Subscriber access provided by CMU Libraries - http://library.cmich.edu Article Surfactant Behaviour of Sodium Dodecylsulfate in the Deep Eutectic Solvent Choline Chloride: Urea Thomas Arnold, Andrew J Jackson, Adrian Sanchez- Fernandez, Daniel Magnone, Ann E. Terry, and Karen J. Edler Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02596 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 7, 2015
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1 2 3 4 5 6 7 8 Surfactant Behaviour of Sodium Dodecylsulfate in 9 10 11 12 the Deep Eutectic Solvent Choline Chloride: Urea 13 14 15 16 1* 2,3 3,4 1† 5 17 T. Arnold , A.J. Jackson , A. Sanchez Fernandez , D. Magnone , A.E. Terry and K.J. 18 19 Edler 4 20 21 22 1 Diamond Light Source, Harwell Campus, Didcot, OX11 ODE, UK 23 24 25 2 26 European Spallation Source, Lund, Sweden 27 28 29 3 Department of Physical Chemistry, Lund University, SE 221 00, Lund, Sweden 30 31 32 4 33 Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK 34 35 36 5 ISIS Spallation Neutron Source, Harwell Campus, Didcot, OX11 ODE, UK 37 38 39 KEYWORDS: Deep Eutectic Solvent, Sodium Dodecylsulfate (SDS), Choline Chloride, Urea, 40 41 42 Micelle, Surfactant, Self assembly, Small angle scattering, X ray reflectometry 43 44 45 46 Deep Eutectic Solvents (DES) resemble ionic liquids but are formed from an ionic mixture 47 48 instead of being a single ionic compound. Here we present some results that demonstrate that the 49 50 surfactant sodium dodecylsulfate (SDS) remains surface active and shows self assembly 51 52 53 phenomena in the most commonly studied DES, choline chloride/Urea. X ray reflectivity (XRR) 54 55 and Small Angle Neutron Scattering (SANS) suggests the behaviour is significantly different 56 57 58 from that in water. Our SANS data supports our determination of the critical micelle 59 60 ACS Paragon Plus Environment 1 Langmuir Page 2 of 35
1 2 3 concentration using surface tension measurements and suggests that the micelles formed in DES 4 5 6 do not have the same shape and size as those seen in water. Reflectivity measurements have also 7 8 demonstrated that the surfactants remain surface active below this concentration. 9 10 11 12 13 14 Introduction 15 16 17 The discovery of new solvents for amphiphile self assembly aids in understanding the 18 19 important solvent properties which enable this self aggregation, allows the systems to be tuned 20 21 22 for particular applications (via control of polarity, surface tension, viscosity, conductivity, 23 24 refractive index and thermal properties) and improves our understanding of the ‘‘solvophobic 25 26 effect’’ to allow further development of useful self organised materials. Micellization, 27 28 29 particularly of non ionic surfactants in protic ionic liquids, has been the subject of several studies 30 31 beginning in the 1980s 1 but with a recent upsurge of interest 2 7. At least 37 protic ionic liquids 32 33 have been identified as solvents where micellization can occur 6. However the hygroscopic nature 34 35 36 of ionic liquids, and the frequent extremes of pH found in these solvents are not conducive to 37 38 long term stability of surfactants in these media and their expense and toxicity prevents their use 39 40 in many applications. 41 42 43 Deep Eutectic Solvents (DES) are an alternative to ionic liquids with several potential 44 45 advantages. In particular they are easily prepared from cheap, non toxic molecules and can be 46 47 48 formed from biodegradable and biocompatible neutral species. DES resemble ionic liquids but 49 50 are formed from an ionic mixture instead of being a single ionic compound. In deep eutectic 51 52 solvents, formation of a liquid at ambient temperatures relies on a large depression in freezing 53 54 55 point coming from a favourable hydrogen bonding interaction between the constituents. The 56 57 freezing point depression is largest at the eutectic point and can be as much as 270 °C. 8 DES 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 35 Langmuir
1 2 3 share with ionic liquids properties which make them highly desirable as “green solvents”; they 4 5 6 have very low volatility, are generally non flammable and have a wider liquid temperature range 7 8 than molecular solvents. DES in general have been shown to be good solvents for a range of 9 10 9 10 11 11 inorganic salts and have been studied most in the electroplating of metals such as zinc, silver 12 12 13 13 and aluminium. DES can also be used to create alloys, and metal nanoparticles as well as in 14 15 the processing of metal oxides. 14 DES have also been investigated for use in pharmaceutical 16 17 15 18 applications and also for selective extractions of, for example, high value species from 19 20 biomass 16 and glycerol from biodiesel. 17 21 22 In general most DES are composed of a quaternary ammonium halide salt mixed with metal 23 24 25 salts or a “hydrogen bond donor”. The strong interaction of this donor with the halide ions 26 27 stabilises the liquid and results in the large depression of freezing point at the eutectic 28 29 composition. The most studied series of DES are those prepared using choline chloride which is 30 31 32 a food additive (choline is an essential nutrient, usually grouped with the B vitamins) with one of 33 34 many other species, including urea 18 or M 2+ ions such as Zn 2+ .19 A wide range of other DES have 35 36 37 also been identified and their synthesis and properties have recently been reviewed by Zhang et 38 20 39 al. This review summarises what is known about the freezing points, density, viscosity, 40 41 polarity, acidity, ionic conductivity, and surface tension for a range of different DES. 42 43 44 Importantly one of the most noticeable properties of many DES is high viscosity. This may result 45 46 from many factors including the presence of an extensive hydrogen bonding network, relatively 47 48 large ion sizes and electrostatic forces within the liquid. 49 50 51 Very recently it has been suggested that sodium dodecyl sulfate (SDS) shows some self 52 53 assembly phenomena in choline chloride / urea DES containing either water or cyclohexane 21 . 54 55 These authors presented surface tension and dynamic light scattering (DLS) measurements for a 56 57 58 59 60 ACS Paragon Plus Environment 3 Langmuir Page 4 of 35
