 0.0029 0.0057 0.0087 0.0025 0.0337 0.0362 6. Amides 0.0000 0.0316 0.0316 0.0005 0.0365 0.0369 7. Glycans 0.0046 0.0000 0.0046 0.0084 0.0396 0.0480 8. Esters 0.0090 0.0177 0.0267 0.0185 0.0293 0.0478 9. Alcohols 0.0105 0.0900 0.1006 0.0105 0.0693 0.0799 10. Sterols 0.0093 0.1343 0.1436 0.0045 0.0785 0.0831 11. Acids 0.0157 0.0200 0.0357 0.0447 0.0762 0.1209 TOTAL 0.0655 0.3494 0.4149 0.1080 0.3981 0.5060

43

Dandaragan South Stirling 0.16 1. Amines 0.14 2. Ketones 3. Alkanes 0.12

4. Aromatics/ring )

2 0.1 compounds 0.08 5. Aldehydes 6. Amides

0.06 7. Glycans Q (mg/m Q 0.04 8. Esters 9. Alcohols 0.02 10. Sterols 0 11. Acids 1 2 3 4 5 6 7 8 9 10 11 List of compounds

Figure 4.3 Total concentrations of volatile SOM (expressed as mg/m2 of soil) identified by GC-MS from Dandaragan (surface area 1.5 m2/g) and South Stirling (surface area 0.79 m2/g) soils

alkaline treatment (isopropanol/ammonia) that is commonly known to be efficient in extracting polar compounds [28, 124].

In particular, the GC-MS results showed that the number of compounds identified in the methanol fraction were more than those in chloroform fraction (Table 4.2). The methanol fraction also contained a larger variety of polar compounds with relatively lower molecular weight as compared to the chloroform fraction which was dominated with molecules with higher molecular weight. Additionally, it was noted that the relative abundance of compounds in the chloroform fraction was significantly higher for each homologous series than in the methanol fraction. These results indicate that the extracts (as demonstrated by the volatile fractions) are richer in polar than non-polar

44

molecules and that the relative abundance of high molecular weight species (as found in the chloroform fractions) though present in both soils, is more prevalent in the severely hydrophobic South Stirling than the lower hydrophobic Dandaragan soil. It can be inferred that polar organic compounds with relatively high molecular weight play a significant role in SWR. These results are consistent with the findings of Morley et al., [120] who demonstrated that similar compounds were present in varying proportions in four sandy soils with different degrees of hydrophobicity and more importantly, that polar compounds with high molecular weight (such as fatty acids with C > 23 and stigmasterol), which were thought to be necessary for exhibiting water repellency, were abundant in the hydrophobic soils while only present in low amounts in the wettable soils.

The GC-MS results further revealed that similar compounds were present (although in varying proportions) in both the Dandaragan and South Stirling soil. It is interesting to note that acids, sterols, alcohols and esters were found in relatively high abundance in the extracts of the two soils (totaling ~ 66% and 75% of the overall detected volatile compounds from Dandaragan and South Stirling respectively). Fatty acids and sterols have been shown to play a significant role in SWR (above) and their distribution in the two W.A soils is discussed below (while the distribution of the other compounds found in significant proportion can be found in Appendix A1.2).

Acids: Most of the acids identified in Dandaragan soil included C7 - C18 as well as benzoic acid and their derivatives, with palmitic (C16) and octadecanoic acids (C18) dominating. These two acids have previously been identified in hydrophobic soils and are associated with their abundance in higher plants [18, 29, 120]. A significant proportion of the shorter-chain acids (C7 – C9), were also detected in Dandaragan soil.

Specifically, both n- and di- (C9) alkanoic acids were detected with the nonanedioic (azelaic) strongly prevailing over nonanoic acid. Azeliac acid and the hydroxy acid,

1, 3, 4, 5-tetrahydroxy-cyclohexanecarboxylic acid (C7) identified, are considered to be

45

products of bacterial degradation [128]. Apart from benzoic acid and its derivatives,

C12, C14, C15, C16, C18 acids were also identified from South Stirling with even numbered carbon atoms dominating over odd, and once again, palmitic acid (C16) detected in high abundance. It has been suggested that variation in soil hydrophobicity can be caused by the ionic state of these acids. When protonated, fatty acid molecules are regarded as being hydrophobic while any changes such as in pH or in moisture content, which favor ionization, may render these acids hydrophilic [27]. These amphiphilic polar molecules are capable of interacting with themselves particularly during the dry seasons where the carboxylate ions orient themselves towards the last remaining water films exposing their hydrophobic moieties thus forming non-polar clusters on soil surfaces resulting in repellency [129, 130]. Importantly, such changes in conformation or even orientation are also dependent on the length and degree of unsaturation of the fatty acids alkyl chain.

Sterols: Three eukaryotic sterols were found in varying proportions in the two W.A soils. Ergosterol was the most predominant sterol identified in Dandaragan soil followed by cholesterol and then stigmasterol while the order was reversed in South Stirling soil. The presence of ergosterol suggests high fungal activity particularly in the Dandaragan soil compared to South Stirling [131]. Stigmasterol on the other hand is a plant sterol while cholesterol originates from animals. Analogs of both stigmasterol and cholesterol with different molecular conformations (having variations in the type of functional groups, their position and orientation in the hydrophobic ring as well as alkyl side chain) were also detected, although in relatively lower proportions.

A closer comparison of the amount of fatty acids and sterols identified in the soils shows that there were far more sterols than acids identified in the South Stirling soils compared to Dandaragan. More importantly, the ratio of palmitic acid to stigmasterol was ~ 1:1 in the severely hydrophobic South Stirling soil (Table 4.3). These values served as guide in the preparation of model hydrophobic surfaces (Section

46

4.3.4) that were used to probe the role played by these SOM components, and in particular the nature of hydrophobic interactions in soil water repellence.

Table 4.3 Two dominant classes of SOM, acids and sterols, found in the two W.A. soils and reported to cause hydrophobicity in soils (calculated as mg/g of soil). Palmitic acid and stigmasterol were found to be relatively abundant and representative of these two key groups

SOM compounds South Stirling soil Dandaragan soil (mg/g soil) (mg/g soil) Acids 0.028 0.181 Palmitic acid 0.011 0.045 Sterols 0.113 0.125 Stigmasterol and Ergosterol 0.015 0.011

Overall the molecular characterization of isopropanol/ammonia extracts from Dandaragan and South Stirling soil indicate both a vegetation- and microbial-dependent distribution of soil organic matter typically associated with water repellency. However, it is noted that the identified compounds may not be the sole contributors of hydrophobicity given that a significant fraction of compounds that still remain unidentified. More importantly, given the marked similarities of molecules present in both the low and severely hydrophobic soils, it is highly probable that the distribution and orientation of specific functionalities of soil organic matter on the soil surface (which dictate their interactions), rather than the absolute quantity of SOM, plays a key role in soil water repellence.

47

4.3.4 Model hydrophobic surfaces

Planar surfaces

Surface microstructure of SOM films on planar quartz: Clean quartz surfaces were hydrophobically modified using model SOM components and their surface microstructures compared to hydrophobic silane treated quartz. The topology of clean untreated quartz surface and of the silane treated hydrophobic substrate (for a scan range of 1 X 1 µm) is shown in Figure 4.4. The untreated quartz surface showed a relatively smooth surface of uniform texture with an average height of ~ 0.8 nm while the silane treatment yielded thin continuous film with undulations of 1.5 - 2.0 nm (Figure 4.4 (a) and (b) respectively) suggestive of a uniformly coated hydrophobic surface.

The SOM components were annealed on the quartz surfaces since these components are usually found not as ‘free’ lipids but bound on the soil surface. Moreover, annealing the SOM components provides the extra advantage of a system that is both mechanically and chemically at equilibrium. Surface topology of the SOM coated quartz surfaces obtained using AFM in tapping mode revealed distinct differences between the film surfaces produced by individual components compared to their 1:1 equimolar mixture.

The AFM images of palmitic acid films deposited at 0.22 µg/cm2 on cleaned quartz surfaces formed a discontinuous film consisting of discrete molecular aggregates (or hemimicelles) of diameter 200 - 400 nm (Figure 4.5). The height profile reveals a film with aggregates between 50 - 100 nm high, suggesting the presence of multilayer packing of the chains (palmitic chain length ~ 1.888 nm from simulation studies). The molecular aggregates form following the annealing process, where a transition from the liquid to crystalline phase takes place. During this transition the interaction between the hydrocarbon chains increase, resulting in tighter packing with long-range ordering. The 48

molecular volume thus decreases creating distinct domains/clusters leaving hydrophilic gaps on the quartz substrate. At room temperature, the interaction between adjacent palmitic acid molecules conforms to highly ordered aggregates as clusters, where the alkyl chains are assumed to be fully extended and perpendicular to the surface. Hence overall, hydrophobic clusters are formed separated by the hydrophilic quartz surfaces.

a b

7.0 7.0

3.5 3.5

Height (nm) Height Height (nm) Height 0.0 0.0 0.0 0.5 1.0 0.0 0.5 1.0 Length (μm) Length (μm)

Figure 4.4 AFM in tapping mode using silicon tips showing; the topography and the corresponding height profile (below the respective topographic image) of nanostructures of; clean untreated quartz substrate (a), and silanized quartz substrate (b). The white lines across the image indicate the transit where measured height variations were recorded.

49

It has been suggested that this arrangement [132] is favoured by the saturated alkyl chains of palmitic acid which align themselves vertically in order to maximize on the intermolecular van der Waals forces (as schematically shown in Figure 4.6a) as saturated fatty acids are known to exist in the condensed phase in monolayers over a wide range of temperatures [133]. As noted previous, similar microstructures have also been reported with mineral soil surfaces before and after treatment with palmitic acid where a difference in moisture adsorption behaviour was shown [40]. Moisture adsorption studies indicate a continuous coating of adsorbed moisture formed on untreated surfaces with an irregular distribution of condensed water droplets adsorbed onto the 0.1% PA treated soil surfaces indicative of discontinuous PA films.

(a) (b)

140.0

70.0 Height (nm) Height

0.0 0.0 0.5 1.0 Length (μm)

Figure 4.5 AFM in tapping mode using silicon tips showing; (a) the topography and (b) height profile of palmitic acid film on quartz (0.22 µg/cm2). The white line across the image indicates the measured height variation transit.

50

(a)

(b)

(c)

Figure 4.6 Schematic representation of SOM molecular alignment represented in a monolayer for simplicity: crystalline phase of palmitic acid (a), stigmasterol (b), and an equimolar mixture of PA and ST (c). The red spheres represent the polar head groups while the black lines represent the hydrophobic moieties of both PA and ST

Topological images of quartz surfaces modified using stigmasterol deposited at 0.35 µg/cm2 also revealed the formation of discontinuous films (Figure 4.7). These films however consisted of discrete molecular aggregates with diameters ranging from 30 - 60 nm. The height profile reveals a height variation of between 2 - 4 nm. It can be inferred that domains of between 1 - 3 multilayers of molecules of stigmasterol were

51

formed on the quartz surface (the length of cholesterol molecule is ~ 1.6 nm [134]). Although the rigid planar ring system found in stigmasterol is able to pack closely together, chain branching (the ethyl group at C24) and the presence of the double bond at the C22-C23 position of the alkyl chain creates a kink or bend at these two positions which creates extra spacing between molecules.

(a) (b)

7.0

3.5 Height (nm) Height

0.0 0.0 0.5 1.0 Length (μm)

Figure 4.7 AFM in tapping mode using silicon tips showing; (a) the topography and (b) height profile of stigmasterol coating on quartz (0.35 µg/cm2). The white line across the image indicates the measured height variation transit

The interaction between the hydrophobic moieties of stigmasterol molecules is therefore reduced and the resulting conformation is one which exhibits a decrease in chain ordering (Figure 4.6b). The overall outcome is the presence of smaller hydrophobic micro-domains with significantly reduced heights, separated by the hydrophilic quartz surface.

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To understand the interactions between the hydrophobic chains of saturated fatty acids and sterols, AFM analysis of a binary equimolar mixture of palmitic acid and stigmasterol was studied. The coating of quartz substrates with this equimolar mixture (0.11 µg PA+ 0.18 µg ST/cm2) resulted in the formation of larger aggregates of 100 - 150 nm, with a continuous film coating between them having a topology consisting of undulations of 3 – 5 nm, suggestive of a gel-like disordered phase separate from the more crystalline aggregates (Figure 4.8).

a b

14.0

7.0 Height (nm) Height

0.0 0.0 0.5 1.0 Length (μm)

Figure 4.8 AFM in tapping mode using silicon tips showing; (a) the topography and (b) height profile of an equimolar palmitic acid and stigmasterol mixture coating (0.11 µg PA + 0.18 µg ST/cm2). The white line across the image indicates the measured height variation transit

This limited solubility behaviour may be attributed to the different conformations of hydrophobic moieties that arise from the different structural architectures of the molecules in the mixture [135, 136]. At 160o C (the annealing

53

temperature, used to yield the equilibrium state), both PA and ST are in liquid state. Since the melting point of ST is higher than that of PA, it may be suggested that ST-rich domains formed followed by PA-rich domains which served as nucleating centres for the formation of highly ordered crystalline structures. As cooling takes places, molecules of individual SOM components experience reduced fluidity as well as changes in phase behaviour and the presence of ST significantly changes the interactions between the PA molecules in the mixture. The relatively rigid ST molecules align themselves alongside the PA molecules with their 3β-OH group oriented towards the carbonyl group providing an opportunity for the polar head groups to interact via hydrogen bonds. However the dominant hydrophobic portion of the molecules interact strongly via van der Waal forces thus limiting the effect of the head group interactions [135, 136]. On the other hand, the fused rigid ring system of ST aligns itself alongside the fatty acids tails resulting in different conformations (trans and gauche) of the fatty acid. Simulation studies of a molecular model constructed using a mixture of palmitic acid and cholesterol at a 1:1 mole ratio for example [137], have shown that cholesterol affects the ordering along the alkyl chain of palmitic acid differently.

