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water mixtures is determined by ionic liquid cation structure
Journal of Colloid and Interface Science 552 (2019) 597–603
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Regular Article
DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure Minh T. Lam a, William D. Adamson a, Shurui Miao a, Rob Atkin b, Gregory G. Warr a,⇑ a b
School of Chemistry and University of Sydney Nano Institute, The University of Sydney, NSW 2006, Australia School of Molecular Sciences, The University of Western Australia, WA 6009, Australia
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 4 April 2019 Revised 24 May 2019 Accepted 25 May 2019 Available online 25 May 2019 Keywords: Cationic surfactant Ionic liquid Counterion binding Mixed micelle Neutron scattering
a b s t r a c t Hypothesis: The high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. Experiments: CMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. Findings: In water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (G.G. Warr). https://doi.org/10.1016/j.jcis.2019.05.082 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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1. Introduction One of the most remarkable properties of ionic liquids (ILs) is their capacity to support amphiphilic self-assembly of both small-molecule surfactants and copolymers into micelles and higher order structures. Prior to the first exploration of surfactant self-assembly in the IL or ‘‘molten salt” ethylammonium nitrate (EAN) by Evans et al. [1–4] only a handful of molecular solvents were known to support such behavior. All such solvents, like water, have the capacity to form dense, three-dimensional H-bond networks necessary to generate a solvophobic effect [5]. Today, hundreds of ILs with diverse structures, many of which do not form three-dimensional H-bond networks, are known amphiphile selfassembly solvents [6]. Protic ILs, prepared by neutralization of a Brønsted acid–base pair, and exemplified by EAN, are particularly effective at promoting solvophobic self-assembly. Nevertheless, the very existence of micelles in ILs is a puzzle. While ILs have sufficiently high cohesive energy densities due to both Coulomb and H-bond interactions for a solvophobic driving force [7], their extreme ionic strength (>12 M in EAN) should alter the balance of opposing forces that stabilizes compact micelles against phase separation [8,9]. High ionic strengths should give rise to very low critical micelle concentrations (CMCs) [10] and to rod-like micelles, bilayers, or insolubility [11]. Instead, surfactants in EAN have higher CMCs than in water and form compact, globular micelles [2,3]. Many ILs are fundamentally unlike molecular solvents in that they are inherently amphiphilically nanostructured [12,13]. Nanostructured ILs contain distinct, segregated polar and apolar domains whose size and spatial disposition depend on cation and anion structure [14]. For example, increasing the alkyl chain length of the cation from ethylammonium in EAN to propylammonium nitrate (PAN) significantly increases the degree of polar/apolar segregation and non-polar volume fraction [15], whereas introducing a hydroxyl functionality to form ethanolammonium nitrate (EtAN) [16] effectively disrupts the nanostructure [17]. IL nanostructure is a determining factor for dissolution and solvation of complex organic solutes [18], the conformation of dissolved polymers [19,20], and the micellization of small surfactants [21–25] and block copolymers [26–29]. Amphiphilic solvent nanostructure also persists, with some modification, upon dilution with water [30,31] or other Hbonding solvents such as glycerol [32] up to at least equal volumes. The polar molecules are incorporated into the IL H-bond network and swell its polar domains. Miscibility with more complex, amphiphilic organic compounds depends strongly on IL nanostructure. Whereas alkanols larger than ethanol are only sparingly soluble in unstructured EtAN, complete miscibility is achieved up to n-hexanol in EAN and at least n-octanol in PAN. The liquid structure in these mixtures is a modification of the underlying IL nanostructure that accommodates the molecular amphiphilic additive, with the ammonium cation stabilizing the internal interface between polar and non-polar domains much like a surfactant monolayer in a bicontinuous microemulsion [31,33,34]. Here