1 2 3 series of choline chloride / urea solutions containing both SDS and water. Crucially they did not 4 5 6 report any results for SDS in the pure DES, so we cannot be sure that the observed structures are 7 8 micelles nor that similar behavior would be observed in the absence of water. The DLS results 9 10 11 presented indicate that the self assembled structures are substantially larger than those observed 12 13 in water, which is an interesting result. For most polar, but non aqueous solvents, SDS is found 14 15 to form smaller micelles are found than for the same surfactant in water 22, 23 . This result, in itself 16 17 18 merits further investigation, but disappointingly these authors did not present any additional 19 20 experimental evidence to refine the details of the observed structures. Typically DLS does not 21 22 give information on particle shape, since the calculations based on diffusion rates assume 23 24 25 spherical objects. They did however present some data (surface tension, DLS, fluorescence 26 27 spectroscopy and small angle x ray scattering (SAXS)) on micro emulsions formed by 28 29 cyclohexane and the DES in the presence of SDS. They used this data to conclude that the 30 31 32 solubility of cyclohexane in this DES was improved by the presence of SDS. This was explained 33 34 by the formation of a micro emulsion, a conclusion that is consistent with the data presented. 35 36 37 However, the interpretation of their SAXS data is unsatisfactory, and we shall discuss this further 38 39 below. 40 41 In this study we have examined the relatively well studied DES based on choline chloride and 42 43 44 Urea (mixed in the molar ratio 1:2). This system is probably the most studied of any DES and is 45 46 therefore an obvious starting place for studies of the behaviour of surfactants within DES. The 47 48 role of water in these systems is also likely to be important for future application and so this has 49 50 51 also been briefly considered. The surfactant chosen for this work is sodium dodecyl sulfate, 52 53 which is a representative anionic surfactant for which extensive data of its behaviour in water 54 55 already exists, for example, from surface tension 24, 25 , small angle scattering 22, 26 and 56 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 35 Langmuir
1 2 3 reflectometry 24, 27, 28 studies. We have performed small angle neutron scattering (SANS) 4 5 6 measurements to demonstrate the existence of micelles formed by SDS in a pure urea choline 7 8 chloride DES and to provide detail on the structure of these self assembled systems. We have 9 10 11 also verified that this surfactant is surface active in the choline chloride / urea mixture and 12 13 determined the structure of the adsorbed surfactant film at the air water interface by means of x 14 15 ray reflectivity. 16 17 18 19 20 Experimental Section 21 22 Materials 23 24 25 We have used the following abbreviations for the materials used in this study: h SDS and d 26 27 SDS are fully protonated and deuterated sodium dodecylsulfate respectively and h urea and d 28 29 urea are similarly fully protonated and deuterated urea. Finally h ChCl refers to fully protonated 30 31 32 choline chloride ((CH 3)3NC 2H4OH Cl) while d ChCl refers to the partially deuterated equivalent 33 34 ((CD 3)3NC 2H4OH Cl). All protonated materials used were ≥ 98% pure and purchased from 35 36 37 Sigma Aldrich while the deuterated materials were purchased from QMX laboratories (98% 38 39 deuteration, 99% purity). The compounds were used without further purification since the 40 41 ultimate applications for these systems are unlikely to use pure compounds. Similarly, although 42 43 44 the samples have been kept in sealed bottles they are hydroscopic and so the precise water 45 46 content of the samples is not known. There is no doubt that the presence of impurities will have 47 48 some effect on the details of the behaviour observed. 49 50 51 We have not attempted to control the humidity nor water content during measurements, but 52 53 have in some cases monitored the behaviour in DES solutions containing 5 wt% water. It is 54 55 potentially possible to measure the water content of the DES, but actually for practical reasons 56 57 58 59 60 ACS Paragon Plus Environment 5 Langmuir Page 6 of 35