The observed limited miscibility can be explained further by the differences in chain saturation and branching present in the hydrocarbon chains of the two molecules. It has previously been shown that saturated and unsaturated hydrocarbons experience poor mixing in the solid phase [136, 138]. The presence of the kink emanating from chain branching and unsaturation in the alkyl chain of ST, interferes with the close packing normally observed with PA molecules creating a tilt angle along the fatty acids backbone to accommodate for the additional space (Figure 4.6c). Such phase formation behaviour is consistent with the binary solubility reported for these compounds where mixing of palmitic acid and cholesterol has been shown to produce phase separation with 80% forming crystalline phase together with remaining 20% existing as liquid-like or gel-like phase in equilibrium [30]. Here the phase separated aggregates are within a continuous coating of PA and ST gel phase. 54

AFM frictional force measurements were also used to determine the surface hydrophobicity of the three model SOM treated surfaces at nonoscale. Figure 4.9 show the 3-D images of the surface topology and the corresponding friction force indicating the spatial distribution of hydrophobicity of the two SOM components forming the SOM surfaces. In surfaces coated with either PA or ST, the surfaces of the hemimicellar aggregates were hydrophobic while the spaces between them remained hydrophilic as demonstrated by the differences in the friction force between the hydrophilic silicon nitride probe tip and the SOM film coated surface. Here the probe tip experienced higher frictional forces on spaces between the hemimicellar aggregates compared to the hemimicellar surfaces. Hence, both PA and ST formed discontinuous films consisting of hydrophobic hemi-micellar aggregates separated by hydrophilic quartz regions. In comparison, the surface coated with an equimolar mixture of PA+ST, exhibited less friction force throughout the surface indicative of formation of continuous hydrophobic film which was distinctly different from the behaviour of the individual SOM components. In summary, these results emphasize the significance of the hydrophobic interactions in the films formed using palmitic acid, stigmasterol and their equimolar mixture. In particular, the marked structural differences between the hydrophobic moieties namely, differences in chain lengths, the presence of carbon chain unsaturation and branching along the hydrocarbon chain can affect mixing of molecules in the solid state and hence the extent of non-wetting coatings on soil particles. This suggests that the molecular structures of components that make up the soil organic matter and which exhibit different structural architectures may significantly dictate their interactions at the soil particle-air interphase generally and hence subsequently soil water repellence.

55

Figure 4.9 3-D images of friction force obtained by contact mode AFM using hydrophilic silicon nitride tips on the model SOM coated planar quartz surfaces imaged in Figures 4.5, 4.7 and 4.8

Wetting characteristics of model SOM coated planar quartz surfaces: Static water contact angles of hydrophobically modified planar surfaces were measured under ambient conditions (20 °C, 40 % RH) using the sessile drop technique. Freshly cleaned (untreated) quartz surfaces exhibited mean water contact angles of 11.2  0.9. Films of either palmitic acid (at the surface coverage of 0.22 µg/cm2) or stigmasterol (0.35 µg/cm2) on the planar quartz surfaces exhibited wetting contact angles that rose to 63.1  1.6 and 78.8  4.0 respectively. In contrast, surfaces coated with an equimolar mixture of palmitic acid and stigmasterol (0.11 µg + 0.18 µg/cm2) were significantly more hydrophobic with contact angles 89.1  1.6. By comparison, silane-coated quartz surfaces exhibited only slightly higher levels of hydrophobicity (contact angles 91.1  0.9). This wetting behaviour of the model hydrophobic surfaces is consistent with the

56

surface microstructures of the films obtained from AFM where the PA and ST formed discontinuous hydrophobic films on quartz and rendered the surface partially hydrophobic while the PA + ST mixture and monolayer silane coatings produced continuous water repellent films making the surface strongly hydrophobic.

Model hydrophobic particulate beds: Capillary imbibition kinetics (capillary suction wetting) of water on model hydrophobic particulate beds prepared using PA, ST and an equimolar mixture of PA + ST on clean quartz sand together with silane treated sand were determined to evaluate the impact of these surfaces on the water repellence in the porous bed regime. The capillary rise (water imbibition) is shown for these particulate bed surfaces in Figure 4.10. Particulate beds coated with individual PA and ST showed a similar initial rate of imbibition within the first 50 s which then slowed down over time. The advancing water contact angles were calculated from the initial slope of the kinetic curves using the Lucas-Washburn equation for PA and ST treated particulate beds (Table 4.4) and showed comparable contact angles to those obtained with the corresponding planar surfaces above. Whereas the rate of water imbibition was considerably lower (~ 7%) in the particulate bed prepared with equimolar PA + ST (25 µg PA + 40 µg ST)/g reaching a maximum height of ~ 2.3 cm after 30 mins

57

6 (a) 5 4

3 (cm)

h h 2 1 0 0 50 100 150 200 250 time (s)

6 (b) 5

4 ) 3

h (cm h 2 1 0 0 50 100 150 200 250 time (s)

(c) 3

) 2

h (cm h 1

0 0 200 400 600 800 1000 1200 time (s)

Figure 4.10 Capillary imbibition of water on model SOM-coated particulate beds: (a) PA (50 µg/g); (b) ST (80 µg/g) and (c) equimolar mixture of PA and ST at concentration (25 µg PA +40 µg ST)/g 58

Table 4.4 Advancing contact angles of model hydrophobic particulate beds with model SOM treatment

Untreated Palmitic acid Palmitic acid + hydrophilic (50 µg/ g Stigmasterol Stigmasterol (25 µg quartz soil)* (80 µg/ g soil)* PA+40 µg ST/m2)* Contact angle o  adv 2.0 ± 1.5 76.6 ± 1.5 80.0 ±1.5 89.6 ±1.5 * Concentrations equivalent to that used for planar surfaces.

The high calculated contact angle in this particulate bed, indicating strong hydrophobicity, was again consistent with that observed on the corresponding PA+ST treated planar surface. This overall behaviour clearly emphasises the role of molecular interactions between SOM components in inducing hydrophobicity and particularly the nanostructural aspects of these components. To investigate the role of particle bed heterogeneity, mixed hydrophobic- hydrophilic particles were assembled. The capillary imbibition rates of particulate beds made up of particle fractions (0, 50, 75 and 100 wt. % hydrophobic silane particles), decreased with increasing weight fraction of the hydrophobic particles (Figure 4.11). Additionally, the wetting behaviour and the advancing contact angle of the clean untreated quartz particulate beds were similar to those of corresponding hydrophilic planar surface (Table 4.5). Particulate beds consisting 100 wt. % of silane treated particles showed no capillary suction consistent with the degree of hydrophobicity.

59

5

4

3

2

1

0 0 100 200 300

Figure 4.11 Capillary imbibition of water in model silane-coated particulate beds with varying degrees of overall bed hydrophobicity: (i) clean untreated quartz (ii), (iii) and (iv); 50 wt. %, 75 wt. % and 100 wt. % hydrophobic particle beds respectively.

Table 4.5 Advancing contact angles of model bed compositions formed from various volume fractions of silane coated quartz particles

Bed Hydrophobicity Untreated 50 wt. % 75 wt. % 100wt. % quartz o 2.0 ± 1.5 65.0 ± 1.5 87.51 ± 1.0 > 90 Contact angle  adv

However, the 50 wt. % and 75 wt. % partially wetting bed compositions showed moderate water imbibition rates indicative of partial wetting behaviour similar to that found in the particles treated with the individual model SOM components. The particulate beds treated with equimolar PA + ST mixture showed similar wetting kinetics to the 100 wt. % silane treated particulates. This behaviour may be attributed to

60

the formation of the continuous hydrophobic film on the particles similar to that observed on the PA+ ST mixture and the 100% silane treated planar surfaces as shown by the AFM images (Figure 4.8).

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Chapter 5

Impact of Non-ionic Surfactants on Water Transport in Non-wetting Soils

5.1 Overview

In this Chapter, the wetting behaviours during water transport and distribution in surfactant-amended soils are analysed in terms of three specific mechanisms namely surfactant adsorption, capillary suction imbibition and gravitational infiltration. The behaviour and solution properties of two non-ionic surfactants; poly (ethylene oxide) poly (propylene oxide) block copolymer and an alcohol ethoxylate during the wetting process and the subsequent water transport is investigated in the two soils from Western Australia used in broad acre cereal production.

5.2 Introduction

Water transport in soils occurs through pores present in the soil matrix and is governed by factors such as the initial soil water content, soil texture and mineralogy, total organic content or even temperature [139, 140]. In poorly- or non-wetting soils, the transport of water is characterized by poor infiltration rates, preferential or fingered flow, ponding and surface run-off that may lead to soil erosion. [41] [140]. Preferential flow, for instance, causes unstable wetting fronts during water infiltration leading to uneven distribution of water to the rhizosphere as well as accelerating the rate of transport of nutrients to ground water. Such unstable wetting fronts commonly occur if the surface water ponding head (ho) is less than the soil-water entry pressure thus requiring high ponding to enhance infiltration. This can be overcome by the use of surfactants [67, 141] which when applied to the upper layer of the soil surface, changes the interfacial tensions (γsv and γsl) increasing the initial water content which plays a significant role at the initial stages of water infiltration. As such, significant research has

62

been directed towards non-wetting soils and the repercussions of soil water repellency on plant growth and production in the last two decades [35, 36]. Several approaches are being evaluated and used in the remediation of soil water repellency of which the use of surfactants remains a low capital intensity alternative.

5.2.1 Surfactants

Non-ionic surfactants have been of interest during the last few decades as they are commonly used to alleviate the effects of water repellence and improve the wetting of soil particles and infiltration of water in soil beds [100-102] particularly in the turf- grass industry. In particular, amphiphilic block copolymers consisting of ethylene oxide and propylene oxide units in various arrangements have attracted attention in the amelioration of soil water repellence and are of particular interest in this study. In a recent study for example, Oostindie et al.,[142] investigated the effects of a soil surfactant, methyl-capped tri-block copolymer, in reducing the effects of water repellency in sandy turf soils using moisture sensors, where it was shown that this triblock copolymer eliminated repellence for a period of twelve months. Further, the surfactant produced a homogenous flow pattern, eliminating the preferential flow previously observed, improving the wettability within the rhizosphere, and providing adequate supply of water and agrochemicals to the turf grass.

Copolymers such as tri-block poly (ethylene oxide) poly (propylene oxide)

(EOxPOyEOz), for example, are non-ionic surfactants whose molecular characteristics depend strongly on the degree of polymerization (x, y and z) and their molecular weight. The degree of polymerization and in particular the number of hydrophobic propylene oxide (PO) units, have been shown to influence their solution behaviour [143]. Additionally, a further group of non–ionic surfactants of interest, belong to the alcohol ethoxylates [144] CxH2x+1 (CH2CH2O)yH commonly denoted as to the CiEOj which consists of an alkyl hydrophobic tail connected to a poly ethylene oxide head 63

group and are typified by their low molecular weight. As previously discussed (Section 2.6), the solution properties of surfactants [143, 145, 146] have been shown to significantly control the wetting, spreading and water imbibition in porous materials and the nature and structural architecture of the surfactants is recognized as greatly influencing their solution properties [143, 147]. In non-ionic surfactants, the degree of polymerization and in particular the number of propylene oxide (PO) units in block polymers, for example, have been shown to affect properties such as solubility (increasing the hydrophobic PO block significantly decreases solubility) as well as the temperature dependence of the micellization process. Alexandridis et al., [143] for example, studied the effect of number of PO units (in block copolymers with similar EO units) on the critical micelle concentration (CMC) at a given temperature and demonstrated that the CMC of these pluronic surfactants [2, 148] decreased with increasing PO units implying that block copolymers with large PO units form micelles at lower concentrations.

Additionally, surfactant blended mixtures have been formulated in order to manipulate the solution properties so as to achieve desired outcomes. When more than one surfactant is present in solution, mixed aggregates are formed at high concentrations and the CMC of the mixture is a function of the surfactant composition in solution.

Under ideal situations, the CMC of the mixture (CMCmix) can be predicted empirically using the expression;

CMCmix = X1 CMC1 + X2 CMC2 5.1

where X1 and X2 are the mole fraction of surfactants in the micelle and CMC1 and

CMC2 are the respective CMC of surfactant 1 and 2 in the mixture.

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5.2.2 Mechanism of surfactant-enhanced wetting and water transport in soils

Surfactant adsorption: Adsorption of surfactant molecules at the solid-liquid interface may be considered to be one of the most important characteristics of surfactant behaviour. Non-ionic surfactants adsorb on surfaces via physical interactions where changes in the surfactants concentration and temperature can have a significant effect on the adsorption process. The mechanism underlying the adsorption of non-ionic surfactants has been examined in a number of studies [70, 149-152]. For instance, Liu et al., [149] studied the adsorption behaviour of four non-ionic surfactants at concentrations below and above their CMC using both surface tension measurements and spectrophotometry and showed that micelle-forming surfactants adsorb differently from surfactants that form bilayer lamellar aggregates. It was further shown that adsorption of surfactants at pre-CMC concentrations followed a simple Freundlich type of isotherm, where micelles do not adsorb on the soil rather it is the surfactant unimers that adsorb, and that the maximum adsorption capacity was independent of the ratio of surfactant solution to soil used.

The adsorption of surfactants on soil has been shown to vary directly with the organic carbon content of the soil [70, 153]. In a study performed to elucidate the behaviour of surfactants on soils with different organic contents, Ussawarujikulchai et al.,[70] demonstrated that the adsorption of the non-ionic surfactant Triton X-100 increased with surfactant concentration as well as the amount of organic content present in the soil. Additionally, it was shown here that the CMC of the surfactant solution following adsorption (termed CMCeff) also increased with increase in organic content. Such studies indicate the absence of a simple single adsorption mechanism (Freundlich) for the adsorption process but rather, influence by a number of factors such as the structure of the surfactant, the nature and amount of soil organic matter together with the soil clay content [151]. Measured adsorption isotherms provide the adsorption capacity of the solid as well as indicating mechanisms of adsorption. Most generally,

65

adsorption of non-ionic surfactants usually follow a Langmuir isotherm [152] although different classification systems representing the adsorption of organic solutes in general also exist [154, 155]. The Brunauer-Deming-Deming-Teller (BDDT) classification system for example, describes isotherms with respect to their initial slope and the sub- groups represent the shape of the curve at high concentrations. Considering the number of variables that influence the wetting process in poorly- and non-wetting soils, it is imperative therefore that the solution properties and surface behaviour that govern soil wetting and water transport in these soils be well defined.

Capillary imbibition: Water transport in poorly- and non-wetting soils is limited by the existing large contact angle (θ > 90o) at the solid-air interface. However, in the presence of surfactants and depending on the surfactant concentration at the wetting front, significant changes at both the solid-liquid and liquid-air interfaces and hence the advancing contact angles take place. During the surfactant-amelioration of water repellent soils, adsorption of surfactant at the three interfaces (Figure 2.3); water-air, solid-air and solid-liquid leads to the reduction in both the surface tension of the water phase and the advancing contact angle. Capillary imbibition of a liquid is generally modelled according to the Washburn equation (Section 2.5) [57, 61] for particle beds o where θa < 90 . In the presence of surfactants the imbibition therefore depends on the dynamics of surfactant adsorption at the three phase line (TPL) which is a function of the nature and concentration of the surfactant, (e.g., above or below the CMC), soil texture (e.g., tortuosity of the liquid pathway) and the nature of the hydrophobic SOM.

Gravitational imbibition: Non-wetting soils require a positive pore entry pressure for water to penetrate the soil matrix. This represents a positive water entry pressure head (depth of water, h) needed to overcome the capillary suction limitation of NWS, e.g,

66

2훾푐표푠휃 ℎp= − 5.2 휌푔푟

where, hp is the water entry pressure head, γ is the liquid-air surface tension, θ is the liquid-solid contact angle, γcosθ is the wetting force (vector), ρ is the density of the liquid, g is the gravitational constant and r is the capillary radius. When θ < 90o i.e., partially wetting soils, hp is a negative pressure [156] while with non-wetting soils where θ > 90o the pore entry pressures are positive. This positive entry pressure can be overcome by the sufficient depth of free water above the soil (ho, ponding) where Feng, et al., [157] found this ratio to be ~ 3. Additionally this positive value of hp can be lowered by surfactants which decrease γ and increase imbibition. At the same time, θ is also reduced which for NWS increases infiltration but as found by Feng et al., may inhibit infiltration in more wettable soils, that is the effect of surfactants is highly dependent on the initial degree of hydrophobicity of the soil.