we examine micelle formation and structure of a representative cationic surfactant, dodecyltrimethylammonium bromide (DTAB) as a function of the composition of IL/water mixed solvents. In molecular solvent mixtures of water with polar, H-bonding cosolvents such as ethylene glycol or glycerol, the CMCs of both ionic and nonionic surfactants increase monotonically with fraction of nonaqueous cosolvent [35–37]. Aliphatic alcohols exhibit complex behavior; The CMC decreases at low additive concentrations (for all but methanol) due to a combination of effects [38], but then increases in water-miscible alcohols as the bulk solvent structure is modified [39–41]. ILs in dilute aqueous solution are
expected to behave as simple electrolytes, whose impact on micelle formation has previously been extensively studied [10,42,43]. At dilute concentrations, the counterion (anion) is expected to dominate behavior. However, as the mixed solvent becomes enriched in IL, how does cation structure and solvent nanostructure affect micelle formation? In this work, the effect of IL:water on DTAB CMCs are determined by a variety of techniques and the results compared, and micelle morphology is determined by small-angle neutron scattering. This allows three key questions to be addressed: (1). How do the CMC and micelle shape of a surfactant change with composition of mixtures of ILs and water? (2). What is the effect of IL amphiphilicity and nanostructure on that behaviour? (3). What techniques are most reliable for determining CMCs in ILs and IL/ water mixtures? 2. Experimental section 2.1. Materials and methods EAN, PAN and EtAN were synthesized following previously published procedures [44], by reacting nitric acid with the relevant akylamine. Ethylamine (Aldrich), propylamine (Adrich), ethanolamine (Merck) and nitric acid (AJAX) were used. The raw products were first dried under low pressure by rotary evaporator and then freeze drying until the water content in each ionic liquid was always less than 0.4 wt% (2% mol/mol) by Karl Fisher titration, and more commonly around 0.1 wt% (0.6% mol/mol). Each ionic liquid – water mixture as well as concentrated stock surfactant solutions was prepared by mass, and molar concentrations calculated using the density data of Allen et al. [45]. The melting point of EtAN is around 53 °C [46], although it readily forms a longlived supercooled liquid. Here only mixtures up to 80 wt% EtAN in water were investigated. Pyrene (Aldrich), sodium bromide (Scharlau Europe), sodium nitrate (Ajax Finechem), deuterium oxide (D2O) (Cambridge Isotope Laboratories), Dodecyltrimethylammonium bromide (DTAB) (Sigma-Aldrich Australia) were all used as received. Water used in this study was purified by a Millipore-Q plus water purification system (Milli-Q water, resistivity 18.2 MO cm). 2.2. CMC measurements Isothermal titration calorimetry (iTC) All iTC experiments were carried out on a TAM III calorimeter (TA Instrument, US). Sample and reference cells each containing approximately 3 g of solvent were accurately weighed (±0.0005 g) before being immersed in a 22 L thermostatic oil bath maintained at 25.0 °C, yielding a temperature stability better than ± 0.1 °C throughout the experiment. DTAB concentration was then incremented by injection (10 s) from a concentrated stock solution of DTAB while being stirred at 50 rpm and the heat flow measured against the reference cell, allowing an interval of 45 min after each injection to allow the signal to return to baseline [47]. Conductivity measurements A concentrated stock solution of DTAB above its CMC in dilute EAN added incrementally to an ionic liquid/water solvent mixture in a 20 mL jacketed beaker thermostatted at of 25.0 ± 0.1 °C [48]. Conductivity was measured using an Oaklon CON 500 benchtop meter. Pyrene Fluorescence All pyrene fluorescence experiments were conducted using an RF-5301PC Spectrofluorophotometer (Shimadzu Scientific Instruments, Inc.) with a 150 W xenon source. The excitation wavelength for pyrene was 332 nm, and emission scanned from 350–450 nm at 0.2 nm resolution yielding a