1 2 3 this is not straightforward. Sample preparation is done in a laboratory and during this process and 4 5 6 during subsequent measurements water is constantly being absorbed from the atmosphere. 7 8 Therefore it would be difficult to take a measurement of the water content at the crucial moment 9 10 11 when it is being measured by another probe. As a result we have taken the approach of accepting 12 13 a certain amount of water content and tried to assess whether this actually significantly changes 14 15 the observed results. To do this we have determined the water content of several representative 16 17 18 samples using the Karl Fischer method (Mettler toledo DL32 Karl Fischer Coulometer Aqualine 19 20 Electrolyte A (Fisher Scientific) Aqualine Catholyte CG A (Fischer Scientific)). Measurements 21 22 were taken 3 times at different sample weights (0.3 and 0.8 g) and averaged. After synthesis of 23 24 25 the DES (2 hours at 350K + 24 h equilibration at 330K) the water content was determined to be 26 27 2142 ± 123 ppm (approximately 0.2 wt%). After freeze drying (24h) it had not changed 28 29 significantly (2039 ± 16 ppm) nor had it after two weeks in a vial (2214 ± 62 ppm) despite 30 31 32 frequent but sporadic opening and closing (this simulates the way in which the mixture was used 33 34 in our other experiments). We therefore are confident that the absorption of water is not rapid on 35 36 37 the timescale of the other measurements performed in this study and that it remains below 0.5 38 39 wt% in all cases.In order to minimise the variability between samples, all the choline chloride / 40 41 urea DES (molar ratio 1:2) were prepared as large stock solutions, from which the surfactant 42 43 44 solutions were subsequently made. For each of the characterisation measurements presented here 45 46 the DES/surfactant samples were prepared in advance and stored in an oven between 40°C and 47 48 80°C. This was to allow for complete dissolution of the surfactant and equilibration at a 49 50 51 temperature several degrees higher than the freezing point for a minimum of 2 3 hours before 52 53 use. 54 55 Methods 56 57 58 59 60 ACS Paragon Plus Environment 6 Page 7 of 35 Langmuir
1 2 3 Surface tension measurements were made using the Drop shape analysis method 29 using a 4 5 6 Kruss DSA100 at Diamond Light Source. This method was used because other methods 7 8 attempted (e.g. du Nouy ring) were found to give less consistent results. The measurement was 9 10 11 done by withdrawing a small quantity of solution into a dispensing needle and mounting the 12 13 needle on the tensiometer. A photograph was taken within 1 minute of removal from the oven. 14 15 So although the temperature could not be controlled, which undoubtedly introduces some 16 17 18 uncertainty in the measurements, we believe that our method allows a self consistent set of data 19 20 to be obtained. Differential Scanning Calorimetry (DSC) measurements were obtained using a 21 22 Perkin Elmer Pyris DSC also at Diamond Light Source. 23 24 25 Small angle neutron scattering measurements were measured on LOQ instrument at ISIS 26 27 Pulsed Neutron Source, UK. 30 . The data were converted from time of flight spectra to 28 29 normalised intensity vs wavevector transfer (Q) using the standard ISIS routines in the Mantid 30 31 31 32 software . Samples were sealed in 1 mm cuvettes and temperature controlled at 30 °C. Samples 33 34 were prepared at different contents of hydrogenated and deuterated components in the solvent or 35 36 37 surfactant to obtain 3 different scattering contrasts; h SDS in d ChCl:d urea, hSDS in h ChCl:d 38 39 urea and d SDS in h ChCl:h urea. Solutions were prepared at surfactant mole fractions below 40 41 and above the critical micelle concentration (CMC) suggested by the surface tension 42 43 44 measurements. The scattering from the appropriate solvent blanks was also measured and 45 46 subtracted as a background from the scattering from the surfactant containing samples. 47 48 The X ray reflectivity measurements were taken on Beamline I07 at Diamond Light Source, 49 50 32 51 using the “double crystal deflector” system at 12.5keV . In this case the samples were enclosed 52 53 in a helium atmosphere and contained within a temperature controlled PTFE trough at 54 55 approximately 45°C. The time between pouring and measurement was approximately 15 30 56 57 58 59 60 ACS Paragon Plus Environment 7 Langmuir Page 8 of 35
1 2 3 minutes. Data was collected by integration over two regions of interest on a Pilatus 100k 4 5 6 detector, one for the specular reflection and the other to approximately subtract the background. 7 8 A “footprint” correction for over illumination was used assuming a Gaussian beam profile and 9 10 11 ignoring meniscus effects. Three attenuation regimes were collected and normalized to the 12 13 critical edge. 14 15 Results 16 17 18 Surface Tension 19 20 The surface tension curve for SDS in choline chloride / urea DES is shown in Figure 1, 21 22 together with the literature values for SDS in water 24, 25 for comparison. Since our conversion to 23 24 3 25 concentration is dependent on the relatively poorly defined density of the DES (1.15g cm 26 27 determined from XRR results below and assuming that the volume of the DES is not influenced 28 29 by the presence of SDS) we have plotted the data against both the mole fraction and 30 31 32 concentration. The result clearly shows that SDS remains surface active in the DES solutions 33 34 because the surface tension shows a clear variation with concentration. Since the surface tension 35 36 37 varies in a similar way to that seen in water, we can also infer the existence of micelles at high 38 39 concentration, which we have confirmed by SANS measurements below. We have also measured 40 41 the surface tension from this DES with the addition of 5wt% water, but this data is not shown 42 43 44 since no significant effect of the added water was observed. This result is consistent with the data 45 46 of Pal et al. 21 who showed some very low resolution SDS surface tension data for water 47 48 containing choline chloride / urea solutions. 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 8 Page 9 of 35 Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 24, 25 46 Figure 1: Surface tension of SDS in Water (blue) from Elworthy & Mysels and in the 47 48 choline chloride / urea DES (black). The x axis is plotted as (a) concentration or (b) mole 49 50 51 fraction of SDS. 52 53 54 55 There are some interesting differences between the behaviour of water and choline chloride / 56 57 58 urea. The surface tension of the pure choline chloride / urea solution is approximately 66±1 mN 59 60 ACS Paragon Plus Environment 9 Langmuir Page 10 of 35
