Overview of Main Techniques Used for Membrane Characterization

Journal of ChemicalBartosz Technology Tylkowski, and Iren Metallurgy, Tsibranska 50, 1, 2015, 3-12

OVERVIEW OF MAIN TECHNIQUES USED FOR MEMBRANE CHARACTERIZATION

Bartosz Tylkowski 1,2, Iren Tsibranska 2

1Universitat Rovira i Virgili, Received 01 October 2014 Departament de Enginyeria Química, Accepted 04 December 2014 Av. Països Catalans, 26 - 43007 Tarragona, Spain E-mail: [email protected] 2Institute of Chemical Engineering, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

ABSTRACT

The main force of membrane technology is the fact that it works without the addition of chemicals, with relatively low energy consumption and easy and well-arranged process conductions in a compact module design. Although a good number of articles and books are available on membrane separation processes, many of which are of excellent quality within their scope, most of them present research-oriented approaches of higher levels, thus making them good references in the specific field of recent membrane processes applications. This publication is focused particularly on the main techniques used for membrane characterization with supporting review of the literature and comparative discussion. Keywords: porous membrane characterization, active pore size, pore size distribution, charaterization techniques.

INTRODUCTION mainly give information on membrane morphology and structure, chemical and physical properties. The dynamic Membrane technologies have been established as techniques are of fundamental importance when inves- an effective and commercially attractive option for tigating membrane performance. Some characterisation separation and purification processes in the chemical techniques are destructive for the membrane, while the and its allied industries dealing with fuel cell [1, 2], non-destructive ones are applied also to monitor the gas separation [3], food chemistry [4], pharmaceutical membrane performance during its use. Except for bub- industry and medicine [5, 6], water treatment [7], con- ble pressure all other reported techniques have not yet centration/separation of extracts [8 - 10] , etc. No doubts, been standardized or harmonized. This fact often causes the membranes are now competitive for conventional confusion and can be misleading. Publications, aimed techniques, with a wide variety of applications, both to classify and comparatively discuss the advances in industrial and scientific. membrane characterization techniques have appeared The aim of this article is to provide a comprehen- recently [11]. Table 1 shows the main characterization sive yet concise overview of main techniques used for tests. membrane characterization. Bubble pressure Characterisation techniques can be classified into This method was initiated by Bechhold in 1908 [12] static and dynamic techniques. The static techniques and is based on the fact that the pressure (P) necessary to

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Table 1. Main characterization test.

Method Characteristic Typology M P dynamic Bubble pressure Maximum pore size non x x destructive dynamic Gas and liquid displacement Pore size distribution non x x methods (GLDP-LLDP destructive statistic Mercury porosimetry (MP) Pore size distribution x destructive Top layer thickness Scanning Electron Microscopy Surface porosity (SEM), statistic Pore size distribution x Transmission Electron destructive Qualitative structure Microscopy (TEM) analysis static Atomic force microscopy Surface porosity non (AFM) destructive dynamic Flux and retention Permeability, non x measurements Selectivity, MWCO destructive static Gas adsorption/desorption Pore size distribution non x (GAD) destructive dynamic Permporometry Pore size distribution non x x destructive SEM + X-Ray microanalysis Chemical analysis static

(EDS) Surface studies destructive X-Ray photoelectron Chemical analysis static

spectroscopy Surface studies destructive Functional group Infrared Spectroscopy (FT-IR, static analysis ATR, Photoacustic) destructive Surface studies non Contact angle measurement Surface studies destructive Stress-Strain measurements distructive blow a gas through a liquid-filled capillary is inversely wetting (the contact angle is equal zero) the pore size proportional to the capillary radius (r). The pore size can has to be calculated from the equation (2): be calculated using the Cantor equation (1): 1.44 r(µm) = (2) 2⋅g P(bar) P = ⋅ cos θ (1) r The main advantages of this test are: where q is a contact angle and g is a surface tension. ● non-destructive method, The bubble point test is a measure of the radius of ● very simple procedure and apparatus, the largest pore, since, according to Cantor equation, ● very useful for integrity test. the gas will pass through it first. For air-water system Unfortunately the test does not give information where the surface tension (g) is 72 mN/m and complete on the membrane pore size distribution and because of 4 Bartosz Tylkowski, Iren Tsibranska