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reproducible vibronic band structure. The excitation and emission slits were set at 10 nm and 1.5 nm, respectively. A small amount of pyrene was added into methanol (10 mL) and stirred until the solution was saturated. Approximately 0.1 mL of this stock was diluted into 1 L of water. A DTAB stock solution in water was prepared and diluted to various concentrations in pyrene-containing water and water/IL mixtures, which were transferred into a 3 mL Spectrosil precision cuvette for fluorescent probing. CMCs were detected as the first point of deviation of the I/III intensity ratio of the vibronic band from its value in the solvent itself [49,50]. Small angle X-ray scattering (SAXS) measurements were performed on an Anton Paar point collimated camera (SAXSess, Graz, Austria) with a Cu Ka (0.1541 nm) sealed tube X-ray source. Scattering was collected on reusable image plates that were converted to a 2D image with a Perkin Elmer CycloneÒ Plus fluorescence reader before being averaged to create 1D data and normalised to the direct beam intensity. All spectra were run for 5 min at 25 °C in 1 mm path length, sealed quartz capillaries, under a vacuum of less than 1 mbar. The spectra were background subtracted using the solvent. The CMC is detected as the onset point of increased the low-angle scattering above solvent background. 2.3. Small angle neutron scattering SANS measurements were performed on the Quokka beamline [51] at the Australian Centre for Neutron Scattering (ACNS). DTAB samples were prepared above their CMC in hydrogenous IL-D2O solvent mixtures by mass. Isotropic scattering patterns were reduced to I(Q) using modified NIST routines [52] in IgorTM and fit to various models in SASView [53]. Details of models are provided in Supplementary Information. 3. Results and discussion 3.1. Critical micelle concentrations Fig. 1 summarises the CMC of DTAB as a function of composition in EAN/water, EtAN/water and PAN/water mixtures determined using all techniques. As ILs are added at low concentrations the CMC decreases markedly from its value in water up to a few weight percent ionic liquid. Beyond this the behavior of EtAN differs from
Fig. 1. CMC of DTAB versus composition in mixtures of water with the ILs EAN (red), EtAN (green) and PAN (blue) determined by various techniques; j conductivity, N iTC, d pyrene fluorescence, SAXS. Literature results for CMCs in pure EAN [23], PAN [54] and water [42] are also shown (r). Inset: expanded low IL concentration region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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that in either EAN or PAN (which was only examined over a limited composition range). In EAN/water mixtures, there is overall good agreement between the various techniques across the composition range yielding a CMC curve with a characteristic minimum followed by a strong increase towards the pure IL. However, each technique has its own idiosyncracies, and can only be used under a limited set of circumstances. These are discussed first. The CMC can only be determined by conductivity for modest IL ionic strengths where that the background conductivity does not swamp the signal from the surfactant. In the present systems with no extraordinary precautions this restricts its use to below 5 wt% IL. The onset of scattering by micelles and good contrast (in part due to the bromide counterion) means that SAXS is effective across a broad range of EAN/water compositions, and best in EAN-rich mixtures. This is mainly due to the relatively high CMCs obtained. At lower concentrations the onset of scattering against background is difficult to distinguish, making SAXS inaccurate for EtAN where CMCs are low. Pyrene fluorescence measurements were effective at determining CMCs across a wide solvent composition range. The I/III intensity ratio of pyrene, which is widely used as a measure of the average polarity of its microenvironment [50], decreased from 1.85 in water to 1.52 in 50/50 water/EAN, then remained constant up to pure EAN. This is consistent with pyrene sensing a less polar average solvent environment as EAN is added to water up to 50%. Beyond this the probe is essentially completely solvated by the IL due to the retention of the underlying amphiphilic nanostructure of EAN [30]. Above the CMC, the I/III ratio is 1.4, independent of solvent composition, which suggests that the pyrene is experiencing a similar microenvironment within the micelles. In EtAN/water mixtures the pyrene I/III ratio surprisingly decreases from 1.85 to 1.1 at 80/20 EtAN/water, below the I/III ratio in micelles. This means that the CMC is more difficult to detect at intermediate EtAN/water compositions by this method, as there is