1 2 3 m 1. This is similar to previous measurements of other DES 20 but lower than the 72 mN m 1 4 5 6 observed for water. In general, the addition of surfactant lowers the surface tension until a 7 8 Critical Micelle Concentration (CMC) is reached. This occurs at a significantly lower 9 10 24, 25 11 concentration than for the same surfactant in water . However, it is an interesting observation 12 13 that when plotted on a scale of mole fraction rather than concentration the CMC is approximately 14 15 the same as that of water. As far as we can tell this is not necessarily the general case for 16 17 33 18 surfactants in DES . It is also interesting to note that CMC’s observed for SDS in other protic 19 20 ionic liquids or non aqueous solvents show an increase in the CMC rather than the decrease seen 21 22 here (see Table 1). 23 24 25 For SDS in the ionic liquid, 1 butyl 3 methyl imidazolium chloride (bmim Cl), although the 26 27 initial surface tension is lower that of water, the limiting surface tension above the CMC is 28 29 similar. 34 However the data reported by Anderson et al 34 shows a smoother decline in surface 30 31 32 tension with increasing concentration, when compared to the discontinuity seen in our data, and 33 34 that commonly seen in surface tension vs concentration data for water. The lack of a 35 36 35 37 discontinuity has been attributed to the formation of pre micelles; aggregates with a low 38 39 number of monomers that continuously increase in number to form larger micelles as 40 41 concentration is increased, rather than the sudden transition seen in water, and also in the DES 42 43 36 34 44 studied here. We also note that in both formamide and bmim Cl (Table 1), the aggregates 45 46 observed above the CMC are described as very large, being more like phase separated 47 48 agglomerations of randomly oriented molecules than traditional micelles where the headgroups 49 50 51 of the surfactant interact with the solvent, and the tails are sequestered in the centre. 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 10 Page 11 of 35 Langmuir
1 2 3 Table 1 Literature CMC values for SDS in a variety of non aqueous solvents compared to the 4 5 6 surface tension of the solvent, the Kamlet Taft solvent parameters and Gordon parameter for 7 8 each solvent. 9 10 11 12 13 SDS CMC Kamlet Taft parameters G /J m 3 14 Surface 6 Temp 15 Conc Mole α (h β (h π* tension 16 / ºC mmol Fraction bond bond polar /mN m 1 17 3 18 dm ** donor) acceptor) isability 19 20 21 21 ~1 ? choline 2 x 10 4 22 37 § 23 chloride / ± 1 x 10 0.675 0.501 1.226 66±1 1.57 2±1 4 24 urea 25 (this 30 26 work) 27 28 24 1.48 x 8.2 4 25 72.8 2.743 29 water 10 1.17 0.47 1.09 30 8.57 38 35 (25ºC) 2.750 31 32 33 64.6 40 60 wt% 1.51 34 14.14 25 (75% glycerol in 1.21 0.51 0.62 (glycer 35 glycerol, 36 water 39 17.6 40 ol only) 37 25ºC) 38 39 hydrazine ‡ 38 22 35 66.67 2.10 40 (H 4N2) 41 42 50.8 42 43 44 60 wt% (0.61 1.20 45 ethylene mole 46 24 25 0.90 0.52 0.92 (EG glycol in fraction 47 41 only) 48 water EG in 49 water, 50 25ºC) 51 52 1 butyl 3 53 methyl 48 ± 4. 48.2 45 0.885 54 imidazolium 4 7.7 x 10 3 ? *** 0.32 0.95 43, 44 1.13 44 55 (25ºC) (25ºC) chloride 56 34 57 (bmim Cl) 58 59 60 ACS Paragon Plus Environment 11 Langmuir Page 12 of 35
1 2 3 formamide 8.69 x 47 1.50 4 23, 46 220 3 60 0.71 0.48 0.97 58.2 5 10 1.70 6 48, 49 7 ** Calculated from literature concentration based on the literature density of the solvents 8 in question and assuming that the dissolution of surfactant does not change the molar volume. 9 3 1 10 § Calculated using the equation given in the text, using a molar volume of 75.3 cm mol 11 calculated from the density (1.15gcm 3) and the average molar mass (86.6 gmol 1). 12 13 ‡ Calculated from surface tension, based on density 1.0036 g cm 3 38 and molar mass of 32.046 14 g mol 1. 15 16 ***bmim Cl is solid at room temperature but is known to supercool. The temperature of the 17 18 measurement is not specified in the reference 19 20 21 22 We have found that the measurement of absolute values of surface tension using our method is 23 24 not repeatable to a very high accuracy (± 1 mmol dm 3). This is most likely to be because of 25 26 27 differences in temperature, atmospheric moisture, impurities and stock solution composition. 