the very high air-water surface tension high pressure closed or bottle ended ones, (which can cause membrane compression) is needed for ● it does not use too high pressures, thus avoiding gas permeation through small pores (in fact the lower mechanical over-stress of the membrane during the limit of the measurable pore diameter is about 13 nm) test that could result in membrane damage or structure [13]. For instance, the bubble points method was used collapse; for three hollow fiber polyphenylsulfone membranes ● it operates quickly which makes analysis simple in iso-propanol to determine the maximum pore size and easily manageable. [13], ranging from 53 to 237 nm at pressures between A pair of immiscible liquids with low interfacial 9 and 2 bar. Close to the lower limit of detection of the tension are used which means that pore sizes can be method are the results with semi-crystalline poly(ether measured at relatively low pressures. The procedure ether ketone) hollow fiber membrane in water, where consists in filling the membrane with the wetting liquid, the largest pore was about 15 nm at applied pressure and then displacing it with the other one. By monitoring 4792 kPa [14]. the pressure and the flow through the membrane, the corresponding pore radius opened at a given applied Liquid-liquid displacement porosimetry pressure can be calculated using the Cantor equation, The liquid-liquid porosimetry (LLDP) is a method provided that contact angle between the liquid-liquid that can be used to provide information on the pore interface and the membrane material can be assumed size distribution of membranes with small pores. The to be zero: procedure is based on the same principles of the air- 2g liquid displacement or extended bubble point technique, ∆p = (4) both methods using the correlation between the applied r pressure and the pore radius rp open to flux as given by By increasing the applied trans-membrane pressure Washburn [15]: stepwise, corresponding pore radius and flow values, 2g cosθ represented as the permeability of the membrane, are ∆p = (3) obtained. Thus a pore size distribution of permeability rp contributions can be evaluated. Assuming cylindrical g being the surface tension and q the contact angle pores, the Hagen-Poiseuille equation can be used to between the permeating interface and the pore material. correlate the volume flow, JV, and the number of pores The high potentiality of this technique in order to per surface unit, N, having a given pore radius, r. For evaluate the active pores in the nano- and subnanometer each pressure step, Δpi, the corresponding volume flow range (usually dp > 1 nm) makes it a very promising measured is correlated with the number of pores thus technique to study the pore size distribution of ultra- opened by [6]: filtration (UF) and nanofiltration (NF) membranes i Nrpπ 4∆ = kk i because of the relatively low applied pressures which JVi ∑ (5) k =1 8ηδ do not cause membrane compaction. Results with polymer membranes with pore sizes from 0.4 μm to 8 where η is the dynamic viscosity of the displacing th nm have proved the usefulness of the LLDP method for fluid, Nk is the number of pores in the class k per optimizing membrane making conditions and accurate unit membrane surface and δ is the pore length. This estimation of performance related capabilities [16]. The could be used to estimate size distributions in num- advantages of LLDP can be viewed in several directions: ber of pores. It should be remembered that Hagen- ● it tests the membrane in the wet state, so can give Poiseuille´s law in that simple form only holds for information very close to the normal operating condi- flow through cylindrically shaped straight pores. Thus, tions of the membrane, the so obtained pore size distributions should be more ● it also evaluates only the open pores, not any model-dependent. 5 Journal of Chemical Technology and Metallurgy, 50, 1, 2015