diminished (or at one composition in principle zero) change in the vibronic spectrum at the CMC. iTC detects a CMC by the difference between the heat of dilution of the injected stock solution when micelles are dissociated into monomers (below the CMC) or simply diluted into the sample solution without dissociation (above the CMC). In water, iTC yields a CMC of 14.7 mM and an enthalpy of micellization of 2.9 kJ mol1, which are both consistent with previous measurements [42,55,56]. Upon increasing EAN content to 10 wt% the CMC first decreases, and then increases at higher EAN content. More significantly however, the negative enthalpy of micellization increases (towards zero, less exothermic) above 10 wt% EAN to such an extent that no heat transition at the CMC was detected at all by iTC in pure EAN (DH°mic = 0) at this temperature. This trend is also seen in aqueous octyltrimethylammonium bromide solutions with added NaBr [57], but is the opposite of that seen by iTC in aqueous dodecyltrimethylammonium chloride (DTAC) solutions with added NaCl. At 298 K DTAC micellisation is endothermic, and decreases with addition of salt [58]. There is a qualitative difference between the CMC of DTAB in mixed solvents containing amphiphilic (EAN and PAN) and nonamphiphilic (EtAN) ILs examined here. At low concentrations of ILs the CMCs decrease from 0.015 M in water, but for EAN and PAN they pass through a minimum at around 5–10 wt% before increasing towards their values in the ILs (0.10 ± 0.02 M in EAN, and 0.3 M in PAN [59]). In EtAN the downward trend continues to higher IL content, and the extrapolated CMC would be much lower than in water. This is consistent with other studies showing that the CMCs of 1-alkyl-3-methylimidazolium and N-alkyl-N,N-dimethylethanolammonium chloride were 6–7 times higher in EAN, but four times smaller in EtAN, than in water [60].
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At sufficient dilution, the ILs seem to be simply acting as an added electrolyte. Fig. 2 compares the CMCs of aqueous DTAB with added ILs with those of NaBr and NaNO3 as a Corrin-Harkins plot [10,43] based on the mass-action model of micellization,
lncmc ¼
DGmic blnðcmc þ csalt Þ RT
where b is the fractional counterion association. Increasing ionic strength by addition of ionic liquid (EAN, PAN, or EtAN) or conventional salt (NaNO3 or NaBr) decreases the CMC to an identical extent up to at least 0.5 M electrolyte due to the association of bromide or nitrate counterion, yielding 0.66 fractional counterion association to DTAB micelles. At higher ionic strength the results diverge. For NaNO3 and NaBr the CMC continues to decrease up to their solubility limits. However the CMCs of EAN/water mixtures go through a minimum and then begin to increase. CMCs in PAN/water mixtures (determined by conductivity) follow the same trend. CMCs in EtAN/water mixtures continue to decrease up to much higher IL content due to its higher solubility before going through a weak minimum, and remain far below even the pure water CMC at the highest concentration measured (80 wt% EtAN, cf. Fig. 1). The amphiphilic cation – the co-ion for DTAB – begins to impact on micelle formation of DTAB in these IL/water mixtures at concentrations as low as 5 wt%. Although the underlying nanostructure of EAN is known to remain upon dilution in water to at least 50 wt% [30,61], it is unlikely to persist to such high dilution. Rather, the increase in CMC observed in EAN and PAN is attributed to the incorporation of the amphiphilic IL cation into the micelle. This was originally proposed by Evans et al. [3] to explain small micelle sizes of alkylpyridinium salts in EAN, and we have recently reported similar phenomena in catanionic surfactant systems in EAN [62]. 3.2. Micelle morphology Fig. 3 shows SANS patterns from 0.09 M solutions of DTAB micelles in water with added NaBr, NaNO3, EAN and EtAN. Lines show best fits to core-shell ellipsoids or cylinders, and include inter-micelle interactions through a screened Coulomb interaction [63], at low electrolyte concentrations. Key best-fit micelle dimensions are shown in Table 1, with full fitting data shown in Supplementary Information.
Fig. 2. Mass-action representation of DTAB micellization in water and various ILs and aqueous salt solutions, log(CMC) vs log(CMC + Csalt). Data from all techniques is combined with literature for DTAB + NaBr or NaNO3 and DTANO3 + NaNO3.[10,43].