28 29 Despite this, we can reasonably assign the CMC in choline chloride / urea to 2 ± 1 mmol dm 3, 30 31 (Mole fraction of SDS (X ) = 2 x 10 4 ± 1 x 10 4) for temperatures in the range 30 50 ºC. 32 SDS 33 34 In contrast to non ionic surfactants, the solubility and formation of micelles by ionic 35 36 surfactants in low melting point mixtures appears to depend sensitively on both the nature of the 37 38 39 solvent and the specific surfactant. In the case of ionic surfactants in ionic liquids the extent of 40 41 headgroup dissociation, and the salts formed between the surfactant ion and components of the 42 43 solvent, as well as the presence of trace amounts of water, are all important factors determining 44 45 50 34 46 solubility in a particular solvent. While aggregates of SDS are reported to form in bmimCl, 47 50 48 SDS does not appear to dissolve easily in bmimPF 6 or emimTf 2N . This was ascribed to either 49 50 the lack of hydration surrounding the SDS headgroup which prevented the solid surfactant from 51 52 50 53 dissolving in emimTf 2N or the nature of the salts formed by statistical mixing (eg (NaTf2N and 54 55 emim dodecyl sulfate in that case). For choline chloride / urea, the salts formed by statistical 56 57 58 mixing would be NaCl and choline dodecyl sulphate. Previous work has shown that urea can 59 60 ACS Paragon Plus Environment 12 Page 13 of 35 Langmuir
1 2 3 51 4 form eutectics with alkali metal halides, with melting points between 30 140 °C, thus 5 6 dissolution of this surfactant in this DES at concentrations sufficient to allow micellisation is 7 8 feasible. 9 10 11 The formation of micelles in a particular solvent depends on both the solvent polarity and the 12 13 cohesive energy density of the solvent. The cohesive energy density can be quantified using the 14 15 Gordon parameter: 16 17 1/3 18 G = γLV /(V m ) 19 20 52 21 where γLV is the liquid air surface tension and V m is the molar volume. We compare the 22 23 liquid air surface tensions, Gordon parameters, and the Kamlet Taft parameters of various liquids 24 25 26 in which SDS has been observed to form micelles (Table 1). A reduction in the cohesive energy 27 28 density in general is expected to lead to an increase in the CMC, while increased solvent polarity 29 30 should increase the tendency to form micelles, thus lower the CMC.39 In the case of choline 31 32 33 chloride /urea although the Gordon parameter is lower than that of water, it is higher than for the 34 35 other solvents where micellization is reported, while the π* solvent polarisability parameter is 36 37 38 even higher than that of water, which would lower solvent interactions with the hydrocarbon tail, 39 40 possibly leading to the low CMC values measured for SDS in this solvent. Compared to the two 41 42 solvents with highest reported CMC values for SDS (which are also those for which unusually 43 44 45 large aggregates are found), in formamide the solvent polarisability parameter is much lower 46 47 than that for the choline chloride / urea DES, while in bmimCl, the cohesive energy density is 48 49 lower than for the DES. In each case however the alternate parameter is roughly similar to that of 50 51 52 the DES, reinforcing the idea that both factors are important for micellization in a given solvent, 53 54 as previously reported by others. 39, 52 55 56 57 58 59 60 ACS Paragon Plus Environment 13 Langmuir Page 14 of 35
1 2 3 Differential Scanning Calorimetry 4 5 6 We have performed a Differential Scanning Calorimetry (DSC) analysis of the melting/glass 7 8 transition of the choline chloride / urea DES as a function of surfactant content. Our data is not 9 10 11 sufficient to determine the order of this transition and so we have not attempted to assign it to 12 13 either melting or a glass transition. This difference is a subtle one and not straightforward to 14 15 distinguish in a viscous eutectic system. We believe that it is probably a glass transition but there 16 17 53, 54 18 is some debate on this in the literature . Figure 2 shows the variation of the transition point of 19 20 the DES solutions with SDS concentration (mole fraction). 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Figure 2: DSC results of solutions of SDS in choline chloride / urea. The CMC position 49 50 determined above is indicated by a dashed line. 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 14 Page 15 of 35 Langmuir
1 2 3 The nominal transition temperature of the pure DES is approximately 293±7K which is very 4 5 6 close to but generally slightly below room temperature in our lab and notably higher than the 7 8 value of 285K reported in the literature 18, 20 . At low surfactant concentrations (particularly below 9 10 11 the CMC defined above) this transition is not significantly changed. However as the 12 13 concentration increases above the CMC, there is a rapid change in the transition temperature up 14 15 to 310±5K where it becomes roughly constant. 16 17 18 Although not shown, the addition of 5wt% water seems to have the opposite effect to that of 19 20 adding SDS, by decreasing the transition temperature by about 5 10K. This small change ensures 21 22 that the DES containing added water is liquid at room temperature. 23 24 25 Small-angle scattering 26 27 Figure 3 (a) shows the integrated SANS intensity as a function of mole fraction of SDS 28 29 molecules , X , for each of the measured contrasts. At low concentrations no significant 30 SDS 31 4 32 scattering above background was observed. However, above XSDS = 2 x 10 , the integrated 33 34 intensity is seen to increase approximately linearly with SDS mole fraction. We can therefore 35 36 37 confirm that micelles exist above this mole fraction and this is consistent with the CMC 38 39 determined earlier. The scattering data from samples above the CMC is shown in Figures 4 and 40 41 S1. This data has been simultaneously fitted, using data from all three contrasts, to a cylinder 42 43 55 56 44 model using the SasView data analysis software , and the modelled parameters are shown in 45 46 Table 3. No inter particle interaction was included in the model as the volume fraction of 47 48 micelles obtained was at most 0.8 vol %. The length and radius were linked between the 49 50 51 contrasts, with the exception of the lowest concentration. In this latter case the h SDS/DD was 52 53 fitted and then the parameters for length and radius fixed for fits of the other two contrasts, since 54 55 56 57 58 59 60 ACS Paragon Plus Environment 15 Langmuir Page 16 of 35