Liquid and air permeability found application in structures like glassy polymers Permeability of a membrane for a certain liquid (examples are known in the study of PIOFG polymers can be considered as a characteristic parameter; often [20], characterization of the support membrane in case the so-called hydraulic radius is calculated from the of thin film composite OSN membranes [21],as well as measured fluxes. In such an analysis, the permeability in the characterization of micro- and mesopore materials is determined, the porosity E, the tortuosity z and the as composite amorphous (ex. TiO2–ZrO2–organic) [23] membrane thickness 1 are estimated (or preferably deter- or semi-crystalline polymer (ex. poly(ether ether ketone) mined) and subsequently the pore size can be calculated PEEK [22] membranes. from the Hagen-Poiseuille equation (5). Permporometry (pore size distribution) It is obvious that such an approach depends largely This technique was initiated by Eyraud and modified on the model as well as on the estimated values used. by Katz et al. [24, 25]. The method is based on the fact Also, the model cannot discriminate between a system that the vapour pressure at the surface of a liquid depends with few large pores and one with a large number of on its curvature. The vapour is capillary-condensed in small pores. membrane pores thus blocking the permeation of the The method can be improved by using gas as non-condensable gas in the feed. The pore filling occurs permeating medium instead of liquid. As the transport according to the Kelvin equation: mechanism for gases is dependent on the overall pressure −4vσθ cos dK = in the system the pore size discrimination between fine RTln( P / PS ) (6) and coarse porous media is possible when the perme- in pores smaller than the Kelvin diameter dK. The latter ability at different pressures is measured. is proportional to the vapour pressure (P) of the con-

Both gas and solute permeation techniques have densable gas and related to the true pore diameter dp by been used for pore size determination in polymeric the thickness t of the adsorbed layer (t): dP= dk+t. The hollow fibers[17] , giving similar trends in the observed rest of the parameters in eq. (6) are the contact angle mean pore size, but the gas permeation has advantages (q), the surface tension (s) and the molar volume (v), considering the influence of the polymer concentration. the temperature (T). The latter greatly decreases the water solution flux, but By measuring the permeability of the non-condensa- has little influence on gas permeability and the value of ble gas as a function of relative pressure, P/Ps (0 to 1), it the measured effective surface porosity over pore length is possible to estimate the pore size distribution. value [18]. Gas permeation technique has proved to be Using this method, the closed pores are said to be useful in characterization of the thin layer in asymmet- excluded, and the pore size distribution of only the active ric membranes, but it does not give any insight into the pores is determined. So, permporometry is considered remaining membrane pore size structure [19]. as a method able to determine accurately the active pore size distribution. The nanopermporometry, based on Nitrogen adsorption/desorption (pore size dis- the capillary condensation of vapor (water or hexane) tribution) and blocking of the permeation of a non-condensable

In this method the pore size distribution is deter- gas (N2), has found numerous applications for charac- mined by the adsorption/desorption isotherms of a gas terization of organic/inorganic hybrid membranes with (usually nitrogen) subjected to adsorption and capillary pores ranging from 1 to several nanometers and used for condensation in the pores. The nitrogen adsorption nanofiltration of organic solutions[23, 26]. BET analysis is very useful for determining surface area and pore size distribution of ceramic membranes Electron microscopy usually in the micro- and meso-size pore range. For The aim of this method is to acquire visual infor- conventional dense polymer membranes (considered mation of the membrane structure and porosity through as “nonporous”) the BET analysis is rarely used. It has magnification by scanning electron microscope (SEM) 6 Bartosz Tylkowski, Iren Tsibranska

or a transmission electron microscope (TEM). coating polymeric hollow fiber membranes in view of However, the high electron beam energy used to their mechanic properties [27], in connection with the observe the specimen at the highest magnification can abrasive resistance of flat sheet membranes[33] , etc. damage the surface of a polymeric membrane. The Stress (σ) is the internal force (F) per unit area procedure for specimen preparation is relatively simple. experienced by the material (So) while strain (ε) is the

The surfaces of the membranes can be observed directly unit change in deformation of the material (100 Δl/lo). while the cross-sections are obtained by a simple cold