In pure water a best-fit is obtained for a model of slightly oblate micelles, with axial and equatorial radii of 14.2 and 22.5 Å, respectively. As has been shown previously, discrimination between slightly eccentric spheroids or polydisperse spheres is difficult on the basis of goodness-of-fit alone; [64,65]. A slightly worse bestfit to a prolate spheroid yields axial and equatorial radii of 26.4 and 16.6 Å, respectively. Both are plausible structures, with each having one dimension less than the expected fully-extended dodecyl chain length of 16.7 Å. For the following discussion we will characterize such micelles, with radius ratios in the range 0.5–2.4 as ‘‘globular.” Addition of NaBr induces a globule-to-rod transition and micelle elongation near 1 M. This can be seen in the increase in scattered intensity at low wave-vector, Q [11,65], yielding scattering patterns that cannot be fit by either sphere or oblate spheroid micelle models above 0.1 M. Best fits correspond first to highly eccentric prolate spheroids, then polydisperse cylinders above 1 M. DTAB with added NaNO3 behaves similarly, but the transition starts at higher concentration after which micelle elongation is more pronounced, and continues to higher concentration due to the higher solubility of the salt. This is probably due to competition between bromide and nitrate ions [66] at the micelle surface affecting micelle growth horizons [67]. Strikingly, there is no significant structure change in DTAB micelles when EAN is added. At all EAN-water mixture compositions, micelles remain globular at all compositions. The reduced intensity relative to background arises from the lower deuterium content in the IL-rich mixtures. The best-fit micelle shape at low EAN concentrations is at first oblate, then transitioning to prolate at 1 M, but above 1 M it is not possible to distinguish between oblate and prolate spheroids with axial ratios in the range 0.55– 1.8, or essentially globular. This is also consistent with previous studies of DTAB and other cationic surfactant micelle structure in pure EAN, which are consistently found to be globular, and smaller than in water [3,23,24,62]. In EtAN-water mixtures there is a clear transition from globular (oblate) to highly eccentric prolate or short cylindrical micelles with an axial ratio somewhat > 2 at around 1 M electrolyte, but never proceeds to form longer rods. At higher EtAN content the micelles instead return to being more globular. The evolution of micelle shape with composition can be understood alongside the CMC behavior. At low electrolyte or IL concentrations in water, the counterion is the dominant influence in all cases. It reduces the CMC by altering the binding equilibrium to the micelles, which screens repulsions between cationic headgroups within the micelle. This lowers the area per molecule and increases the surfactant packing parameter, v/a0lc [9], which favours a less-curved, elongated structure – a globule-cylinder or sphere-rod transition. At higher concentrations the co-ion comes into play to different extents. Whereas sodium is a simple, strongly-hydrated cation that has little impact, ethylammonium is an amphiphilic cation that can be incorporated into micelles. In addition to the bulk amphiphilic nanostructure of EAN itself [17,68], it is known to act as a stand-alone surfactant and to also stabilise bicontinuous structures of cosurfactants like alkanols [34]. This gives rise to the minimum in the CMC, as association of the alkylammonium cation is akin to the formation of mixed cationic micelles of DTAB with a less hydrophobic species. Incorporation of such a small, amphiphilic co-ion opposes the screening of head-group repulsion that underlies the globule-to-cylinder transition. Although EtAN does not itself exhibit bulk amphiphilic nanostructure, previous studies of the air-IL interface have revealed that it preferentially orients itself with cation methylene groups towards the non-polar phase [69]. Thus, like conventional amphiphilic IL cations EAN and PAN, which also preferentially orient their alkyl moieties towards air [70,71], EtAN is thus able to act
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Fig. 3. SANS patterns of 0.09 M DTAB in D2O with added (a) NaNO3, (b) NaBr, (c) EAN, (d) EtAN, at various concentrations.
Table 1 Dimensions of hydrocarbon cores of micelles of 0.09 M DTAB in water/salt and water/IL solutions obtained from best-fit to either core-shell spheroids or coreshell cylinders. Grey shading denotes elongated micelles with an axial ratio exceeding 2.4, and dark grey denotes fits to core-shell cylinders rather than spheroids.