5.3 Results and discussion

5.3.1 Single surfactant systems

Surface tension and CMC: A plot of surface tension (mN/m) plotted against the surfactant concentration (g/L) for aqueous solutions of the block copolymer and the alcohol ethoxylate surfactants are given in Figure 5.1. Two distinct regions exist. At very low surfactant concentrations a sharp drop in surface tension occurs with very small increases in surfactant concentration up to a definite point of inflection. This initial region marks the reduction of surface tension as surfactant molecules in their unimeric state adsorb at the liquid-air interface. Beyond the point of inflection, any increase in surfactant concentration results in a minimal decrease in surface tension due to the formation of micelles (Section 2.5). The results show that the alcohol ethoxylate is capable of decreasing the surface tension of water significantly more than the block copolymer surfactant and requires higher surfactant concentrations to form micelles

67

80 80 60

mN/m 40 20 60 0 1 2 3 4 5 g/l

40 Surface (mN/m) tension Surface

20 0 0.05 0.1 0.15 0.2 0.25 Concentration (g/L)

Figure 5.1 Surface tension of surfactants; (■) block copolymer and (●) alcohol ethoxylate; C13EO8 as a function of Concentration

(CMC = 0.035 g/L) compared to the block copolymer (CMC= 0.02 g/L). The low CMC attained by the block copolymer may be attributed to its relatively higher hydrophobic character emanating from the long poly (propylene oxide) units (PO = 30). Additionally, given that the number of EO units in the head group are relatively less in the block copolymer, they may contribute in the lowering of the CMC although not as significantly as the hydrophobic effect. Furthermore, both ethylene oxide and propylene oxide units have different degrees of solvation making the entropy-driven micellization of the block copolymer strongly depend on the structure (i.e., degree of polymerization) and the concentration of the surfactant in solution [158]. 68

Surface adsorption: The first mechanism in surfactant enhanced wetting, adsorption, involves the partitioning of surfactant molecules from the liquid phase to the solid (soil surface) phase. Adsorption of a surfactant on soil surfaces is governed mainly by the nature of the surfactant together with that of the soil (hydrophobic or hydrophilic), and is significant during wetting and water transport dynamics in soils since it influences the overall transport mechanisms; capillary suction and gravitational imbibition. Adsorption of surfactant molecules on each soil was carried out using the batch equilibrium technique (Section 3.6.2) over a range of surfactant concentrations (below and above the CMC). Plots of the surface tension of surfactant solutions before and after adsorption on South Stirling and Dandaragan soils, at concentrations below the CMC using the bubble pressure tensiometer at 20 ± 2o C, are given in the Appendices Figures A2.1 – A2.6. Here, the changes in surface tension of surfactant solutions before and after adsorption on the two soils are plotted on the initial concentration scale to indicate the depletion of surfactant concentration in solution by the adsorption process on soil surfaces. The surface tension of the supernatants obtained from the aqueous soil systems were higher than for pure surfactant solutions at a particular initial concentration in the batch experiment indicative of adsorption. A sharp decrease in surface tension (from ~73- 50 mN/m) with increasing concentration is observed in the pre-CMC region and is indicative of the quantity of surfactant unimers adsorbed on the soil. In the second region (post-CMC region), the surface tension remains almost constant with increasing surfactant concentration. This implies that the concentration of surfactant unimers in the solution remains constant and any further increase in surfactant concentration results in further micelle formation. Surfactant adsorption can be seen to be complete at or near the CMC. The values of CMC, obtained from supernatants of the surfactant solutions, following adsorption at a constant ratio of surfactant solution to weight of soil are higher (Table 5.1), reflecting the lowering of solution concentration due to adsorption and consistent with the findings of Laha et al., [66].

69

Table 5.1 CMC of surfactant solutions before and after adsorption on South Stirling and Dandaragan soils

CMC (g/L) after adsorption

Surfactant CMC (g/L) South Stirling soil Dandaragan soil

Block copolymer 0.020 0.022 0.055

Alcohol ethoxylate 0.035 0.060 0.120

Additionally, the higher surface tension (and CMC) values obtained from the surfactant/Dandaragan soil systems can be attributed to the relatively higher organic and clay content in the Dandaragan soil that acts as further adsorption sites for surfactant unimers. These results are consistent with those obtained by Zheng and Obbard [150] who demonstrated using both GC-MS and surface tension techniques that the CMCs of three non-ionic surfactants, Tween 80, Triton X-100 and Brij 35 increased in soil/aqueous systems and attributed this elevation to the adsorption of surfactant molecules, which was shown to be proportional to the organic content.

2 The quantity of surfactants adsorbed (Qads mg/m ) onto the two W.A soils was determined using Expression 3.2 and the adsorption isotherms of surfactant solutions on the NWS were then generated. The adsorption isotherms for the two non-ionic surfactants on the severely hydrophobic South Stirling and low hydrophobic Dandaragan soil, over the full range of surfactant concentrations used in the batch experiments, are given in Figure 5.2 and 5.3 where data is expressed on an area (m2) basis. In general, the isotherms showed different adsorption characteristics which varied depending on the soil and nature of the surfactant. Isotherms obtained with pre-CMC

70

concentrations indicate a Freundlich type of isotherm (Appendix Table A2.1) where all initially adsorbed surfactant unimers are in a simple physical equilibrium with their solution counterpart molecules. These results are consistent results obtained by various authors who have shown that the adsorption of non-ionic surfactants on soil at concentrations below the CMC follows the Freundlich isotherm [149, 151, 159]. Over the larger range of surfactant concentrations (up to and post-CMC) Langmuir isotherm behaviour gives distinct monolayer plateaus. The adsorption of the block copolymer surfactant on South Stirling soil (Figure 5.2 A) was characterized by a steep linear slope at low surfactant equilibrium concentrations (with an adsorption capacity, K = 2.85, Appendix Table A2.1) which implies that surfactant molecules are filling the available adsorption sites at a rapid rate indicating a strong interaction between the surfactant molecules and the soil surfaces compared to the surfactant-solvent (water) interaction, which could be attributed to the strong hydrophobic matching between the surface SOM and the PO units in the surfactant monomers. As the concentration in solution increases, adsorption of surfactant molecules becomes increasingly impeded as adsorption sites begin to fill up and the rate of adsorption forms a plateau reaching a maximum (Q max ~1.0 mg/m2) at surfactant equilibrium concentration of ~ 0.93 g/L. The initial surfactant concentration (in the batch experiment) that corresponds to the onset of the plateau is higher than the CMC of surfactants suggesting that the unimers physically adsorb in the extended configuration with PO units attached to the soil surface in the form of tightly coiled loops and trains (segments of surfactant molecules in contact with the soil) [160, 161]. The plateau observed at high equilibrium concentrations suggests saturation of the original adsorption sites and indicates the formation of a monolayer and surfactants unimers in this region are then in equilibrium with micelles.

71

(A) 0.3 0.2 0.1 0 0 0.01 0.02 2.5

) 2 2 1.5

Q (mg/m Q 1

0.5

0 0 1 2 3 4

C eqb (g/L)

(B) 1 0.5

0 0 0.01 0.02 0.03 2.5

2 )

2 1.5

1

Q (mg/m Q 0.5

0 0 1 2 3 4

C eqb (g/L)

Figure 5.2 Adsorption isotherm of non-ionic surfactants: block copolymer (A) and alcohol ethoxylate (B) on the severely hydrophobic South Stirling soil over the complete range of concentrations used. The inset shows the isotherms generated with pre-CMC concentrations

72

A similar adsorption behaviour was observed with the alcohol ethoxylate isotherm on the severely hydrophobic South Stirling soil (Figure 5.8 B) although the initial slope at very low equilibrium concentrations was steeper and more vertical (with a significantly larger adsorption capacity, K = 24.90, Appendix Table A2.1), indicative of the initial surfactant molecules having a very high affinity for the adsorption sites on the soil surface. The initial slope was not only steeper but persisted longer than that in the block copolymer signifying a higher adsorption density of surfactant molecules per 2 unit area of the soil (Q max ~ 1.4 mg/m ). As with Langmuir isotherms, when the adsorption sites begin to fill up, the surface adsorption density slows down resulting in a plateau indicative of a monolayer up to an equilibrium concentration of 2g/L where the point of inflection indicates the onset of “co-operative adsorption” and the commencement of a multilayer. Similar results were obtained by Sánchez-Martín et al., [151] who studied the adsorption of three surfactants that included a non-ionic surfactant (tween X-100) on clay soils and demonstrated that the adsorption of Tween X showed co-operative behaviour and attributed this behaviour to the type of mineralogy present in the clay soils. Here it is shown to be a more general adsorption behaviour occurring readily on a highly hydrophobic soil. This suggests that cooperative surfactant adsorption and hence multilayer formation is dictated by the particular soil surface- surfactant relationship.

The adsorption isotherms for the non-ionic surfactants on Dandaragan soil are illustrated in Figure 5.3. At low equilibrium concentrations (pre-CMC values), the adsorption isotherm of the block copolymer surfactant indicate a Freundlich isotherm, that is concave to the equilibrium concentration suggesting very a low affinity binding constant (as shown by its low K value in Appendix Table A2.1), which is then seen up to and beyond the CMC suggesting the beginning of molecular association. On the other hand, the adsorption isotherm of the alcohol ethoxylate on Dandaragan soil indicates a Freundlich isotherm at pre-CMC concentrations with a relatively higher affinity binding

73

(A) 0.4

0.2

0 4 0 0.01 0.02 3.5

) 3 2 2.5 2

Q (mg/m Q 1.5 1 0.5 0 0 1 2 3 4 C (g/L) eqb

(B) 0.4

0.2

0 4 0 0.01 0.02 0.03 3.5

3 ) 2 2.5 2

1.5 Q (mg/m Q 1 0.5 0 0 1 2 3 4 C (g/L) eqb Figure 5.3 Adsorption isotherm of non-ionic surfactants: block copolymer (A) and alcohol ethoxylate (B) on the low hydrophobic Dandaragan soil over the complete range of concentrations used. The inset shows the isotherms generated with pre-CMC concentrations 74

constant and begins to correspond to a Langmuir isotherm up to and beyond 2 CMC with Q max ~ 2.2 mg/m . In general, more surfactant was adsorbed on Dandaragan soil given its high organic content compared to the severely hydrophobic South Stirling soil. Similar results have previously been reported by Rodríguez-Cruz et al., [159] who demonstrated that in using the non-ionic surfactant TX-100 the adsorption behaviour on 18 different soils varied with the total organic matter (OM) and the clay content. Soils with very high organic matter content (> 5%) followed the S type of isotherm in which adsorption increase with increasing surfactant concentration while soils having medium or low OM and low clay content corresponded to Langmuir type adsorption and soils with low OM but high clay content corresponded to the H (high affinity) types of isotherm.

Capillary imbibition: Capillary rise experiments in packed beds prepared using both South Stirling and Dandaragan soils (Section 3.6.3) were conducted using surfactant solutions at the conventional 4 g/L (together with water as control) to determine the effect of surfactants on capillary suction and transport. Curves showing capillary rise for water and the two non-ionic surfactants as a function of penetration time on South Stirling and Dandaragan soils are given in Figures 5.4 and 5.5 respectively. South Stirling soil shows a clear difference in capillary suctional wetting with the block copolymer surfactant having a considerably lower wetting rate (indicating the influence of adsorption on the TPL) compared to the alcohol ethoxylate (Figure 5.4). The adsorption of the alcohol ethoxylate surfactant on the severely hydrophobic South Stirling soil (Figure 5.2B) is more favoured than that of the block copolymer, which also has a very low diffusion coefficient reducing its transport to the [162]. It has previously been determined that block copolymers particularly those with long hydrophobic poly (propylene oxide) units are capable of forming thermodynamically stable micelles [163] which would slow the relaxation τ2 times during the release of the

75

copolymer unimers from the micelles [160, 162] and which may further significantly slow down the adsorption of surfactant unimers along the advancing meniscus.

6 4 6 2 0 5 0 600 1200 1800 2400

4

3 h (cm) h

2

1

0 0 60 120 180 240 300 time (s)

Figure 5.4 Capillary imbibition of non-ionic surfactants at a concentration of 4 g/L; (■) block copolymer, (●) alcohol ethoxylate and (▲) water on the severely hydrophobic South Stirling soil

Both surfactants appear to reach a similar terminal plateau in capillary rise due to the terminal height being a balance between the capillary suction pressure and gravity. In the low hydrophobic Dandaragan soil, the effectiveness of the surfactants are more similar, but with the alcohol ethoxylate solution advancing at a slightly higher rate at the initial stages (Figure 5.5). To address the underlying mechanism of this behaviour, the respective contact angles and interfacial tensions were analysed. From the respective kinetic data, the contact angles were calculated according to the Lucas-Washburn

76

equation (Section 2.5 and Appendix Figure A2.7). Table 5.2 provides the surface tensions and contact angles from these plots together with the respective calculated wetting force (vector) γcos θ (Section 5.2.2).

6 4 2 0 0 300 600 900 1200 6

5

4

3 h (cm) h 2

1

0 0 60 120 180 240 300 time (s)

Figure 5.5 Capillary imbibition of non-ionic surfactants at a concentration of 4 g/L; (■) block copolymer, (●) alcohol ethoxylate and (▲) water on the low hydrophobic Dandaragan soil

From Table 5.2, it can be seen that with the severely hydrophobic South Stirling soil the force advancing the wetting front in the presence of the alcohol ethoxylate surfactant is about 10 times that of the block copolymer. Whereas with the significantly less hydrophobic Dandaragan soil, the wetting force for the alcohol ethoxylate is about the same as the block copolymer surfactant. This significant difference of surfactant effectiveness and hence efficiency, points to the importance of the relative behaviour of

77

γ and θ (cos θ), particularly on less hydrophobic surfaces where the cos θ function is particularly sensitive.

Table 5.2 Advancing contact angles for surfactants, 1:1 wt. ratio surfactant mixture and water on South Stirling and Dandaragan soils calculated using Washburn’s equation *Surface tension measurements taken at 20.3 ± 0.5 oC

Surfactant *surface Advancing Soil tension contact o (4 g/L) (γ) cos θ γ cos θ angle (θ )

Block copolymer 40.9 0.0136 0.56 89.2

South Stirling Alcohol ethoxylate 29.2 0.1937 5.66 78.8

Block copolymer- 26.2 0.1095 2.87 83.7 alcohol ethoxylate

Nil (water) 72.9 > 90

Block copolymer 40.9 0.3670 15.0 68.5

Dandaragan

Alcohol ethoxylate 29.2 0.5734 16.7 55.0

Block copolymer- 26.2 0.4285 11.2 64.6 alcohol ethoxylate

Nil (water) 72.9 0.0780 5.69 85.5

78

An examination of the surfactant adsorption isotherms (Figure 5.2 and 5.3) indicates that the relative binding strength as shown by the initial slopes suggest surfactant adsorption is strongest on the severely hydrophobic (South Stirling) soil, as may be expected through Van der Waals bonding, and that the alcohol ethoxylate surfactant (showing a rectilinear isotherm coincident with the y-axis) exceeds the block copolymer. This also corresponds to the relative wetting forces (5.66 and 0.56 mN/m). Whereas on the low hydrophobic (Dandaragan) soil although the overall wetting forces were greater due to a component from the inherent wettability of the soil), the relative difference was less (16.7 and 15.0 mN/m).