(liquid N2) fracture. The SEM observation does not give σ=F/So (7) a deeper insight on finer porosity since it has a lower resolution (≈ 10-20 nm) than TEM. The stress-strain relationships can then be used to The TEM has a much higher resolution (0.2 nm) than establish the compressive or tensile yielding strength, SEM and is therefore particularly useful for studying the modulus of elasticity and the ultimate strength. the very thin skin layer of asymmetric membranes. The Fig. 1 presents a typical stress-strain curve for a procedure for the preparation of TEM cross-sections is structural mild steel specimen subjected to tensile test difficult and tedious (drying, embedding into epoxy, cold under normal conditions. The specimen elongation is ultramicrotomy) since samples thin enough (50 nm or plotted along the horizontal axis and the corresponding less) for electrons to penetrate must be prepared. Using stresses are indicated by the ordinates of the curve 0 A this kind of microscopy the membrane surface cannot B C D. This diagram will be used to explain some of be directly observed. The procedure for preparing the the following nomenclature. so-called surface replica is also difficult and tedious the In the region 0A, the stress and the strain are propor- risk of producing artifacts is very high. tional and the stress at A is the proportional limit. If upon Membrane characterization by SEM is a common removal of the stress (load), the strain in the specimen technique for analysis, used for polymeric hollow returns to zero as the stress goes to zero, the material is fibers [27], in order to characterize the predominant said to remain perfectly elastic. morphology of polymeric asymmetric and mixed ma- The constant of proportionality in the straight-line trix membranes [19, 28 - 32], used in organic solvent region 0A is called the modulus of elastic or Young’s nanofiltration, to measure the thickness of the top- and modulus (E). Geometrically, it is equal to the slope of the support layers and their role in the control of the the stress-strain relationship in the region 0A. membrane mass-transport rates, as well as eventual σ E = changes in the membrane before and after nanofiltration ε (8) [4, 9]. The limits of traditional microscopy techniques were enlarged in direction 3D characterization of po- Upon loading beyond the proportional limit, the rous membranes by incorporating other modifications. elongation increases more rapidly and the diagram An example is the X-ray ultramicroscopy, based on integration with TEM, with a resolution of 100 nm and capability to characterize membrane samples in both dry and wet states [11]. Stress - strain property Most materials are subjected to stress and the ac- companying deformation during processing and use. The mechanical properties of the membranes characterised by stress-strain tests are needed to confirm the potential suitability of this membrane in applications like: nanofiltration [33], the appropriate chemical treatment in Fig. 1. Typical stress-strain curve. 7 Journal of Chemical Technology and Metallurgy, 50, 1, 2015

Fig. 2. Water drop behavior and contact angle values at solid of different hydrophobicity. becomes curved. At point B, a sudden elongation of the terms introduced to describe relative affinity of solids specimen takes place without significant increase in the to water spreading on their surfaces. Wettability is due applied load and the material has yielded. The value of to unbalanced molecular interactions when at least two stress at point B is called yield stress or yield strength. materials are brought in a contact. A measure of this The deformation of the material prior to reaching the unbalance of forces is the contact angle (θ). Its value yield point creates only elastic strains, which are fully is related to surface energy values of the materials. It recovered if the applied load is removed. However, is generally, but rather arbitrarily assumed that for θ once the stress in the material exceeds the yield stress, < 90° indicates that the solid is partially wetted by a permanent (plastic) deformation begins to occur. The liquid (for example water). Surfaces characterized by strains associated with this permanent deformation are the contact angle of water smaller than 90° are usually called plastic strains. When the material has passed termed hydrophilic. Hydrophilic means literally „water through the yielding point, stress continues to increase preferring”, and drops of water placed at such surfaces with strain, but at a slower rate than in the elastic range, should spread spontaneously forming a thin water film until a maximum value is reached which is termed the at the surface when θ ≈ 0° (Fig. 2). The contact angle ultimate strength (point C). The increase in stress upon θ > 90° indicates non-wetting. Such surfaces are called yield stress is due to material strain hardening. Beyond hydrophobic. Drops of water tend to form „beads” on point C, the stress decreases until the specimen ruptures hydrophobic („water rejecting”) solid surfaces. Surfaces at point D. on which the water contact angle is above 140° are Yield strain and tensile strain are defined differently termed superhydrophobic and can be obtained from the in polymers than in metals and the degree of plastic hydrophobic ones by appropriate modifications such as deformation varies with polymer type and concentra- roughening, micro patterning, machining or etching. tion. For instance results with PAN membranes show Water spreading and forming drops of various con- that tensile strength and elongation can be enhanced tact angles is showed in Fig. 2. On the other hand, if the by reasonably increasing PAN concentration. The me- contact angle of water is higher than zero, then the work chanical properties of the membranes depend on their of spreading, WS, is negative. microstructure. Greater tensile strength and elongation W = W −W are observed when cellular-like pores in comparison S A C (9) with sheet- or needle-like pores [34]. where WA is the work of adhesion and WC is the work Contact angle of cohesion. The works of adhesion and cohesion are The hydrophobic/hydrophilic characteristics of sol- related to solution surface tension as follows: ids plays a key role in many processes such as: wetting, W =σθ(1 + cos ) flotation, enhanced oil recovery, cleaning technologies, A LV (10) superhydrophobicity, liquid spreading, plants protec- tion, etc. Hydrophilicity and hydrophobicity are general WC= 2σ LV (11)