1 2 3 their weak contrast resulted in significantly noisier data. The scattering length densities for each 4 5 6 component were calculated (Table 2) and held during fitting. 7 8 9 10 11 Table 2 Scattering length densities (SLD) used in fitting of SANS and XRR data. 12 13 6 2 57 6 2 14 Neutron SLD / x10 Å X ray SLD / x10 Å 15 16 d ChCl: d Urea 6.23 10.8 (fitted to critical edge) 17 18 h ChCl: d Urea 3.52 19 20 h ChCl: h Urea 1.10 21 22 d SDS Tail 6.25 n/a 23 24 h SDS Head 12.6 58 25 26 Tail 0.3 7.86 58 27 28 29 30 To fit this data we have used a model that assumes the micelles of SDS are cylindrical in 31 32 33 shape. This is not the only possible shape that the micelles could adopt, but we have not been 34 35 able to obtain acceptable fits to the data using different shapes. To illustrate this, Figure 4 (b) 36 37 3 38 shows the data from X SDS = 1.7 x 10 in the h SDS in d ChCl:d urea contrast together with the 39 40 best calculated fit for three possible micelle shapes: cylindrical, spherical and ellipsoid. It is clear 41 42 from this that only the cylindrical model gives a fit that has the correct shape to match the 43 44 45 observed data. This is in notable contrast to the behaviour of SDS in water where spherical 46 47 micelles are seen at equivalently low concentrations 22 . 48 49 The radius of the micelles is constant within error at an average of 14.7 ± 1.9 Å. This radius is 50 51 52 determined from the scattering of the surfactant tails only, because, in terms of the scattering 53 54 lengths, the solvated SDS head groups are almost indistinguishable from the solvent (at least in 55 56 terms of the statistical quality of the data presented here). This distance is roughly consistent 57 58 59 60 ACS Paragon Plus Environment 16 Page 17 of 35 Langmuir
1 2 3 with the radius expected from dodecyl chains close to fully extended and is similar to the radius 4 5 22, 58 6 of spherical micelles in water . Figure 3 (b) also shows the results of fitting the micelle 7 8 length, plotted as a function of mole fraction and indicates an approximately linear increase in 9 10 11 the length of the micelles with increasing surfactant concentration. Our fits suggest an increase 12 4 4 13 from 80 ± 11 Å at XSDS = 3.5 x10 to 268 ± 12 Å at XSDS = 1.7 x10 , but the errors in these fits 14 15 are substantial and it would be difficult to draw firm conclusions from them, other than to state 16 17 18 that the micelles are clearly cylindrical and due to this, are substantially larger than those seen in 19 20 water. SDS in formamide 36 and BMIM Cl 34 is also reported to form much larger aggregates than 21 22 those in water, however in those cases the aggregates reported are spherical, rather than 23 24 25 elongated as observed here, and thus they exceed the dimensions expected for the normal 26 27 packing of surfactant molecules in a spherical micelle. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 17 Langmuir Page 18 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Figure 3 (a) A plot of the normalised summed scattering intensity vs mole fraction of SDS for 44 45 46 the three contrasts measured: h SDS in d ChCl:d urea (black squares), h SDS in h ChCl/d urea 47 48 (red open circles) and d SDS in h ChCl/h urea (blue triangles). The plots are normalised to the 49 50 51 maximum intensity measured for each contrast, the dashed lines are a guide to the eye and an 52 53 arrow indicates the approximate CMC as determined from Figure 1. (b) the micelle length vs 54 55 mole fraction of SDS, as determined by fits to the SANS data shown in Figures 4 and S1. The 56 57 58 line is a linear fit to the data. . 59 60 ACS Paragon Plus Environment 18 Page 19 of 35 Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 Figure 4 (a) Plots showing the scattering data (on an absolute scale) and fits for increasing 45 46 concentration of SDS for the h SDS in d ChCl:d urea system. The error bars are neglected for 47 48 clarity and only data for the highest mole fractions are shown (i.e. the other samples showed no 49 50 51 small angle scattering). A similar plot for the other contrasts is shown in Figure S1, in the 52 53 supporting information. The parameters used for all of the fits are listed in Table 3. All 54 55 measurements were made at 303K (b) A comparison of the best possible fits obtainable assuming 56 57 58 59 60 ACS Paragon Plus Environment 19 Langmuir Page 20 of 35