Gravitational infiltration: Water infiltration in packed beds prepared using both South Stirling and Dandaragan soils pre-treated with surfactant solutions and with water as control, was carried out to determine the effect of surfactants on gravitational infiltration and transport. Results for the gravitational infiltration of water as a function of penetration time on South Stirling and Dandaragan soils following treatment with non-ionic surfactants modelling agricultural practices (surfactant deposition and drying, Section 3.6.5) are given in Figure 5.6 and 5.7 respectively. In South Stirling soil, the gravitational imbibition can be divided into two regions; the first 2 cm of packed bed which corresponded to the depth of administered surfactant treatment and where both adsorbed surfactants showed similar infiltration kinetics (Figure 5.6). This implies that the pre-adsorbed surfactant molecules made the top layer of the soil wettable and o lowered the pore entry pressure hp since θ is now less than 90 . This region therefore serves to provide an additional depth of wetting front, deff (adding an effective pressure head to the initial 5mm head of ponding), while the second region (the lower ~ 8 cm of packed bed consists of the untreated soil);

deff = a + d treatment 5.3

79

where a is the initial 5mm depth of ponding and d treatment is the depth comprising of the initially pre-deposited surfactant solution, assuming no significant pressure drop across this distance.

As water enters this second (severely hydrophobic) region, the rate of infiltration slows down and the alcohol ethoxylate wetting front advances at a significantly higher rate compared to the block copolymer. Here the initial infiltration rate is dominated by the capillary forces where the wetting force, γcos θ, for the alcohol ethoxylate surfactant is ~ 10 times that of the block copolymer and contact angle θ is lower. These results show that the pre-adsorption of these initially high concentrations (> 10 X CMC) do not influence the re-dissolution rate and hence the surfactant unimer concentration that is

12

10

8

6 d (cm) d

4

2

0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 time (s) Figure 5.6 Gravitational infiltration of water following application of surfactants; (■) block copolymer, (●) alcohol ethoxylate and (▲) water (control) on the severely hydrophobic South Stirling soil at a constant 5 mm ponding head 80

available at the TPL for wetting advancement. The rate of infiltration is higher for the alcohol ethoxylate surfactant since the alcohol ethoxylate adsorption binding is stronger and hence more effective at the TPL as shown, for instance, by the penetration depths after 500 s., i.e. 8.5 cm vs 3.3 cm. Mechanistically, the desorption of polymeric surfactants such as the block copolymer is made difficult by the multiple contacts the surfactant polymer molecule has with the hydrophobic surface as described by Berg [161] thus delaying the re-solubilization of the block copolymer unimers for further adsorption at the TPL. Over time, capillary forces are overcome by gravity and increases in the depth of wetting front creating a higher effective hydraulic head which increases the water content of the soil and further continuing gravitational infiltration.

Figure 5.7 shows that on Dandaragan soil, although both surfactants lower the water infiltration rate over time as discussed in Section 5.2.2, the pre-adsorbed surfactants on the top layer of the soil did not change the inherent wettability of the soil (up to a depth of ~ 4 cm) not providing the additional depth of water front seen in South Stirling soil. Additionally, the respective adsorption isotherms of both surfactants on Dandaragan soil (Figure 5.4) show a significantly lower binding efficiency compared to the severely hydrophobic South Stirling soil (Figures 5.3) as indicated by the initial gradients at low coverage. Further, the wetting force vector (γcosθ) for the alcohol ethoxylate is similar to that of the block copolymer. The net effect of this, to a first approximation, is that both surfactants hinder water infiltration within this soil equally producing 7.5 cm infiltration in 500 s. This impediment may be attributed to the redistributed of surfactant molecules which adsorb on the soil surface with the hydrophilic EO units tethered to the soil surface and the hydrophobic moieties oriented towards the liquid-air interphase creating higher contact angles at the wetting fronts compared to the water control. Water infiltration into soils that have been treated with surfactant solution in the top portion modelling banded application and drying during the season prior to the follow-up rainfall are shown schematically below in Figure 5.8.

81

Here two cases are given for the declining surfactant availability profile with depth. Both cases can influence the water kinetics curves in Figures 5.6 and 5.7.

10 9 8 7 6 5

d (cm) d 4 3 2 1 0 0 200 400 600 800 1000 1200 1400 time (s) Figure 5.7 Gravitational infiltration of water (triangles) following application of surfactants; (■) block copolymer, (●) alcohol ethoxylate and (▲) water (control) on the low hydrophobic Dandaragan soil at a constant 5 mm ponding head

82

A

-0.5 W 0 Co Sa 2 h (cm)

P

10 Surfactant concentration Surfactant

B Depth of soil

-0.5 W 0 Co Sa h (cm) 2

P

10 Surfactant concentration Surfactant

Depth of soil

Figure 5.8 Schematic representation of a linear (A) and curved (B) concentration gradient of surfactant along the advancing water front in a particulate soil bed (P) during gravitation infiltration at constant head of ponding (W). Surfactant solutions were pre-deposited on the top layer of the soil at an initial concentration Co and pre-adsorbed to a depth, Sa

83

5.3.2 Blended surfactant systems

Surface tension and CMC: Blending of surfactants in agricultural practice is common [100-102]. The surface tension behaviour of the 1:1 wt. ratio mixture of the block copolymer/alcohol ethoxylate was compared to those of the individual surfactants at 20.8 ± 0.3o C and are given in Figure 5.9. The surface tension of the surfactant mixture at low concentrations follows closely the concentration dependence of the alcohol ethoxylate surfactant up to 0.25 g/L despite being higher at a given surfactant concentration. The presence of the alcohol ethoxylate component in the mixture further reduces the surface tension of the solution beyond the CMC of the block copolymer (0.020 g/L) by continuously adsorbing at the liquid-air interface prior to forming mixed micelles at a CMC of ~ 0.06 g/L.

Mechanistically, the presence of the alcohol ethoxylate in solution leads to the dehydration of the EO head group of the block copolymer due to interaction between water and the alcohol ethoxylate. This delays micellization of block copolymer resulting in larger mixed micelles and CMC values. These results suggest that the presence of the alcohol ethoxylate in the mixture results in an interaction with the block copolymer that enhances the surface activity of the block copolymer and are in good agreement with measurements on similar systems [164, 165]. Lam and Zhang [158] for example, demonstrated using surface-tension measurements the existence of synergistic interactions between binary mixtures of a block copolymer F127 (EO97PO67EO97) surfactant and either a cationic (tetradecyltrimethylammonium bromide; TTAB) or a non-ionic (C12EO6) surfactant, and further confirmed this increased synergistic behaviour in binary surfactant systems with the more hydrophobic F127 graft copolymer.

84

80

80 60

40

20 0 1 2 3 4 5 60

40 Surface (mN/m) tension Surface

20 0 0.05 0.1 0.15 0.2 0.25 0.3 Concnetration (g/L)

Figure 5.9 Comparison of surface tension of surfactants; (■) block copolymer, (●) alcohol ethoxylate and their (○) blend (1:1 wt/wt,) as a function of concentration

Adsorption of surfactant mixture: The adsorption isotherms for the 1:1 wt. ratio block copolymer/alcohol ethoxylate surfactant blend mixture (and its components) on the two soils over the full range of concentrations used in the batch adsorption are given in Figure 5.10 where data is expressed on an area (m2) basis. Isotherms obtained with pre- CMC concentrations indicate a Freundlich type of isotherm (Appendix Figure A2.8 and Table A2.1) where all initially adsorbed surfactant unimers are in physical equilibrium with their solution counterpart molecules. The surfactant molecules in the mixture

85

(A) 4 3.5

3 ) 2 2.5 2

Q (mg/m Q 1.5 1 0.5 0 0 0.5 1 1.5 2

C eqb (g/L)

(B) 4 3.5

3 )

2 2.5 2

Q (mg/m Q 1.5 1 0.5 0 0 0.5 1 1.5 2

C eqb (g/L)

Figure 5.10 Adsorption isotherm of surfactants: (■) block copolymer, (●) alcohol ethoxylate and (∆) block copolymer/alcohol ethoxylate blend (1:1 wt. ratio) on (A) South Stirling and (B) Dandaragan soil over the complete range of concentrations used

86

are seen to have a higher affinity for the South Stirling soil (K = 12.48) than the block copolymer. Over the larger range of surfactant concentrations (to and post the CMC) the adsorption profile on South Stirling soil (Figure 5.10A) is characterized by an extensive 2 linear slope with Q max (~ 4 mg/m ) extending well above the CMC indicative of a thermodynamically favoured disintegration of mixed micelles (shorter relaxation time

τ2) and the availability of unimers for further adsorption via cooperative interaction. At higher concentrations (post-CMC) the Langmuir adsorption isotherm behaviour gives a distinct monolayer plateau. On the other hand, the adsorption isotherm for the 1:1 wt. ratio blend mixture on Dandaragan soil illustrated in Figure 5.10B shows that a Freundlich isotherm (with an initial slope similar to that of the alcohol ethoxylate, Appendix Figure A 5.8B) is seen up to and beyond CMC values, similar to the block copolymer.

Capillary imbibition and advancing contact angle: The impact of the 1:1 wt. ratio surfactant mixture on capillary suction imbibition (at 4 g/L) in packed beds prepared using South Stirling and Dandaragan soils are given in Figure 5.11 and 5.12 respectively. In the severely hydrophobic South Stirling soil, the blended solution achieves a similar capillary wetting behaviour to the alcohol ethoxylate (Figure 5.11) o with a calculated advancing contact angle of θadv = 83.7 . This shows the influence of the alcohol ethoxylate in the availability of surfactants for adsorption at the soil surface at the TPL despite it having an equal weight of the block copolymer surfactant which o had a considerably higher contact angle (θadv = 89.2 ). Over time, capillary imbibition of the blend solution forms off to a terminal plateau due to the balance between the capillary suction pressure and gravity. Additionally, the high adsorption (binding) affinity (K = 12.5) of the 1:1 wt. ratio surfactant mixture (Appendix Table A2.1) on South Stirling soil favours adsorption at the TPL through disintegration of mixed micelles, which are much less tightly formed, (insert Figure 5.9), thermodynamically favouring low τ2 relaxation times and availability of unimers at the TPL.

87

3 2 1 0 0 30 60 90 120

5

4

3

h (cm) h 2

1

0 0 300 600 900 1200 1500 1800 2100 2400 time (s)

Figure 5.11 Capillary imbibition of; (■) block copolymer, (●) alcohol ethoxylate, (♦) block copolymer-alcohol ethoxylate surfactant blend (1:1 wt. ratio) and (▲) water on South Stirling soil

On Dandaragan soil, the effectiveness of the blended mixture is similar to the individual surfactant systems particularly at the initial imbibition stages (Figure 5.12). The interfacial tension and contact angle measurements of the blend mixture on the two soils, were calculated by the Lucas-Washburn equation (Appendix Figure A2.9) and used to generate the wetting force, γcos θ (Table 5.2), to inform the mechanisms involved in the capillary suction behaviour. Here, it can be seen that for the severely hydrophobic South Stirling soil, the force advancing the wetting front for the blend solution (2.87 mN/m) is about half that of the alcohol ethoxylate surfactant and about 5 times that of the block copolymer. Whereas with the significantly lower hydrophobicity 88

Dandaragan soil, the wetting force for the blended mixture was 11.23 mN/m which was about 66 % and 75 % that of the alcohol ethoxylate and the block copolymer respectively. Here, small changes in surface tension results in significant changes in the contact angle. Overall, for the 1:1 surfactant mixture on Dandaragan soil, the dynamics of wetting via capillarity is similar to the individual surfactant components and is significantly driven by the inherent wettability of the soil.

2

1

0 0 30 60

5

4

3

h (cm) h 2

1

0 0 200 400 600 800 1000 time (s)

Figure 5.12 Capillary imbibition of; (■) block copolymer, (●) alcohol ethoxylate, (♦) block copolymer-alcohol ethoxylate surfactant blend (1:1 wt. ratio) and (▲) water on Dandaragan soil

89

Gravitational infiltration: The rates of gravitational infiltration of water in packed beds of South Stirling soil and Dandaragan soil pre-treated with the 1:1 wt. ratio of surfactant mixture are given in Figures 5.13 and 5.14 respectively. The infiltration rates for both the blend and its individual components on the South Stirling soil were similar within the first 2 cm of packed bed reflecting the pre-adsorbed surfactants. Water infiltration rates were significantly reduced in South Stirling soil beds pre-treated with the blend mixture compared to the components alone on entering the untreated lower hydrophobic region. This reduction in infiltration corresponds to the increased surface

12

10

8

6 d (cm) d

4

2

0 0 500 1000 1500 2000 2500 3000 time (s)

Figure 5.13 Gravitational infiltration of water following application of; (■) block copolymer, (●) alcohol ethoxylate, (♦) block copolymer-alcohol ethoxylate surfactant blend (1:1 wt. ratio) and ▲water (control) applied on South Stirling soil at 5 mm constant ponding head

90

adsorption of the 1:1 mixture compared to its components as seen in Figure 5.10. Since re-dissolution is required for subsequent gravitational infiltration beyond the deposition depth, the availability of the 1:1 surfactant mixture falls below that of its individual components. Additionally, advancement of the water front is indicative of a linear surfactant concentration gradient illustrated in Figure 5.8 where its behaviour at the initial stages is significantly influenced by the block copolymer component as shown in the first 500 s of infiltration.

12

10

8

6 d (cm) d

4

2

0 0 500 1000 1500 2000 time (s)

Figure 5.14 Gravitational infiltration of water following application of; (■) block copolymer, (●) alcohol ethoxylate, (♦) block copolymer-alcohol ethoxylate surfactant blend (1:1 wt. ratio) and (▲) water on Dandaragan soil at 5 mm constant ponding head

91

In the case of the less hydrophobic Dandaragan soil, the blended mixture, lowered the water infiltration rate (Figure 5.14), similar to the individual surfactant systems as discussed in Section 5.2.2. Here, the effect of lowing the surface tension results in a more positive water entry pressure, hp ((Equation 5.2) similar to that created by the individual components (the alcohol ethoxylate and the block copolymer surfactant), and thus the impediment observed in gravitational infiltration could have been caused by the adsorption of surfactant molecules with hydrophilic portions towards the surface or forming partial bilayer marginally increasing the effective surface hydrophobicity.