8 Bartosz Tylkowski, Iren Tsibranska

where σ LV is the liquid/vapor surface energy. Hence: so-called homogeneous wetting regime [35] i.e. when the liquid completely penetrates scratches, grooves and

WS=σ LV (1 + cos θσ ) −= 2LV σ LV (cos θ − 1) (12) cavities (Fig. 3). Another situation, when air is entrapped inside the grooves underneath the liquid, is termed the The first approach to characterize the equilibrium heterogeneous wetting (see Fig.3.2C), and is described in a solid/liquid/vapor system was introduced by Young by the Cassie-Baxter equation:

(1805) and his equation describes the mechanical bal- '' cosθθ=rf' cos +− f 1 ance at the line of the three phase contact (TPC) on an CB f (16) ideal (smooth, homogeneous, rigid and insoluble) solid where θCS is the Cassie-Baxter contact angle, f’ is the surface as: fraction of the projected area that is wet by a liquid and

rf’ is the roughness ratio of the wet area. When f’ = 1 σSV= σσ SL + LV cos θ (13) then rf’ is equal r and Cassie-Baxter equation (16) simpli- fies to Wenzel equation (15). As recently discussed[36, where σSL is the solid/liquid surface energy, and σSV is the 37], the phenomenon of higher values of contact angle solid/ vapor surface energy. The fundamental problem on modified (roughened, micro patterned, machined or associated with the equilibrium contact angle is related etched hydrophobic surfaces is caused by inhibition to the structure and topography of the solid surface, as of the liquid spreading into grooves, scratches and/or the real solids are rough. Surface roughness can affect cavities on the rough surface. Moreover, a spreading of strongly wettability and values of the apparent contact such drop can be “arrested” by the edges of the grooves. angles. To characterize the geometrical non-ideality of Contact angle is a widely used analysis for char- a surface, roughness parameter r was introduced and acterization of membrane hydrophilic/hydrophobic defined as: behaviour [31], the effect of chemical modification like A cross-linking on the latter [38] or by introducing hydro- r = real A (14) philic or hydrophobic solvent stable polymeric materi- geometrical als as support membranes in the search of optimal flux where (Areal) and (Ageometrical) are the real and geometrical without sacrificing selectivity [22]. It is observed that areas of the surface. For r > 1 Young equation was modi- contact angle and thickness of the hollow-fiber decrease fied by Wenzel. The Wenzel equation: similarly against air gap distance. Contact angle is related to the roughness parameter of the outer membrane cosθ = r ⋅ cosθ W (15) surfaces [18]. It is considered as key feature for deeper is a generalization of the Young’s equation, where θW is understanding of other techniques like permporometry, the Wenzel contact angle. The Wenzel equation refers to related to the sorption layer thickness [11].

Fig. 3. Water drop behavior and contact angle on smooth and rough hydrophobic surface.