1 2 3 three possible micelle shapes: The data is the highest concentration shown in (a) and the lines 4 5 6 correspond to fits for Cylindrical (red), Spherical (blue) and Ellipsoid (green) models. 7 8 9 Table 3: Fitting parameters for LOQ SANS Data shown in Figures 4 and S1. 10 11 12 Contrast Average Length /Å Radius /Å Volume fraction 13 Mole fraction of micelles x10 4 14 4 15 of SDS x10 16 17 h SDS in d ChCl: d Urea 5.6 ± 0. 8 18 19 h SDS in h ChCl: d Urea 3.5 ± 0.03 80 ± 12 16.6 ± 1.8 12.2 ± 1.1 20 21 d SDS in h ChCl: h Urea 9.7 ± 0.6 22 23 h SDS in d ChCl: d Urea 26.6 ± 19.8 24 25 h SDS in h ChCl: d Urea 6.8 ± 0.2 127 ± 73 12.0 ± 5.3 37.5 ± 33.4 26 27 d SDS in h ChCl: h Urea 36.6 ± 28.4 28 29 h SDS in d ChCl: d Urea 45.2 ± 11.2 30 31 h SDS in h ChCl: d Urea 10.3 ± 0.1 214 ± 95 15.1 ± 2.3 42.7 ± 15.8 32 33 d SDS in h ChCl: h Urea 39.3 ± 11.2 34 35 h SDS in d ChCl: d Urea 67.2 ± 11.1 36 37 38 h SDS in h ChCl: d Urea 17.2 ± 0.8 268 ± 12 15.1 ± 0.1 85.1 ± 18.4 39 40 d SDS in h ChCl: h Urea 52.4 ± 11.3 41 42 X-ray reflectivity 43 44 The XRR data and corresponding fit of the pure DES is shown in Figure 5. Fits shown were 45 46 calculated using Motofit 59 which uses the Abeles optical matrix method to simulate reflectometry 47 48 49 data. The model used to fit the pure DES data has only the following variables: the SLD, the 50 51 surface roughness and a residual background level. In all our data the background has been 52 53 10 54 subtracted so only a residual background remains due to imperfect subtraction (at R ≈ 1 × 10 ). 55 56 57 58 59 60 ACS Paragon Plus Environment 20 Page 21 of 35 Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 4 45 Figure 5: (a) XRR plotted on the RQ scale. This way of displaying the data highlights the 46 47 interference fringes at high Q where the most significant differences in the data are visible. It 48 49 10 50 also means that the background of R = 1 x 10 is not directly displayed on the plot. As 51 52 described in the text, an approximation to the true background has been subtracted and the value 53 54 of 1 x 10 10 is the residual background that results from the fact that this is not a perfect 55 56 57 58 59 60 ACS Paragon Plus Environment 21 Langmuir Page 22 of 35
1 2 3 subtraction. (b) The corresponding fits for pure choline chloride / urea DES (blue) and 4 5 5 5 6 containing SDS at a mole fraction of 4x10 (purple) and 6x10 (red) at 303K. 7 8 9 10 11 The SLD of choline chloride / urea can be determined from the position of the critical edge and 12 13 14 depends on the precise composition of the mixture (including any water that may have been 15 16 absorbed from the atmosphere). We can also calculate the SLD based on the chemical 17 18 formulation of the mixture and the measured density of the solution. The density of our choline 19 20 3 21 chloride /urea mixture was measured as 1.15±0.05 g cm . Using this density we calculate the 22 23 real part of the SLD = 10.7 × 10 6 Å 2 at 12.5keV 57 for a perfect 2:1 mixture. This is in 24 25 6 2 26 agreement with the value of 10.8 ± 0.1 × 10 Å determined from the position of the critical 27 28 edge. (Note that our density is slightly different to the reported 20 value of 1.25g cm 3, which 29 30 6 2 31 would give a calculated SLD of 11.6 × 10 Å .) 32 33 The roughness of the pure DES is found to be 2.7 ± 0.05 Å and, within the resolution and 34 35 reproducibility of our measurements, does not vary with the temperature nor water content. This 36 37 60, 61 38 value is similar to the capillary wave roughness of water (~3Å at 298K). This roughness is 39 40 mostly due to thermally induced capillary waves and the magnitude of the roughness is related to 41 42 43 the surface tension, and indirectly the viscosity of the liquid. As mentioned above, the surface 44 45 tension of the DES is of a similar order of magnitude to that of water, so it is not surprising that 46 47 the measured roughness is comparable. 48 49 50 Figure 5 also shows the XRR data and corresponding fits for two DES containing different 51 52 mole fractions of SDS (below the CMC determined above). This data is plotted on the 53 54 Reflectivity x Momentum transfer RQ 4 scale since this is very sensitive to the differences 55 56 57 observed at high angle. Qualitatively, the presence of a thin layer of SDS can simply be observed 58 59 60 ACS Paragon Plus Environment 22 Page 23 of 35 Langmuir
1 2 3 by noting the appearance of an interference fringe as the concentration is increased. The best fits 4 5 6 to the data are obtained using a simple two layer model which includes separate layers for the 7 8 SDS head and tail groups, and uses literature values for the SLDs (see Table 2). This is not the 9 10 11 only possible model that can be used to “acceptably” fit the data, an alternative single layer 12 13 model can also give good fits (see supporting information). The SLD profiles for these models 14 15 differ minimally, so we can be confident that in terms of the overall film thickness and interfacial 16 17 18 roughness our fits are reasonably accurate, but with uncertainty over the fine details of the 19 20 models. We have measured XRR for a range of SDS mole fractions from 2x10 6 up to the CMC 21 22 at 2x10 4. Broadly this data shows a trend of increasing film thickness (from ~12 to ~20Å) and 23 24 25 interfacial roughness with increasing SDS content. Although each individual XRR measurement 26 27 is of a good quality, there is some discrepancy between samples that we believe is due to the 28 29 dynamics of surfactant adsorption after pouring. This means that the trend in increasing thickness 30 31 32 with SDS content shows considerable scatter despite consistent sample preparation and 33 34 measurement procedures. As such we cannot be confident that the measured thicknesses are at 35 36 37 equilibrium. The kinetics of adsorption are likely to be highly dependent on the viscosity, 38 39 temperature and water content of the DES. We are therefore wary of over analyzing the results at 40 41 this stage. However this data does clearly demonstrate that the SDS is surface active and that the 42 43 44 thickness of the film formed is similar to that seen on water. 45 46 47 48 Summary 49 50 51 This study clearly demonstrates the existence of micelles in a pure DES and shows that there 52 53 are considerable differences between the behaviour in this DES and that seen in water. We have 54 55 used both surface tension and SANS to determine the CMC to be 2 ± 1 mmol dm 3, (X = 2 x 56 SDS 57 58 59 60 ACS Paragon Plus Environment 23 Langmuir Page 24 of 35