92

Chapter 6

Impact of Additives on the Wetting Behaviour of Surfactants on Non-wetting Soils

6.1 Overview

In this chapter, the impact of hydrotropic additives on the efficiency of non- ionic surfactants during surfactant amelioration of NWS is addressed on the basis of the three techniques previously described in Chapter 5; surface adsorption, capillary imbibition, and gravitation infiltration. Four hydrotropes: an alkyl polyglucoside (APG; with alkyl carbon C8 – C10), hexyl glycol ether (C6E1), butyl glycol ether (C4E1) and butyl diglycol ether (C4E2) were used to formulate mixtures of surfactant-hydrotrope at a 1:1 wt. ratio. Their solution properties and effectiveness during water transport and distribution was then investigated in the two Western Australia soils.

6.2 Introduction

The effectiveness of a surfactant in the amelioration of SWR can be modified by the use of surfactant-hydrotropic mixtures since hydrotropes have solution properties modulate surfactant behaviour [76, 80-84, 164, 166] (Section 2.6: solution properties of hydrotropes). Addition of hydrotropes to surfactants, for example, result in the formation of mixed micellar aggregates in solution while blend formulation may exhibit different solution properties which may be superior to those of individual surfactants. As well as providing the advantage of requiring less surfactant for a given application, hydrotropes in surfactant mixtures can provide a synergistic effect which significantly influences the surfactant properties and their mechanisms. Addition of hydrotropes to surfactant systems have been shown to improve infiltration and water transport in hydrophobic soils. Kostka and Bially [100, 101] for example, demonstrated that a blended solution of a non-ionic ethylene oxide propylene oxide block copolymer (L 64) 93

surfactant and the hydrotrope C8-C10 APG increased water infiltration rates in a model and a synthetic and naturally occurring water repellent sand. These studies however did not describe the mechanisms involved in surfactants-hydrotropic action in the amelioration of water repellent soils.

During treatment of hydrophobic soils with surfactant-hydrotropic mixtures, concentrations above the surfactants CMC are commonly used [100, 101] where molecules in the mixture exist as mixed micelles and unimers in a state of dynamic equilibrium and where mixed micelles are continuously disintegrating and reforming [167]. Hence, the assembly of surfactants and hydrotropes at the liquid-soil interface during adsorption is strongly influenced by the changes in the total concentration above the CMC which is dictated by the surfactant-hydrotrope ratio in this mixed micelle and unimeric state.

6.3 Results and discussion

6.3.1 Surface tension and CMC

The impact of hydrotropic agents on the surface tension of the 1:1 wt. ratio mixtures of the non-ionic surfactants was examined with hydrotropes: alkyl polyglucoside (APG; with alkyl carbon C8 – C10), hexyl glycol ether (C6E1), butyl glycol ether (C4E1) and butyl diglycol ether (C4E2). Figure 6.1 gives the concentration dependence of surface tension of each of these hydrotropes where all hydrotropes apart from the APG, exist as unimers over a large range concentrations and is consistent with the solution behaviour of hydrotropes where a large increase in hydrotrope concentration results in a small decrease in surface tension [82, 166] confirming that aggregation of these hydrotropes takes place at relatively high concentrations.

94

80

70

60

50 mN/m 40

30

20 0 1 2 3 4 5 6 7 8 9 10 11 C (g/L)

Figure 6.1 Surface tension of hydrotropic agents as a function of concentration in aqueous solutions; (▲) APG, (x) C6E1, (●) C4E1 and (■) C4E2

The variation of surface tension with concentration for aqueous solutions of mixtures consisting of the non-ionic surfactants (block copolymer or alcohol ethoxylate) and the hydrotropic additives in a 1:1 wt. ratio at 20.8 ± 0.3o C are given in Figure 6.2 and 6.3 respectively. The decrease in surface tension of the block copolymer- hydrotropic mixtures (Figure 6.2) at low concentrations (below 0.1 g/L) was significantly influenced by the hydrotropic component in the mixture where the loss in surface activity at a given concentration may be attributed to the weak behaviour of the hydrotropes at the liquid-air interface as demonstrated in Figure 6.1. At higher concentrations the surface tension followed closely the concentration dependence of the block copolymer surfactant. At high concentrations, (beyond ~ 0.1 g/L), the surface tension of block copolymer-hydrotropic mixtures, with the exception of the APG, followed closely the concentration dependence of the block copolymer surfactant indicative of a liquid-air interface that is saturated with the block copolymer molecules

95

and a bulk solution consisting of mixed micelles (formed at higher CMCs, Table 6.1) where unimers of the mixture are in a dynamic equilibrium. It has previously been shown that co-solvents such as methanol and ethanol having less polarity and acting as hydrotropes, in mixtures with PEO-PPO-PEO block copolymers, can increase the CMC

80 75 70 60 70 50 40 65 30 60 20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

mN/m 55

50

45

40 0 0.05 0.1 0.15 0.2 0.25 0.3 C (g/L)

Figure 6.2 Surface tension of the (♦) block copolymer surfactant and its 1:1 wt. ratio hydrotropic blends; (▲) APG, (x) C6E1, (●) C4E1 and (■) C4E2 as a function of concentration

Table 6.1 CMC of aqueous solutions (in g/L) of 1:1 wt. ratio surfactant-hydrotropic mixtures at 20.8 ± 0.3o C

APG C6E1 C4E1 C4E2

Block copolymer 0.025 0.060 0.063 0.061

Alcohol ethoxylate 0.09 0.06 0.08 0.10

96

and cloud point of the solution by acting as good solvents for both EO and PO units [168, 169]. On the other hand, the hydrotropic effect on surface tension was prolonged in the block copolymer-APG mixture as shown by the behaviour beyond 0.1 g/L. Hence the effectiveness of APG as a hydrotrope appears to arise from destabilization of unstable mixed micelles forming at higher surfactant concentration by decreasing the micelle relaxation time τ2 and increasing the availability of both un-associated block copolymer and APG molecules for interfacial adsorption (Sections 2.5 and 2.7).

Surface tension curves of mixtures comprising of the alcohol ethoxylate and hydrotropic additives were similar over the range of concentrations studied (Figure 6.3) although the surface tension of the mixtures remained considerably higher than the surfactant itself in the low concentration region (below 0.1 g/L).

80

60 75 70 40 65 20 60 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 55 50

mN/m 45 40 35 30 25 0 0.05 0.1 0.15 0.2 0.25 0.3 g/L

Figure 6.3 Surface tension of (♦) alcohol ethoxylate surfactant and its 1:1 wt. ratio hydrotropic blends; (▲) APG, (x) C6E1, (●) C4E1 and (■) C4E2 as a function of concentration 97

Here, the higher surface tensions arise from the dominance of the hydrotropic unimers at the liquid-air interface. At higher concentrations, the surface tension of the mixtures formed a plateau due to the formation of mixed micelles at higher CMC. These results are consistent with those previously obtained by Cantu et al.,[170] who showed using surface tension and light scattering measurements, that the CMC of a binary mixture consisting of a non-ionic ionic surfactant C12EO8 and a hydrotrope glycerol, was higher than the surfactant itself.

6.3.2 Adsorption of surfactant-hydrotrope mixtures

The surface adsorption on the two Western Australian soils from surfactant- hydrotropic mixtures was carried out using the batch equilibrium technique (Section 3.6.2) and the impact of the hydrotrope in the surfactant-hydrotropic mixtures on the adsorption process was assessed on the basis of the structure of the hydrotrope. Plots of the surface tension of surfactant-hydrotropic mixtures before and after adsorption at concentrations below the CMC using the bubble pressure tensiometer at 20 ± 2o C gave 2 the surfactant-hydrotrope adsorption Qads mg/m (Appendices Figures A3.1 – A3.8) as described in Section 5.3.2. The surfactant-hydrotrope supernatants obtained after adsorption had higher surface tensions at low concentrations and higher CMC values (Appendix Table A3.1) compared to the initial surfactant-hydrotropic mixtures at each concentration in the batch experiment indicative of adsorption. This difference was more pronounced in systems involving the low hydrophobicity Dandaragan soil, which notably has a relatively higher total organic content than the severely hydrophobic 2 South Stirling soil. The degree of surfactant-hydrotrope adsorption (Qads, mg/m ) onto these non-wetting soils was determined using Expression 3.2 and the adsorption isotherm generated over the concentration range examined.

The isotherms for the surfactant-hydrotropic mixtures on the severely hydrophobic South Stirling soil are given in Figures 6.4 to 6.7. Here, it can be seen that

98

the isotherms showed a range of adsorption characteristics depending on the surfactant- hydrotropic mixture. The adsorption isotherms derived from concentrations below the CMC (inserts in Figures 6.4 to 6.7) indicate a Freundlich type of isotherm where the adsorbed molecules are in a simple physical equilibrium with those in solution. The increase in the surface binding with increases in concentration on the severely hydrophobic soil were predominantly linearly (n ~ 1) with a few exceptions (Appendix Table A3.2).

South Stirling-block copolymer-hydrotrope mixtures: The adsorption of the block copolymer-APG mixture (Figure 6.4A), for example, had a significantly higher binding affinity (n = 0.59) and showed the highest capacity (K = 235) for the severely hydrophobic South Stirling soil while mixtures comprising of the block copolymer and glycol ether hydrotropes exhibited lower affinities. This may be attributed to the long hydrophobic carbon alkyl chain length (C8-C10) in the APG hydrotrope which enables it to leave the solution and adsorb on the hydrophobic soil surface with increasing concentration. This enhanced adsorption resulting from an increased hydrophobic effect is also observed in mixtures comprising of the block copolymer and glycol ethers as the alkyl chain length increases from C4 to C6 in C4E1 and C6E1 respectively. At very low concentrations, adsorption on the severely hydrophobic soil is a non-cooperative process but as concentrations increases up to the CMC of the mixture, the hydrophobic effect is complemented by increasing lateral interactions between the adsorbed molecules which increases with increasing tail length.

Over the full range of concentrations (to and post CMC), the equilibrium adsorption isotherm of the block copolymer mixtures on South Stirling soil exhibited Langmuir monolayer-multilayer isotherm behaviour. The block copolymer-APG mixture (Figure 6.4A), shows a moderately steep and extended initial linear slope with a 2 significantly higher binding capacity (with Qmax ~ 36.8 mg/m ) compared to the block 2 copolymer surfactant itself (Qmax ~ 1 mg/m , Figure 5.2A) indicative of disintegration of the mixed micelles and the availability of unimers for cooperative adsorption and the 99

onset of multilayers Further, this implies that adsorption on the severely hydrophobic soil is thermodynamically more favourable than the micellization process.

A 0.5 0.4 0.3 0.2 0.1 0 0.000 0.010 0.020 0.030 40 35

30 )

2 25 20 15

Q (mg/m Q 10 5 0 0 1 2 3 4 C eqb (g/L)

B 2 1.5 1 12 0.5 0

) 10

2 0 0.02 0.04 0.06 0.08 0.1 8

6 Q (mg/m Q 4 2 0 0 1 2 3 4 C eqb (g/L)

Figure 6.4 Adsorption isotherms of the 1:1 wt. ratio mixture on South Stirling soil: (A) block copolymer-APG, (B) alcohol ethoxylate-APG. The inserts shows the isotherms generated at pre-CMC concentrations 100

A similar behaviour is observed with the block copolymer mixture comprising of C6E1 2 (Figure 6.5A) which shows a moderately high adsorption capacity (Qmax ~ 6 mg/m ) on

South Stirling soil. When blended with C4E1 which has a shorter hydrophobic chain (Figure 6.6A), the block copolymer mixture shows a significantly lower binding efficiency due to the molecules weak affinity (K ~ 0.5) for the hydrophobic soil surface and may be attributed to the polar nature of C4E1, which increases the solubility of the surfactant in the aqueous solution thus limiting adsorption to sites on the soil surface.

Here, increasing the head group size from E1 to E2 (in C4E1 and C4E2 respectively) does not appear to have a significant impact on the adsorption efficiency particularly at high equilibrium concentrations. These results suggest that the adsorption of block copolymer-hydrotropic mixtures on the severely hydrophobic South Stirling soil is enhanced when the hydrophobic chain length of the hydrotrope increases and that the area per molecule is relatively independent of the chain length effect.

South Stirling-alcohol ethoxylate-hydrotrope mixtures: The equilibrium adsorption isotherms of the alcohol ethoxylate-hydrotropic mixtures on South Stirling soil are given in Figures 6.4B to 6.7B. Similar to the block copolymer-hydrotropic mixtures, isotherms obtained with pre-CMC values indicate a Freundlich type of isotherm with n ~1 (inserts Figures 6.4B to 6.7B and Appendix Table A3.2). The adsorption profiles of the alcohol ethoxylate-hydrotropic mixtures on South Stirling soil at low surfactant equilibrium concentrations are characterized by relatively moderate linear slopes (compared to the steep initial slope obtained with the individual surfactant, (Figure 5.3B). This implies that the surfactant and hydrotropic molecules are filling the available adsorption sites with a lower binding strength compared to the alcohol ethoxylate

101

2 A 1.5 1 0.5 8 0 0 0.02 0.04 0.06 0.08 0.1

6

) 2 4

Q (mg/m Q 2

0 0 1 2 3 4 C eqb (g/L)

B 2 1.5 1 8 0.5 0

6 0 0.02 0.04 0.06 0.08 0.1

) 2

4 Q (mg/m Q 2

0 0 1 2 3 4

C eqb (g/L)

Figure 6.5 Adsorption isotherms of the 1:1 wt. ratio mixture on South Stirling soil over the complete range of concentrations used: (A) block copolymer- C6E1, (B) alcohol ethoxylate-C6E1. The inserts shows the isotherms generated at pre-CMC concentrations

102

0.5 A 0.4 0.3 0.2 0.1

0 )

2 12 0 0.02 0.04 0.06 0.08 0.1 10

8 Q (mg/m Q 6 4 2 0 0 1 2 3 4

C eqb (g/L)

B 2 1.5 1 0.5 0 12 0 0.02 0.04 0.06 0.08 0.1

10 ) 2 8 6

4 Q (mg/m Q 2 0 0 1 2 3 4

C eqb (g/L)

Figure 6.6 Adsorption isotherms of the 1:1 wt. ratio mixture on South Stirling soil over the complete range of concentrations used: (A) block copolymer- C4E1, (B) alcohol ethoxylate-C4E1. The inserts shows the isotherms generated with pre-CMC concentrations

103

A 0.5 0.4 0.3 0.2 0.1 12 0 0 0.02 0.04 0.06 0.08 0.1

10 ) 2 8 6

4 Q (mg/m Q 2 0 0 1 2 3 4

C eqb (g/L)

B 2 1.5 1 0.5 12 0 0 0.02 0.04 0.06 0.08 0.1 10

8

) 2 6 4 Q (mg/m Q 2 0 0 1 2 3 4

C eqb (g/L)

Figure 6.7 Adsorption isotherms of the 1:1 wt. ratio mixture of on South Stirling soil over the complete range of concentrations used: (A) block copolymer-C4E2, (B) alcohol ethoxylate-C4E2. The inserts shows the isotherms generated with pre-CMC concentrations

104

surfactant suggesting that this decreased affinity for the soil surface results from high solubilization of the surfactant-hydrotrope system. At higher concentrations (post- CMC) isotherm behaviour gives distinct monolayer plateaus in alcohol ethoxylate mixtures with APG and C4E1 and sequential (multilayer) adsorption behaviour in alcohol ethoxylate mixtures consisting of C6E1 and C4E2 suggestive of enhanced cooperative adsorption and the formation of multilayers.