Salt Conc. (M) 0 0.1 0.5 1.0 2.0a 4.0b 6.0c 7.5d a
Core Equatorial/Axial radii or cylinder half-length (Å) NaNO3 NaBr EAN EtAN 22.5/14.2 23.9/14.5 14.0/14.7 24.6/14.6 24.2/14.6 23.6/14.5 16.7/32.6 24.6/14.4 24.9/14.0 16.7/33.8 16.7/44.6 16.7/36.1 16.7/41.3 16.7/60.0 15.4/29.9 24.7/14.0 16.7/45.2 16.1/68.0 16.4/49.5 21.0/14.2 16.7/42.0 16.3/79.9 16.5/61.6 18.7/12.6 16.7/34.0 16.3/114 insoluble 16.5/11.4 16.0/36.1
2.1 M in EtAN. 4.1 M in EtAN. 6.1 M in EAN and EtAN. d 7.6, 7.7 M in EAN, EtAN, respectively. b c
as a small bolaform cosurfactant or hydrotrope at the micelle surface and to some extent inhibit the globule-to-cylinder transition. This is also reflected in its impact on the CMC of DTAB. Although the CMC in concentrated (or even pure) EtAN is lower than in water, there is still an identifiable minimum (Fig. 2) in the CMC as a function of solvent composition.
4. Conclusions Although consistent values for CMCs in ILs and their mixtures were determined by multiple techniques, no single technique was found to be optimal. Pyrene fluorescence and iTC were the most versatile, but both were unable to detect CMCs over certain (different) solvent composition ranges due to vanishing signals. Conductivity is only applicable at high water content, and SAXS requires a sufficiently high CMC that a reliable sub-CMC baseline can be determined.
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The evolution of CMC and micelle shape of DTAB with composition of IL-water mixed solvents can be broken into three broad domains. At low IL or salt concentrations (<2 wt%, or approx. 0.2 M), counterion binding is the dominant effect. This lowers the CMC by reducing the electrostatic repulsion between charged surfactant head groups. This lowers the area per molecule at the micelle surface and increases the surfactant packing parameter, enabling a sphere-rod transition. At intermediate IL concentrations (2–10 wt%, or approx. 0.2 – 1 M), the structure and particularly the amphiphilicity of the cation comes into play. Although not themselves able to form micelles in aqueous solution, the short alkyl tails on amphiphilic cations such as ethyl- or propyl-ammonium enables their association with cationic DTAB micelles. This co-ion uptake offsets the effect of counterion binding on head-group repulsions and both opposes further reduction in CMC and interrupts the sphere-rod transition. Ethanolammonium is less effective than either alkylammonium examined. At still higher concentrations of IL (>10 wt%, or approx. 1 M), its effect as a component of the solvent becomes dominant. Even though the ionic strengths and degrees of counterion binding are similar in EAN, PAN and EtAN, the CMCs and hence the free energies of micellization have diverged. In this region the long alkyl chains of DTAB experience an average solvent structure very different from water, which corresponds to a solvophobic driving force for micellization that is different in water and between different ILs [21]. In the ILs examined here, amphiphilic nanostructure in EAN and PAN enhances alkyl chain solubility [33] (chemical potential), raising the CMC more than the ionic strength effect decreases it. In EtAN-rich mixtures, the CMC is much lower than in water primarily due to this lack of this nanostructure [17]. Although not a direct consequence of IL nanostructure, the stabilisation of globular micelles in ILs and IL-water mixtures has the same origin: amphiphilicity of the cation. Changing IL cation or anion structure thus enables CMC and micelle morphology in these solvents to be controlled independently. This will have broad implications for performance of various complex fluids, from solubilization capacity to rheology [72]. We would also expect the effect on anionic surfactants in IL-based solvents to depend differently on cation type [62]. Acknowledgments This work was supported by funding from the Australian Research Council. The authors acknowledge the support of ANSTO in providing the neutron beamline facilities used in this work, and also thank Dr Paul FitzGerald and Dr Kathleen Wood for assistance with the SANS experiments, and Dr R.J. Clarke for the use of his spectrofluorimeter. This work benefited from the use of the SasView application, originally developed under NSF award DMR0520547. SasView contains code developed with funding from the European Union’s Horizon 2020 research and innovation programme under the SINE2020 project, grant agreement No 654000. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.082. References [1] D.F. Evans, E.W. Kaler, W.J. Benton, Liquid crystals in a fused salt: b,cdistearoylphosphatidylcholine in N-ethylammonium nitrate, J. Phys. Chem. 87 (4) (1983) 533–535.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure Minh T. Lam a, William D. Adamson a, Shurui Miao a, Rob Atkin b, Gregory G. Warr a,⇑ a b