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CONCLUSIONS tion in proton transport of a polymer film based on an oriented self-organized columnar liquid-crystalline The field of membrane technology is currently polyether, Materials Science and Engineering: C, 32, enjoying a great deal of interest and its field of appli- 2012, 105-111. cation is expanding rapidly. Every day, both in public 2. J. Park, D. Kim, Effect of cerium/18-crown-6-ether laboratories (universities, research centers, etc) and in coordination complex OH quencher on the properties private ones (R&D centers of industrial companies) of sulfonated poly(ether ether ketone) fuel cell elec- variety of techniques and approaches are routinely used trolyte membranes, Journal of Membrane Science, for characterizing the physical and chemical properties 469, 2014, 238-244. of membranes. 3. Y. Okamoto, H. Zhang, F. Mikes, Y. Koike, Z. He, T.C. Concerning the applications of porous membranes in Merkel, New perfluoro-dioxolane-based membranes separation processes the importance of reliable characteri- for gas separations, Journal of Membrane Science, zation of pore sizes and distributions requires: increased 471, 2014, 412-419. measurement resolution and capability to define the active 4. B. Tylkowski, B. Trusheva, V. Bankova, M. Giamber- part of the pore structure, which really contributes to the ini, G. Peev, A. Nikolova, Extraction of biologically permeability; discrimination between artefacts induced active compounds from propolis and concentration by the characterization technique itself, swelling effects, of extract by nanofiltration, Journal of Membrane etc.; dealing with pronounced asymmetric pore-size dis- Science, 348, 2010, 124-130. tributions, including bimodal ones; characterization of 5. Q. Fan, K.K. Sirkar, B. Michniak, Iontophoretic three dimensional morphology of porous membranes. In transdermal drug delivery system using a conducting the scope of the actual problematic of depth profile ex- polymeric membrane, Journal of Membrane Science, amination, non-destructive sample preparation, wet-state 321, 2008, 240-249. characterization, etc., recent standard methods for char- 6. I.H. Tsibranska, B. Tylkowski, Concentration of etha- acterizing membrane surfaces, cross-section and porosity nolic extracts from Sideritis ssp. L. by nanofiltration: are presented and discussed. This review is motivated by Comparison of dead-end and cross-flow modes, Food the improvement of researchers’ ability to evaluate the and Bioproducts Processing, 91, 2013, 169-174. role(s) of membrane properties in determining membrane 7. K. Szymanska, A.I. Zouboulis, D. Zamboulis, Hybrid performance, preferably for a wide range of applications ozonation–microfiltration system for the treatment of with accent on examples with porous polymeric mem- surface water using ceramic membrane, Journal of branes. In the focus are conducting and interpreting results Membrane Science, 468, 2014, 163-171. from bubble pressure, liquid-liquid porosimetry, nitrogen 8. I. Tsibranska, B. Tylkowski, R. Kochanov, K. Al- adsorption/desorption, permporometry, scanning electron ipieva, Extraction of biologically active compounds microscope, transmission electron microscope, contact from Sideritis ssp. L, Food and Bioproducts Process- angle and permeability methods. Porosity and pore size ing, 89, 2011, 273-280. distributions studies together with factors as tortuosity, 9. B. Tylkowski, I. Tsibranska, R. Kochanov, G. Peev, M. pore shape and connectivity give link to recent devel- Giamberini, Concentration of biologically active com- opments (including 3D morphology characterization) pounds extracted from Sideritis ssp. L. by nanofiltration, important to the performance of membranes in various Food and Bioproducts Processing, 89, 2011, 307-314. applications areas. 10. I. Tsibranska, I. Saykova, Combining nanofiltration and other separation methods (review), Journal of REFERENCES the University of Chemical Technology and Metal- lurgy, 48, 2013, 333-340. 1. B. Tylkowski, N. Castelao, M. Giamberini, R. Garcia- 11. K.-L. Tung, K.-S. Chang, T.-T. Wu, N.-J. Lin, K.-R. Valls, J.A. Reina, T. Gumí, The importance of orienta- Lee, J.-Y. Lai, Recent advances in the characteriza- 10 Bartosz Tylkowski, Iren Tsibranska

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