Dandaragan-block copolymer-hydrotropic mixtures: The equilibrium adsorption isotherms for the surfactant-hydrotropic mixtures on the less hydrophobic Dandaragan soil are given in Figures 6.8 - 6.11. According to the initial slope of the isotherms, block copolymer blends with APG, C6E1 and C4E2 (inserts Figure 6.8A, 6.9A and 6.11A respectively) showed a weak adsorbent-adsorbate interaction at very low surfactant concentrations compared to the low value of C4E1 (as shown by their K values in Appendix Table A3.2). The low adsorption observed at lower blend concentrations can be attributed to the weak affinity for the more hydrophilic surfaces of Dandaragan soil and further suggests that surfactant and hydrotrope molecules adsorb as isolated molecules which is more apparent with the APG hydrotrope mixtures. At higher concentrations (to and beyond the CMC), the amount of adsorption significantly increases. Here, increased lateral interaction between the adsorbed molecules enhances cooperative adsorption forming multilayers or perhaps micelle-like aggregates on the soil surface. Notably, the adsorption profile of the block copolymer-C4E1 mixture (Figure 6.10A) showed Langmuir type of isotherm with an S-shape which suggests the formation of a monolayer at low equilibrium concentration (~1.3 g/L) followed by cooperative multilayer patches which then diminished at even higher concentrations where complete multilayers formed. These results indicate that block copolymer- hydrotropic mixtures enhance equilibrium adsorption on the low hydrophobic Dandaragan soil compared to the adsorption of the single surfactant particularly at higher concentrations (Figure 5.3A), and with those hydrotropes having a long

105

A 0.1

0.05 10 0

8 0 0.01 0.02 0.03 ) 2 6

4 Q (mg/m Q 2

0 0 1 2 3 4

C eqb (g/L)

B 1.5 1 0.5 0 10 0 0.02 0.04 0.06 0.08 0.1

8

) 2 6

4 Q (mg/m Q 2 0 0 1 2 3 4 C eqb (g/L)

Figure 6.8 Adsorption isotherms of the 1:1 wt. ratio mixtures of the block copolymer-APG (A) and alcohol ethoxylate-APG (B) on Dandaragan soil over the complete range of concentrations used. The insets shows the isotherms generated with pre-CMC concentrations

106

A 0.5 0.4 0.3 0.2 0.1 0 10 0 0.02 0.04 0.06 0.08 0.1

) 8 2 6

Q (mg/m Q 4

2

0 0 1 2 3 4

Ceqb (g/L)

B 1.5 1 0.5 10 0 0 0.02 0.04 0.06 0.08 0.1

8 ) 2 6

4 Q (mg/m Q 2

0 0 1 2 3 4 C (g/L) eqb

Figure 6.9 Adsorption isotherms of the 1:1 wt. ratio mixtures of the block copolymer-C6E1 (A) and alcohol ethoxylate-C6E1 (B) on Dandaragan soil over the complete range of concentrations used. The inset shows the isotherm generated with pre-CMC concentrations

107

A 0.5 0.4 0.3 0.2 0.1 10 0 0 0.02 0.04 0.06 0.08 0.1

8 ) 2 6

4 Q (mg/m Q 2

0 0 1 2 3 4 C eqb (g/L)

B 0.5 0.4 0.3 0.2 0.1 10 0 0 0.02 0.04 0.06 0.08 0.1

8 ) 2 6

4 Q (mg/m Q 2

0 0 1 2 3 4 C (g/L) eqb

Figure 6.10 Adsorption isotherms of the 1:1 wt. ratio mixtures of the block copolymer-C4E1 (A) and alcohol ethoxylate-C4E1 (B) on Dandaragan soil over the complete range of concentrations used. The insets shows the isotherms generated with pre-CMC concentrations

108

A 0.5 0.4 0.3 0.2 0.1 10 0 0 0.02 0.04 0.06 0.08 0.1

8 ) 2 6

4 Q (mg/m Q 2

0 0 1 2 3 4 C eqb (g/L)

B 1.5 1 0.5 10 0 0 0.05 0.1 0.15 0.2

8 ) 2 6

4 Q mg/m Q 2

0 0 0.5 1 1.5 2 2.5 3 3.5 4 C (g/L) eqb

Figure 6.11 Adsorption isotherms of the 1:1 wt. ratio mixtures of the block copolymer-C4E2 (A) and alcohol ethoxylate-C4E2 (B) on Dandaragan soil over the complete range of concentrations used. The insets shows the isotherms generated with pre-CMC concentrations

109

hydrocarbon chain enhancing the equilibrium adsorption more significantly. It is apparent that an increase in alkyl chain length increases the ability of the molecules to pack more closely with stronger inter-molecular van der Waals bonds thus increasing this cooperative effect. Increased cooperativity is also observed in block copolymer mixtures where the size of the hydrotropic head group is increased from C4E1 to C4E2 and is similar to the effect the sugar head group comprising of several OH units in the APG mixture. The adsorption of surfactant and hydrotrope molecules on the more hydrophilic surfaces of Dandaragan soil can be attributed to hydrogen bonding between the OH units on the head groups and the soil surface. These results show the effect of incorporating a co-surfactant, having several OH groups on its head group, into the block copolymer mixture compared to the solvo surfactants (having one or two EO units), which has an insignificant effect on adsorption particularly at higher concentrations on the Dandaragan soil surface.

Dandaragan-alcohol ethoxylate-hydrotropic mixtures: The adsorption isotherms for the alcohol ethoxylate-hydrotropic mixtures on Dandaragan soil (Figure 6.8B - 6.11B) on the other hand, indicate that the adsorption behaviour of all alcohol ethoxylate blends showed relatively steeper and more vertical slopes at very low equilibrium concentrations (with relatively larger adsorption capacities, K, Appendix Table A3.2), indicative of the alcohol ethoxylate-hydrotropic blend having a higher affinity for adsorption sites on Dandaragan soil. Over the larger range of equilibrium concentrations (beyond CMC), the Langmuir type behaviour indicates the formation of multilayers is dominant. Notably, increasing the size of the head group in the solvo-surfactants from

E1 to E2 (Figure 6.10 and 6.11) results in increased adsorption in both the alcohol ethoxylate and block copolymer mixtures. Increasing the hydrophobic character of the hydrophobe from C4E1 to C6E1 does not appear to have a significant impact on the overall adsorption behaviour of the alcohol ethoxylate mixtures.

Overall, these results show that the presence of hydrotropic agents in surfactant- hydrotropic mixtures at a 1:1 wt. ratio enhances equilibrium adsorption on both South 110

Stirling and Dandaragan soils particularly at the higher concentrations commonly used in agricultural practice, and that the enhancement is dependent on both the structure of the hydrotrope and the surfactant.

6.3.3 Capillary imbibition and contact angle

Capillary suction into packed beds of both South Stirling and Dandaragan soils (Section 3.6.3) were conducted using aqueous solutions of the surfactant-hydrotropic mixtures (at 4 g/L and 1:1 wt.) to determine the effect of these additives on the efficiency of surfactants. Plots of capillary rise (h versus t) for the block copolymer and its hydrotropic mixtures on South Stirling soil are given in Figures 6.12A.

South Stirling-block copolymer-hydrotropic mixtures: All surfactant-hydrotropic mixtures enhanced capillary imbibition rates on the severely hydrophobic South Stirling soil. In particular, block copolymer mixtures comprising of the solvo-surfactants (C6E1,

C4E1 and C4E2) were more efficient in enhancing capillary imbibition rates than the hydrotropic surfactant APG, which exhibited a similar trend in capillarity to that of the individual surfactant. These observations were consistent with the contact angles (Table 6.2) derived from the Lucas-Washburn equation (Section 2.5). Surface tension and contact angles, together with the calculated wetting force (vector) γcosθ (Section 5.2.2), are given in Appendix Table A3.3 and were used to address the primary mechanism involved in the capillarity behaviour. Here, it can be seen that the force advancing the wetting front of the block copolymer mixtures containing solvo-surfactants (C6E1, and

C4E1) are ~3 times greater than the block copolymer-APG mixture which was similar to that of the block copolymer-C4E2 mixture and about 1 ½ times that of the individual surfactant. On the contrary, the adsorption capacity (K = 235) obtained from the equilibrium adsorption isotherm of the block copolymer-APG mixture (Appendix Table A3.2) is significantly higher but inconsistent with the high contact angle (88.1o, Table 6.2) and the low imbibition kinetics shown in Figure 6.12A. This high adsorption K 111

value is indicative of APG being strongly adsorbed on the surface but not in a dynamic equilibrium with the advancing TPL. Further, the low CMC (0.025g/L) of the block copolymer-APG mixture suggests that the surfactant and hydrotropic molecules prefer to be in the mixed micellar state and potentially indicating that the APG mixture forms stable mixed micelles with slow relaxation times τ2 lowering the number of unimers for dynamic adsorption at the TPL making wetting slower. Hence, the low concentration of unimers along the advancing meniscus results in high interfacial tensions which lead to high contact angles and reduced imbibition rates.

South Stirling-alcohol ethoxylate-hydrotropic mixtures: The alcohol ethoxylate surfactant and its blends also enhanced water transport via capillary suction in South Stirling soil (Figure 6.12B). Although when present as blends, capillary imbibition was significantly reduced particularly at longer times which is consistent with the low calculated γcosθ values (Appendix Table A3.3). Here the force advancing the wetting front of the alcohol ethoxylate mixtures containing C6E1 and C4E1 are slightly lower (70% and 60%) compared to the alcohol ethoxylate surfactant itself while alcohol ethoxylate mixtures containing either APG or C4E2 are lower.

Overall, surfactant-hydrotropic mixtures, comprising APG and C4E2 were less efficient in enhancing capillary imbibition rates in the severely hydrophobic soil compared to C6E1 and C4E1 which is consistent with their measured contact angles. Additionally, since capillary imbibition is a function of the adsorption kinetics, the slope and length of the linear region particularly at the initial stages in the equilibrium adsorption plot may be indicative of the stability of the mixed micelles in the respective blends.

112

6 A 4 2 0 0 600 1200 1800 2400 3 2.5 2

1.5 h (cm) h 1 0.5 0 0 30 60 90 120 time (s)

B 6 4 2 0 0 600 1200 1800 2400 3 2.5 2

1.5 h (cm) h 1 0.5 0 0 30 60 90 120 time (s)

Figure 6.12 Capillary imbibition of (x) surfactants: (A) block copolymer and (B) alcohol ethoxylate, and their 1:1 wt. ratio hydrotropic mixtures containing: (♦) APG, (■) C6E1, (▲) C4E1 and (●) C4E2, and (○) water (control), on South Stirling soil

113

Table 6.2 Contact angles of solutions derived from h2/t plot at initial capillary rise stages on South Stirling and Dandaragan soils

Treatment South Stirling Dandaragan

o o θadv θadv

Block copolymer 89.2 68.5

Alcohol ethoxylate 78.8 55.0

Block copolymer/APG 88.1 54.6

Block copolymer/C6E1 85.9 62.3

Block copolymer/C4E1 85.9 60.4

Block copolymer/C4E2 88.6 62.7

Alcohol ethoxylate/APG 84.8 78.9

Alcohol ethoxylate/C6E1 81.6 50.3

Alcohol ethoxylate/C4E1 82.2 55.1

Alcohol ethoxylate/C4E2 84.2 69.2

Water >90 85.5

114

5 APG blends 4 3

2 h (cm) h 1 0 0 200 400 600 800 1000 time (s)

5 C6E1 blends 4 3

2 h (cm) h 1 0 0 200 400 600 800 1000 time (s) 5 C4E1 blends 4 3

2 h (cm) h 1 0 0 200 400 600 800 1000 time (s)

5 C4E2 blends 4 3

2 h (cm) h 1 0 0 200 400 600 800 1000 time (s) Figure 6.13 Capillary imbibition of; (■) block copolymer–hydrotropic blends, ( ) alcohol ethoxylate-hydrotropic blends and (○) water (control), on South Stirling soil

115

Dandaragan-surfactant-hydrotropic mixture: On the less hydrophobic Dandaragan soil, the block copolymer surfactant-hydrotropic mixtures and the surfactant itself showed a similar behaviour during capillary suction (Figure 6.14A) with rates slightly higher within the first 2 min, consistent with the corresponding wetting forces (Appendix Table A3.3) calculated from the measured surface tension and contact angles. This implies that the nature and rate of dynamic adsorption of the block copolymer surfactant and its respective hydrotropes are similar at the advancing meniscus. Although all alcohol ethoxylate surfactant-hydrotropic mixtures also increased capillary infiltration (Figure

6.14B), the mixture comprising C6E1, having a wetting force slightly higher than the single surfactant, gave a significant improvement in the capillary infiltration rate.

Further comparison, revealed that the surfactant mixtures comprising of APG and C4E2 with wetting forces significantly less than the alcohol ethoxylate surfactant itself, were less efficient in enhancing capillary imbibition rates which was similar to the block copolymer blends above. Unlike the severely hydrophobic South Stirling soil where both surfactants and surfactant-hydrotropic mixtures significantly improved capillary suction, results obtained on the lower hydrophobicity Dandaragan soil (Figure 6.15), imply that the underlying forces involved in the dynamic adsorption of the surfactant unimers and the disintegration of mixed micelles only marginally improves capillary imbibition of water, although the overall wetting forces were greater due to the inherent wettability of the soil itself.

116

A 6

5

4

3 h (cm) h 2

1

0 0 60 120 180 240 300 360 time (s)

B 6

5

4

3 h (cm) h 2

1

0 0 60 120 180 240 300 360 time (s)

Figure 6.14 Capillary imbibition of (x) surfactants: (A) block copolymer and (B) alcohol ethoxylate, and their 1:1 wt. ratio hydrotropic mixtures containing: (♦) APG, (■) C6E1, (▲) C4E1 and (●) C4E2, and (○) water (control), on Dandaragan soil

117

6 APG blends

4

h (cm) h 2

0 0 60 120 180 240 300 time (s)

6 C6E1 blends 4

h (cm) h 2 0 0 60 120 180 240 300 time (s)

6 C4E1 blends 4

h (cm) h 2

0 0 60 120 180 240 300 time (s) 6 C4E2 blends 4

h (cm) h 2

0 0 60 120 180 240 300 time (s)

Figure 6.15 Capillary imbibition of; (■) block copolymer–hydrotropic blends, □ alcohol ethoxylate-hydrotropic blends and (○) water (control), on Dandaragan soil

118

6.3.4 Gravitational infiltration

The rates of gravitational infiltration of water following treatment with aqueous solutions of surfactant-hydrotropic blends on the two Western Australian soils are given in Figure 6.16 and 6.17. Similar to the behaviour observed in beds pre-treated with the individual surfactants (Figure 5.6), gravitational infiltration rates in South Stirling soil pre-treated with surfactant-hydrotropic mixtures showed similar infiltration kinetics within the first 2 cm of the packed bed corresponding to the depth of the initially deposited surfactants and where the hydrostatic pressure deff in this initial bed regime (Equation 5.3), effectively adds to the ponding height of 5 mm lowering the pore entry pressure hp.