School of Chemistry and University of Sydney Nano Institute, The University of Sydney, NSW 2006, Australia School of Molecular Sciences, The University of Western Australia, WA 6009, Australia
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 4 April 2019 Revised 24 May 2019 Accepted 25 May 2019 Available online 25 May 2019 Keywords: Cationic surfactant Ionic liquid Counterion binding Mixed micelle Neutron scattering
a b s t r a c t Hypothesis: The high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. Experiments: CMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. Findings: In water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (G.G. Warr). https://doi.org/10.1016/j.jcis.2019.05.082 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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1. Introduction One of the most remarkable properties of ionic liquids (ILs) is their capacity to support amphiphilic self-assembly of both small-molecule surfactants and copolymers into micelles and higher order structures. Prior to the first exploration of surfactant self-assembly in the IL or ‘‘molten salt” ethylammonium nitrate (EAN) by Evans et al. [1–4] only a handful of molecular solvents were known to support such behavior. All such solvents, like water, have the capacity to form dense, three-dimensional H-bond networks necessary to generate a solvophobic effect [5]. Today, hundreds of ILs with diverse structures, many of which do not form three-dimensional H-bond networks, are known amphiphile selfassembly solvents [6]. Protic ILs, prepared by neutralization of a Brønsted acid–base pair, and exemplified by EAN, are particularly effective at promoting solvophobic self-assembly. Nevertheless, the very existence of micelles in ILs is a puzzle. While ILs have sufficiently high cohesive energy densities due to both Coulomb and H-bond interactions for a solvophobic driving force [7], their extreme ionic strength (>12 M in EAN) should alter the balance of opposing forces that stabilizes compact micelles against phase separation [8,9]. High ionic strengths should give rise to very low critical micelle concentrations (CMCs) [10] and to rod-like micelles, bilayers, or insolubility [11]. Instead, surfactants in EAN have higher CMCs than in water and form compact, globular micelles [2,3]. Many ILs are fundamentally unlike molecular solvents in that they are inherently amphiphilically nanostructured [12,13]. Nanostructured ILs contain distinct, segregated polar and apolar domains whose size and spatial disposition depend on cation and anion structure [14]. For example, increasing the alkyl chain length of the cation from ethylammonium in EAN to propylammonium nitrate (PAN) significantly increases the degree of polar/apolar segregation and non-polar volume fraction [15], whereas introducing a hydroxyl functionality to form ethanolammonium nitrate (EtAN) [16] effectively disrupts the nanostructure [17]. IL nanostructure is a determining factor for dissolution and solvation of complex organic solutes [18], the conformation of dissolved polymers [19,20], and the micellization of small surfactants [21–25] and block copolymers [26–29]. Amphiphilic solvent nanostructure also persists, with some modification, upon dilution with water [30,31] or other Hbonding solvents such as glycerol [32] up to at least equal volumes. The polar molecules are incorporated into the IL H-bond network and swell its polar domains. Miscibility with more complex, amphiphilic organic compounds depends strongly on IL nanostructure. Whereas alkanols larger than ethanol are only sparingly soluble in unstructured EtAN, complete miscibility is achieved up to n-hexanol in EAN and at least n-octanol in PAN. The liquid structure in these mixtures is a modification of the underlying IL nanostructure that accommodates the molecular amphiphilic additive, with the ammonium