1 2 3 10 4 ± 1 x 10 4). Above this concentration the transition temperature of the solution increases by 4 5 6 approximately 10K and micelles are directly observed in the small angle scattering. These 7 8 micelles appear to be cylindrical rather than spherical in shape and their length increases with 9 10 11 increasing concentration. 12 21 13 As discussed earlier, Pal et al. were able to show the existence of self assembled structures in 14 15 solutions of DES containing SDS and either water or cyclohexane. Only fluorescence data is 16 17 18 given to suggest that SDS self assembly occurs in the neat DES. Our results confirm that the 19 20 observed self assembled structures are indeed larger than the equivalent structures observed in 21 22 pure water. However in our study we have added significant detail to this generic conclusion. In 23 24 25 particular we note that Pal et al were unable to fit their SAXS data from the cyclohexane/SDS 26 27 emulsions to obtain size and shape information. They specifically stated that they observed “a 28 29 single broad correlation peak, followed by a q −4 decay at higher q values.” As far as we can tell, 30 31 32 this is an erroneous interpretation of the data shown in Figure 6B of reference 21. Unusually the 33 34 logarithmic scale in this figure covers a range of q from 0.25 to 10 Å 1, so is not really within the 35 36 37 range of small angle scattering. Even accounting for a mislabeling of this axis, we believe that 38 39 the “peak” observed is in fact a result of a large beam stop that truncates the data at low q. Given 40 41 these shortcomings, we cannot make a direct comparison with our data and our results therefore 42 43 44 represent the first quantitative observation of micelles in a pure DES. 45 46 We have also shown for the first time that the SDS remains surface active at concentrations 47 48 below the CMC. XRR measurements are consistent with a layer of surfactant between 12 and 20 49 50 27, 28 51 Å thick, which is broadly consistent with the behaviour observed on water . We have also 52 53 now begun a more comprehensive study of general surfactant behaviour in DES systems, 54 55 56 57 58 59 60 ACS Paragon Plus Environment 24 Page 25 of 35 Langmuir
1 2 3 including cationic and non ionic surfactants in choline chloride / urea, as well as other DES 4 5 6 systems. 7 8 9 10 11 Supporting Information . 12 13 14 Details of SANS and XRR fit parameters can be found in the electronic supplementary 15 16 information. This material is available free of charge via the Internet at http://pubs.acs.org. 17 18 19 20 AUTHOR INFORMATION 21 22 23 Corresponding Author 24 25 * [email protected] 26 27 28 29 Present Addresses 30 31 † School of Earth Atmospheric and Environmental Sciences and Williamson Research Centre for 32 33 34 Molecular Environmental Science, University of Manchester, Oxford Road, Manchester, M13 35 36 9PL, UK. 37 38 39 Author Contributions 40 41 42 The manuscript was written with principal contributions from TA, AJJ, ASF and KJE. DM 43 44 undertook some preliminary work that enabled this study, particularly early surface tension data. 45 46 47 AET was the local contact for neutron beamtime at ISIS. All authors have given approval to the 48 49 final version of the manuscript. 50 51 52 ACKNOWLEDGMENT 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 25 Langmuir Page 26 of 35
1 2 3 We would like to thank RK Thomas & PX Li for their assistance regarding the surface tension of 4 5 6 SDS on water. We would also like to thank ISIS and Diamond Light Source for awarded 7 8 beamtime (experiment numbers RB1410460 and SI9748 respectively). We also gratefully 9 10 11 acknowledge DLS for funding DM and access to offline equipment and funding from the 12 13 University of Bath Alumni Fund and the European Spallation Source for ASF. This work 14 15 benefited from use of the SasView software, originally developed by the DANSE project under 16 17 18 NSF award DMR 0520547 19 20 21 22 23 REFERENCES 24 25 1. Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z., Micelle Formation in 26 27 28 Ethylammonium Nitrate, a Low Melting Fused Salt. J. Colloid Interface Sci. 1982, 88 (1), 89 96. 29 30 31 2. Araos, M. U.; Warr, G. G., Self Assembly of Nonionic Surfactants into Lyotropic Liquid 32 33 Crystals in Ethylammonium Nitrate, a Room Temperature Ionic Liquid. J. Phys. Chem. B 2005, 34 35 36 109 (30), 14275 14277. 37 38 39 3. Araos, M. U.; Warr, G. G., Structure of nonionic surfactant micelles in the ionic liquid 40 41 ethylammonium nitrate. Langmuir 2008, 24 (17), 9354 9360. 42 43 44 4. Chen, L. G.; Bermudez, H., Solubility and Aggregation of Charged Surfactants in Ionic 45 46 47 Liquids. Langmuir 2012, 28 (2), 1157 1162. 48 49 50 5. Greaves, T. L.; Drummond, C. J., Ionic liquids as amphiphile self assembly media. 51 52 Chemical Society Reviews 2008, 37 (8), 1709 1726. 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 26 Page 27 of 35 Langmuir
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