South Stirling soil-block copolymer-hydrotropic mixtures: Pre-treatment of the severely hydrophobic South Stirling soil with block copolymer-hydrotropic mixtures (Section 3.6.5), significantly enhanced infiltration rates in the subsequent untreated soil region compared to the individual block copolymer surfactant (Figure 6.16A). Hydrotropes

C6E1 and C4E2 with the block copolymer surfactant provide the highest rate of infiltration (among the solvo-surfactants). These marginally exceed the block copolymer-APG infiltration rate, which although adsorbed more strongly (K = 235) does not enhance infiltration also. In general, the APG results are in agreement with those of Kostka et al., [100] who demonstrated that gravitational infiltration on hydrophobic soils increased in the presence of a blended solution of a non-ionic (ethylene oxide propylene oxide copolymer) surfactant and an alkyl glucoside hydrotrope (C8-C10 APG).

South Stirling soil-alcohol ethoxylate-hydrotropic mixtures: The effectiveness of a surfactant-hydrotropic mixture in enhancing gravitational infiltration rates, is further seen in hydrotropic mixtures of the alcohol ethoxylate surfactant. Here, hydrotropic mixtures increase and decrease infiltration rates compared to the individual alcohol ethoxylate surfactant, with mixtures of APG and C6E1, enhancing the rate which also

119

A 10 9 8 7 6 5

d (cm) d 4 3 2 1 0 0 300 600 900 1200 1500 1800 2100 2400 time (s)

B 10 9 8 7 6 5

d (cm) d 4 3 2 1 0 0 300 600 900 1200 1500 1800 2100 2400 time (s)

Figure 6.16 Gravitational infiltration of water following pre-application of (■) surfactants: (A) block copolymer and (B) alcohol ethoxylate, and their 1:1 wt. ratio hydrotropic mixtures containing: (▲) APG, (x) C6E1, (♦) C4E1 and (●) C4E2 at 4L/Ha equivalent, and (○) water (control), on South Stirling soil at 5 mm constant ponding head

120

have the higher adsorption capacities K compared to the alcohol ethoxylate alone. Again the very high K value of APG is seen not to translate to water infiltration kinetics. On the other hand, the alcohol ethoxylate-C4E1 mixture having a lower adsorption binding affinity had a relatively lower infiltration rate, similar to the block copolymer-C4E1. The discontinuities observed in the infiltration profiles (Figures 6.16 A and B) may be due to the different re-solubilization and adsorption kinetics of the hydrotropes and the surfactant which may form distinct micro-domains during the pre- treatment stage, which Hamley, [171] refered to as ‘surface islands’. Overall, pre- treatment of the severely hydrophobic South Stirling soil with block copolymer- hydrotropic mixtures (1:1 wt. ratio) yielded improvements in infiltration compared to those with the alcohol ethoxylate surfactant (Figure 6.17) which may be attributed to the adsorption dynamics along the TPL following re-solubilization of the pre-adsorbed surfactant-hydrotropic molecules.

Dandaragan-surfactant-hydrotropic mixtures: The gravitational infiltration in the lower hydrophobic Dandaragan soil pre-treated with these mixtures (Figure 6.18), showed infiltration profiles that were significantly different compared to the severely hydrophobic soil. As seen in Figure 5.7, addition of both block copolymer and alcohol ethoxylate surfactants inhibit gravitational infiltration as discussed in Section 5.3.1. Consistent with the block copolymer and alcohol ethoxylate data above, all hydrotropic mixtures did not restore or enhance water infiltration. Additionally, the effect of capillarity was insignificant in Dandaragan soil. These results indicate that the use of hydrotropes in surfactant mixtures significantly increase the water entry pressure hp and the effect of the additional head of ponding provided by the top layer (~ 2 cm) of soil seen in South Stirling soil is absent.

In general all hydrotropes enhanced the infiltration of the block copolymer surfactant (Figure 6.16A) on the highly hydrophobic soil as demonstrated by the wetting depths of between 5.2 – 9.2 cm compared to 2.5 cm when using the block copolymer alone, while only the APG and C6E1 enhanced the wetting efficiency of the alcohol 121

APG blends 10 8 6 4 d (cm) d 2 0 0 200 400 600 800 1000 (time (s)

C E blends 10 6 1 8 6 4 d (cm) d 2 0 0 200 400 600 800 1000 time (s)

C E blends 10 4 1 8 6 4 d (cm) d 2 0 0 200 400 600 800 1000 time (s)

C4E2 blends 10 8 6 4 d (cm) d 2 0 0 200 400 600 800 1000 time (s) Figure 6.17 Gravitational infiltration of water following pre-application of; 1:1 wt. ratio (■) block copolymer-hydrotropic blends, (□) alcohol ethoxylate- hydrotropic blends at 4L/Ha equivalent and (○) water on South Stirling soil at 5 mm constant ponding head

122

A 10 9 8 7 6 5 d (cm) d 4 3 2 1 0 0 300 600 900 1200 1500 time (s)

B 10 9 8 7 6 5 d (cm) d 4 3 2 1 0 0 300 600 900 1200 1500 time (s) Figure 6.18 Gravitational infiltration of water following pre-application of (■) surfactants: (A) block copolymer and (B) alcohol ethoxylate, and their 1:1 wt. ratio hydrotropic mixtures containing: (▲) APG, (x) C6E1, (♦) C4E1 and (●) C4E2 at 4L/Ha equivalent, and (○) water (control), on Dandaragan soil at 5 mm constant ponding head 123

ethoxylate surfactant by increasing the wetting front to ~ 7.7 cm compared to 5.5 cm when using the alcohol ethoxylate alone (Figure 6.16B). On the other hand, all hydrotropes did not help overcome the impediment to infiltration seen with surfactants alone in the lower hydrophobic soil (Figure 6.18) but rather contributed to further impediment to infiltration. The high K value of the APG hydrotrope showed it was readily adsorbed on the severely hydrophobic soil but without proportionally contributing to the advancement of the wetting front irrespective of the surfactant structure.

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Chapter 7

Conclusion and Future Work

7.1 Conclusion

Solvent extraction of soil organic matter (SOM) from these non- and poorly- wetting soils allowed ten major classes of organic compounds to be identified, where sterols and fatty acids (palmitic acid and stigmasterol) were dominant. These were used as model SOM components in studies of wetting mechanisms. Atomic force microscopy (AFM) and contact angle measurements indicated that both the composition of the surface and its nanoscale topology determined wetting dynamics on both planar and particulate SOM surfaces.

a) Palmitic acid and stigmasterol films when independent, form hydrophobic hemimicelle aggregates on quartz surfaces, with hydrophilic quartz areas between them, hence moderate hydrophobicity b) Whereas, palmitic acid plus stigmasterol form these aggregates embedded in a continuous hydrophobic gel-like film on the quartz surfaces, through limited solubility of these SOM components and hence gave severe hydrophobicity

This indicates that the combination and specific interactions between SOM components, rather than their absolute quantities determines the degree of soil hydrophobicity. Additionally, air entrapped within these surface nanostructures provides additional hydrophobicity due to the added air-water interface on the surface.

The impact of two non-ionic surfactant types on wetting, water transport and water distribution in the two WA soils of contrasting hydrophobicities, was evaluated in terms of three specific mechanisms: surfactant adsorption on soil particle surfaces, capillary water suction and gravitational infiltration. The non-ionic surfactants used were a block copolymer, EOxPOyEOz and an alcohol ethoxylate, C13E8. Overall the 125

efficiency of the surfactant was shown to be dependent on the nature of the soil coating (hydrophobicity), structure of the surfactant and its adsorption behaviour. Wetting and water transport were measured by the kinetics of capillary suction and gravitational infiltration. The capillary suction of water in the severely hydrophobic soil was significantly enhanced by the low molecular weight alcohol ethoxylate surfactant compared to the conventionally used block copolymer. These surfactants have similar head groups, poly (ethylene oxide) but different hydrophobic tails (CH2- and long chains of poly (propylene oxide) respectively) that would adsorb with different affinities on the soil SOM surface. When blended into a 1:1 wt. ratio, the surfactant mixture reduced water infiltration rates below that achieved by both surfactants separately, an important aspect in the potential use of blended surfactants.

The adsorption of the block copolymer and alcohol ethoxylate surfactants as well as its 1:1 wt. mixture on the two soils indicated a simple dynamic equilibrium at pre-CMC concentrations suggesting that the driving force is the increased interaction between the surfactant molecules (unimers) and the soil surface. Agricultural practice requires the application of surfactants at relatively high concentrations, usually above the CMC, where the kinetics of micellar disintegration to unimers becomes a significant parameter in the overall adsorption and wetting processes. It was shown that the adsorption capacities, obtained from the isotherms generated from pre-CMC concentrations, and not the overall adsorption behaviour over the full range of concentrations studied) served as an appropriate indicator of the efficiency of non-ionic surfactants in the severely hydrophobic soil.

In the less hydrophobic Dandaragan soil, both the surfactants and their 1:1 wt. mixture impeded gravitational infiltration rates in a similar way (Figure 5.12) and capillary suction did not have an impact on the overall imbibition. These results indicate that the sensitivity of the non-ionic surfactants effectiveness is highly dependent on the initial degree of soil hydrophobicity.

126

The addition of hydrotropic agents to surfactant solutions mediate solvency in general, as such they influence the solution behaviour (surfactant-surfactant interactions and thus the CMC) as well as surface behaviour (surfactant adsorption). Four hydrotropes, consisting of 3 glycol ethers (C6E1, C4E1, C4E2) and an alkyl poly glucoside APG were evaluated as 1:1 wt. mixtures with the two non-ionic surfactants. All hydrotropes increased the adsorption of surfactants onto both soil types. Adsorption of block copolymer-hydrotropic blends at concentrations greater than the CMC increased in the order APG >>> C6E1 ~ C4E2 > C4E1 (Figures 6.4 - 6.7) consistent with the adsorption capacities obtained from pre-CMC isotherms. The adsorption of alcohol ethoxylate-hydrotropic blends at concentrations greater than the CMC increased in the order C4E1> C4E2 > C6E1 > APG however the adsorption capacities obtained using pre-

CMC equilibrium concentrations (Table A3.2) indicate a reverse order APG > C6E1>

C4E2 > C4E1, indicating adsorption post CMC is dominated by cooperative effects and multilayer formation. As a consequence, APG unimers are less available for transport to the advancing wetting front lowering its effectiveness in water infiltration.

In general, hydrotrope addition to surfactants only significantly improved the capillary wetting where soil surfaces were significantly hydrophobic and where the effectiveness of the single surfactant was low (e.g. block copolymer).

a) All hydrotropes improved the efficiency of the block copolymer in

capillary suction (Figure 6.12 and Table A3.3) in the order C4E1 ~ C6E1

>C4E2 >> APG. Importantly, addition of all hydrotropes to the alcohol ethoxylate surfactant reduced its capillary wetting rate, an important aspect for practical application. b) In comparison, addition of hydrotropes to both surfactants made an insignificant difference to capillary wetting of the lower hydrophobic Dandaragan soil compared to the surfactant alone. This indicates that hydrotropes do not provide a universal improvement to the limited ability of surfactants to enhance wetting of lower hydrophobicity soils 127

Hydrotrope additives, again, were most effective in enhancing gravitational infiltration into soils that were significantly hydrophobic and where surfactants alone were not effective (e.g. block copolymers), consistent with the capillary suction behaviour.

a. On the severely hydrophobic soil, all hydrotropes, with the block copolymer surfactant significantly improved the infiltration rates

C4E2 ~ APG ~ C6E1 > C4E1 (Figure 6.16) while only C6E1 and APG increased the efficiency of the alcohol ethoxylate surfactant (Figure 6.16) due to the relatively higher wetting efficiency of the alcohol ethoxylate itself. b. Importantly, on the less hydrophobic Dandaragan soil, all hydrotropes reduced the gravitational infiltration of the primary surfactants (block copolymer and alcohol ethoxylate).

Overall, both APG and C6E1 provided the highest enhancements in surfactant efficiency in wetting and water transport on the severely hydrophobic soil especially where the efficiency of the surfactant was initially low (e.g the block copolymer), suggesting that hydrotropes with longer hydrophobic chains are better surfactant enhancers. Surface adsorption (particularly at pre-CMC concentrations) is a significant parameter that can be used to predict the wetting efficiency of a soil system. Importantly, that the surfactant–hydrotropic systems are better at enhancing wetting and water transport than the mixed surfactant systems conventionally used.

In summary, the underlying mechanisms relating the parameters that influence the effectiveness of surfactants and particularly their blends and mixtures commence with the nature (composition and texture) of the soil surface, and progress through their solution behaviour at high concentration (micelle stability) and finally impact on their adsorption behaviour at the advancing wetting front. These are illustrated in terms of the

128

forces at the advancing three phase contact line during the advancement of wetting of a surface in Figure 7.1.

γLA

γ θ SL γSA

a) Surfactant surfactant molecule τ2

+

b) Surfactant-hydrotrope mixture hydrotrope molecule τ2

+

Figure 7.1 Schematic representation of the adsorption mechanism along the advancing wetting front on a hydrophobic soil illustrating the disintegration of micelles and the effect of hydrotrope on the relaxation time τ2

129

7.2 Future Work

The advancement of a wetting front, if measured by the progression of this front in a packed bed, does not address the issue of homogeneity of wetting, that is, there may be significant areas of non-wetted soils behind the advancing front. This is well known in soil science and hydraulic transport as fingering. Although, surfactants have been shown to alleviate this qualitatively, the effectiveness of surfactant types and wetting processes have not been addressed in terms of quantitative mechanisms. Although this may be addressed by simultaneous infiltration-gravimetric measurements, the newer technologies of X-ray and neutron tomography show highest promise.

As noted in this thesis, the practical application of surfactants in broad-acre non- and poorly wetting soils involves their deposition (and drying) in a concentrated area. There has not been a quantitative study of their re-solubilization and subsequent transport to the three phase line of the wetting front. This could be achieved by systematic molecular analysis such as size exclusion chromatography and GC-MS.

Additionally, the impact of varying the surfactant-hydrotrope molecular ratios would allow a greater understanding of their impact on the state of surfactant micelles in solution and their disintegration kinetics generating unimers able to adsorb at the wetting front at various soil depths. This may be achieved with adsorption studies together with dynamic light scattering monitoring micelle size and concentration.

130

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Appendices

Appendix 1

Figure A1.1 GC-MS chromatograms (TIC) of methanol and chloroform soluble fractions obtained from Dandaragan; CH3OH fraction (A), CHCl3 fraction (B) and South Stirling; CH3OH fraction (C) and CHCl3 fraction (D), water repellent soil ample. Samples were extracted with an isopropanol 15.7 M: ammonia (7:3, v/v) solution.

141

Appendix A1.2

Alcohols: Long chain alcohols identified in Dandaragan soil included C16, C20, C22, C24,

C25, C30 and benzendiol derivatives with C30 being most predominant. This was the case with the South Stirling soil from which Benzene ethanol, C12, C13, C16, C18, C20, C22, C26 and C30 and their derivatives were detected with alcohols ranging between C20 – C30 dominating (similar to Dandaragan). In general, odd numbered homologues were found in lower concentrations compared to the even numbered members. Alcohols are commonly found unbound or as esters in plants and the even-numbered alcohols with C > 20 commonly found in soils, have been associated with leaf waxes from higher plants [172, 173].

Alkanes: Only long chains alkanes were identified in the two soils (C19, C20, C24, C30 and C54 in Dandaragan and C16, C20, C24 and C30 in South Stirling), with C24 and C30 being the majority in Dandaragan and South Stirling respectively.

Esters: Analysis of the Dandaragan soil revealed that overall, short chain alkyl esters of hexadecanoic acid and 1-tetradecanol acrylate where the major esters identified. The esters identified from South Stirling soil were of a similar nature to those found in Dandaragan soil, only found in lower proportions.

Glycans: These polar saccharides were found in relatively high proportions in the Dandaragan soil compared to South Stirling soil particularly in the methanol fraction existing mainly as 1, 6-Anhydro-β-D-glucopyranose (Levoglucosan). It has been suggested that soil glycans originate from both plants and microorganisms and may be key participants in SWR since they bind to soil constituents via hydrogen and covalent bonds forming aggregates. However being a source of energy for microbial activity in the soil, these aggregates are liable to fast degradation that may result in exposing hydrophilic surfaces [174-176].

142

Amides: The origin of soil organic matter is mainly associated with the vegetation and microbial activity within the soil. The absence of amides in higher plant lipids leads to the speculation that this group of compounds may have been impurities most likely from the extraction process (due to the presence of ammonia) [120].

In general, higher concentrations of identified compounds were present in the Dandaragan soil than South Stirling (Table 4.2) indicating that the concentration and distribution of volatile compounds are higher in Dandaragan than in South Stirling soil. This may be due to the differences in the physical distribution patterns of SOM in the two soils. Since Dandaragan soils contains more clay content, a significant fraction of the extracted and analyzed SOM may have been entrapped as interstitial matter in the high surface area clay structure and not necessarily found on the surfaces where it would be in direct contact with water during water transport. This is supported by the fact that Dandaragan soil is less hydrophobic as seen in measurements such as capillary rise and water transport during infiltration. The SOM here if adsorbed on the soil surface (particularly clay) may have done so orienting themselves in a fashion that enhances hydrophilicity. This may have been made possible by the thought that the charged functionalities of SOM which are capable of attaching themselves on hydroxyl groups found in charged regions on the soil surface [114].

143

Appendix 2

Changes in surface tension of surfactant solutions before and after adsorption on South Stirling and Dandaragan soil plotted on the initial concentration scale illustrating the depletion of surfactant in solution by adsorption on soil surface

80 90 75 70 70 50 65 30 60 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

55 mN/m 50 45 40 0 0.05 0.1 0.15 0.2 0.25 C (g/L)

Figure A2.1 Changes in surface tension of the block copolymer; (●) before and (○) after adsorption on South Stirling soil

80 80 70 60 40 60 20

/m 50 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 mN 40

30

20 0 0.05 0.1 0.15 0.2 0.25 C (g/L)

Figure A2.2 Changes in surface tension of the alcohol ethoxylate; (●) before and (○) after adsorption on South Stirling soil 144

80 90 75 70 70 50 30 65 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

60 mN/m 55 50 45 40 0.0 0.1 0.1 0.2 0.2 0.3 C (g/L)

Figure A2.3 Changes in surface tension of the block copolymer; (♦) before and (◊) after adsorption on Dandaragan soil

80 80 60 70 40 60 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

50 mN/m 40

30

20 0.0 0.1 0.1 0.2 0.2 0.3 0.3 C (g/L)

Figure A2.4 Changes in surface tension of the alcohol ethoxylate; (♦) before and (◊) after adsorption on Dandaragan soil

145

80 80 60 70 40 60 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

50 mN/m 40

30

20 0.0 0.1 0.1 0.2 0.2 0.3 0.3 C (g/L) Figure A2.5 Changes in surface tension of block copolymer/alcohol ethoxylate mixture (1:1 wt. ratio) (●) before and (○) after adsorption on South Stirling soil

80 80 60 70 40

60 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

50 mN/m 40

30

20 0.0 0.1 0.1 0.2 0.2 0.3 0.3 C (g/L) Figure A2.6 Changes in surface tension of block copolymer-alcohol ethoxylate mixture (1:1 wt. ratio) (♦) before and (◊) after adsorption on Dandaragan soil 146

A 9 8 7 6

2 5 h 4 3 2 1 0 0 30 60 90 120 150 180 time (s)

B 9 8 7 6

2 5 h 4 3 2 1 0 0 30 60 90 120 150 180 time (s)

Figure A2.7 Kinetics plots of capillary suction used to determine the advancing contact angles for the non-ionic surfactants on; (A) the severely hydrophobic South Stirling soil using; (■) block copolymer and (●) alcohol ethoxylate, and (B) the low hydrophobic Dandaragan soil using; (□) block copolymer and (○) alcohol ethoxylate surfactant

147

1 A

0.8 ) 2 0.6

0.4 Q (mg/m Q 0.2

0 0 0.01 0.02 0.03 0.04 0.05 0.06 C eqb (g/L)

B 1

0.8

) 2 0.6

0.4 Q (mg/m Q

0.2

0 0 0.01 0.02 0.03 0.04 0.05 0.06 C eqb (g/L)

Figure A2.8 Adsorption isotherm of surfactants: (■) block copolymer, (●) alcohol ethoxylate and (∆) block copolymer-alcohol ethoxylate blend (1:1 wt. ratio) on (A) South Stirling and (B) Dandaragan soil for pre-CMC concentrations

148

Table A2.1 Freundlich isotherm parameters (K and n) and the correlation coefficient R2 for the adsorption of surfactants on the W.A. soil systems for equilibrium concentrations below the CMC

Soil Surfactant (1/n) n log K K R2 Block copolymer 0.6883 1.453 0.4548 2.85 0.930

Alcohol ethoxylate 0.9638 1.038 1.396 24.90 0.9905 South Stirling Block copolymer- 0.9132 1.095 1.096 12.48 0.9816 alcohol ethoxylate mixture

Block copolymer 0.2221 4.502 -0.673 0.2126 0.9825

Alcohol ethoxylate 0.8467 1.181 0.95 8.913 0.9896 Dandaragan

Block copolymer- 1.1217 0.8915 1.369 23.36 0.9924 alcohol ethoxylate mixture

149

h2 vs t 9 8 7 6

2 5 h 4 3 2 1 0 0 30 60 90 120 150 180 time (s) Figure A2.9 Kinetics plots of capillary suction used to determine the advancing contact angles for the block copolymer/alcohol ethoxylate blend (1:1 wt. ratio) on (□) severely hydrophobic South Stirling soil and (●) low hydrophobic Dandaragan

150

Appendix 3

Figure A3.1 Changes in surface tension of; (A) block copolymer-APG blend, (B) alcohol ethoxylate-APG blend (1:1 wt. .ratio) (●) before and (○) after adsorption on South Stirling soil

A 75 80 70 60 65 40 60 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 55

mN/m 50 45 40 35 0.0 0.1 0.2 0.3 C (g/L)

B 100 80 50 70 0 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

50 mN/m 40

30

20 0.0 0.1 0.2 0.3 0.4 0.5 0.6 C (g/L)

151

Figure A3.2 Changes in surface tension of (a) block copolymer-APG blend, (b) alcohol ethoxylate -APG (1:1 wt. ratio) (●) before and (○) after adsorption on Dandaragan soil

A 120 75

70 70

65 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 60

mN/m 55

50

45

40 0.0 0.1 0.2 0.3 C (g/L)

B 80 80 60 70 40 60 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

50 mN/m 40

30

20 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 C (g/L)

152

Figure A3.3 Changes in surface tension of (a) block copolymer-C6E1 blend, (b) alcohol ethoxylate-C6E1 blend (1:1 wt. ratio) (●) before and (○) after adsorption on South Stirling soil

A 80 75 70 60 65 40 60 0 1 2 3 4 5

mN/m 55 50 45 40 0 0.1 0.2 0.3 0.4 0.5 0.6 C (g/L)

B 80 70

70 20 60 0 1 2 3 4 5

50 mN/m 40

30

20 0 0.1 0.2 0.3 0.4 0.5 0.6 C (g/L)

153

Figure A3.4 Changes in surface tension of (a) block copolymer-C6E1 blend, (b) alcohol ethoxylate-C6E1 blend (1:1 wt. ratio) (●) before and (○) after adsorption on Dandaragan

A 80 80 75 60 70 40 65 0 1 2 3 4 5

60 mN/m 55 50 45 40 0 0.05 0.1 0.15 0.2 0.25 0.3 C (g/L) i

B 120 80 70 70 20 60 0 1 2 3 4 5

50 mN/m 40

30

20 0 0.2 0.4 0.6 0.8 1 Ci (g/L)

154

Figure A3.5 Changes in surface tension of (a) block copolymer-C4E1, blend, (b) alcohol ethoxylate-C4E1 blend (1:1 wt. ratio) (●) before and (○) after adsorption on South Stirling soil

A 75 80 70 60 65 40 60 0 1 2 3 4 5

mN/m 55

50

45

40 0 0.2 0.4 0.6 C (g/L)

B 80 80 60 70 40 20 60 0 1 2 3 4 5

50 mN/m

40

30

20 0 0.2 0.4 0.6 C (g/L) 155

Figure A3.6 Changes in surface tension of (a) block copolymer-C4E1, blend, (b) alcohol ethoxylate-C4E1 blend (1:1 wt. ratio) (●) before and (○) after adsorption on Dandaragan soil

A 75 80 70 60 65

60 40 0 1 2 3 4 5

mN/m 55

50

45

40 0 0.2 0.4 0.6 C (g/L)

B80 120

70 70

60 20 0 1 2 3 4 5

50 mN/m 40

30

20 0 0.2 0.4 0.6 C (g/L)

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Figure A3.7 Changes in surface tension of (a) block copolymer-C4E2 blend, (b) alcohol ethoxylate-C4E2 blend (1:1 wt. ratio) (●) before and (○) after adsorption on South Stirling soil

A 80 75 60 70

65 40 0 1 2 3 4 5 60

mN/m 55

50

45

40 0 0.1 0.2 0.3 C (g/L)

B 80 80

70 60 40 60 20 0 1 2 3 4 5

50 mN/m 40

30

20 0 0.2 0.4 0.6 C (g/L)

157

Figure A3.8 Changes in surface tension of (a) block copolymer-C4E2 blend, (b) alcohol ethoxylate-C4E2 blend (1:1 wt. ratio) (●) before and (○) after adsorption on Dandaragan soil

A 80 75

70 60

65 40 0 1 2 3 4 5 60

mN/m 55

50

45

40 0 0.05 0.1 0.15 0.2 0.25 0.3 C (g/L)

B 120 80 70 70 20 60 0 1 2 3 4 5

50 mN/m 40

30

20 0 0.2 0.4 0.6 0.8 1 1.2 C (g/L)

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Table A3.1 CMC of 1:1 wt. ratio surfactant hydrotropic mixtures (in g/L) before and after adsorption at 20.8 ± 0.3o C

Surfactant-hydrotropic CMC before CMC after CMC after mixture adsorption adsorption adsorption on on South Dandaragan Stirling soil soil

Block copolymer-APG 0.025 0.03 0.04

Block copolymer-C6E1 0.06 0.12 0.10

Block copolymer-C4E1 0.06 0.07 0.07

Block copolymer-C4E2 0.06 0.092 0.08

Alcohol ethoxylate-APG 0.09 0.22 0.32

Alcohol ethoxylate-C6E1 0.06 0.17 0.30

Alcohol ethoxylate-C4E1 0.08 0.18 0.30

Alcohol ethoxylate-C4E2 0.10 0.14 0.32

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Table A3.2 Freundlich isotherm parameters (K and n) and the correlation coefficient R2 for the adsorption of surfactant-hydrotrope mixtures on the W.A. soils for equilibrium concentrations below the CMC A. South Stirling soil

Surfactant --hydrotrope slope mixture (1/n) n log K K R2 Block copolymer (BCP) 0.6883 1.453 0.4548 2.850 0.93 BCP-APG 1.688 0.5924 2.371 235.0 0.9678 BCP-C6E1 0.644 1.553 0.4365 2.732 0.9818 BCP-C4E1 0.3488 2.867 -0.28 0.5448 0.9700 BCP-C4E2 0.9549 1.047 0.8023 6.343 0.9879 Alcohol ethoxylate (AE) 0.9638 1.038 1.396 24.90 0.9905 AE-APG 1.259 0.7944 1.701 50.23 0.9637 AE-C6E1 1.137 0.8795 1.504 31.92 0.9963 AE-C4E1 0.8608 1.162 1.1775 15.05 0.9268 AE-C4E2 1.083 0.9236 1.331 21.42 0.9913

B. Dandaragan soil

Surfactant /hydrotrope slope mixture (1/n) n log K K R2 Block copolymer (BCP) 0.2221 4.503 -0.6725 0.2126 0.9825 BCP-APG 0.1214 8.237 -1.1208 0.0757 0.3825 BCP- C6E1 0.3145 3.180 -0.2237 0.5974 0.8716 BCP-C4E1 0.8148 1.2273 0.4643 2.913 0.9937 BCP-C4E2 0.5788 1.728 0.0973 1.251 0.9835 Alcohol ethoxylate (AE) 0.8467 1.181 0.9500 8.913 0.9896 AE-APG 0.7874 1.270 0.9731 9.399 0.9797 AE-C6E1 1.0914 0.9163 1.615 41.19 0.9994 AE-C4E1 0.9966 1.003 0.6629 4.602 0.9877 AE-C4E2 0.8545 1.170 1.114 12.99 0.9930

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Table A3.3 Advancing contact angles for water and surfactant-hydrotropic blends on South Stirling and Dandaragan soils calculated using Washburn’s equation *Surface tension measurements taken at 20.3 ± 0.5 oC

Soil Surfactant - hydrotrope mixture γ cos θ γcosθ θo South BCP-APG 28.8 0.0327 0.94 88.1 Stirling BCP -C6E1 40.9 0.0786 2.9 85.9 BCP-C4E1 41.0 0.0718 2.94 85.9 BCP-C4E2 40.9 0.0246 1.00 88.6 AE-APG 26.9 0.0911 2.45 84.8 AE-C6E1 27.1 0.1470 3.98 81.6 AE-C4E1 26.3 0.1365 3.59 82.2 AE-C4E2 26.3 0.1009 2.65 84.2 Dandaragan BCP-APG 28.8 0.5813 16.7 54.5 BCP-C6E1 40.9 0.4653 19.0 62.3 BCP-C4E1 41.0 0.4940 20.3 60.4 BCP-C4E2 40.9 0.4595 18.8 62.7 AE-APG 26.9 0.1927 5.18 78.9 AE-C6E1 27.1 0.6392 17.3 50.3 AE-C4E1 26.3 0.5725 15.1 55.1 AE-C4E2 26.3 0.3545 9.32 69.2

161