cation stabilizing the internal interface between polar and non-polar domains much like a surfactant monolayer in a bicontinuous microemulsion [31,33,34]. Here we examine micelle formation and structure of a representative cationic surfactant, dodecyltrimethylammonium bromide (DTAB) as a function of the composition of IL/water mixed solvents. In molecular solvent mixtures of water with polar, H-bonding cosolvents such as ethylene glycol or glycerol, the CMCs of both ionic and nonionic surfactants increase monotonically with fraction of nonaqueous cosolvent [35–37]. Aliphatic alcohols exhibit complex behavior; The CMC decreases at low additive concentrations (for all but methanol) due to a combination of effects [38], but then increases in water-miscible alcohols as the bulk solvent structure is modified [39–41]. ILs in dilute aqueous solution are
expected to behave as simple electrolytes, whose impact on micelle formation has previously been extensively studied [10,42,43]. At dilute concentrations, the counterion (anion) is expected to dominate behavior. However, as the mixed solvent becomes enriched in IL, how does cation structure and solvent nanostructure affect micelle formation? In this work, the effect of IL:water on DTAB CMCs are determined by a variety of techniques and the results compared, and micelle morphology is determined by small-angle neutron scattering. This allows three key questions to be addressed: (1). How do the CMC and micelle shape of a surfactant change with composition of mixtures of ILs and water? (2). What is the effect of IL amphiphilicity and nanostructure on that behaviour? (3). What techniques are most reliable for determining CMCs in ILs and IL/ water mixtures? 2. Experimental section 2.1. Materials and methods EAN, PAN and EtAN were synthesized following previously published procedures [44], by reacting nitric acid with the relevant akylamine. Ethylamine (Aldrich), propylamine (Adrich), ethanolamine (Merck) and nitric acid (AJAX) were used. The raw products were first dried under low pressure by rotary evaporator and then freeze drying until the water content in each ionic liquid was always less than 0.4 wt% (2% mol/mol) by Karl Fisher titration, and more commonly around 0.1 wt% (0.6% mol/mol). Each ionic liquid – water mixture as well as concentrated stock surfactant solutions was prepared by mass, and molar concentrations calculated using the density data of Allen et al. [45]. The melting point of EtAN is around 53 °C [46], although it readily forms a longlived supercooled liquid. Here only mixtures up to 80 wt% EtAN in water were investigated. Pyrene (Aldrich), sodium bromide (Scharlau Europe), sodium nitrate (Ajax Finechem), deuterium oxide (D2O) (Cambridge Isotope Laboratories), Dodecyltrimethylammonium bromide (DTAB) (Sigma-Aldrich Australia) were all used as received. Water used in this study was purified by a Millipore-Q plus water purification system (Milli-Q water, resistivity 18.2 MO cm). 2.2. CMC measurements Isothermal titration calorimetry (iTC) All iTC experiments were carried out on a TAM III calorimeter (TA Instrument, US). Sample and reference cells each containing approximately 3 g of solvent were accurately weighed (±0.0005 g) before being immersed in a 22 L thermostatic oil bath maintained at 25.0 °C, yielding a temperature stability better than ± 0.1 °C throughout the experiment. DTAB concentration was then incremented by injection (10 s) from a concentrated stock solution of DTAB while being stirred at 50 rpm and the heat flow measured against the reference cell, allowing an interval of 45 min after each injection to allow the signal to return to baseline [47]. Conductivity measurements A concentrated stock solution of DTAB above its CMC in dilute EAN added incrementally to an ionic liquid/water solvent mixture in a 20 mL jacketed beaker thermostatted at of 25.0 ± 0.1 °C [48]. Conductivity was measured using an Oaklon CON 500 benchtop meter. Pyrene Fluorescence All pyrene fluorescence experiments were conducted using an RF-5301PC Spectrofluorophotometer (Shimadzu Scientific Instruments, Inc.) with a 150 W xenon source. The excitation wavelength for pyrene was 332 nm, and emission scanned from 350–450 nm at 0.2 nm resolution yielding a
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reproducible vibronic band structure. The excitation and emission slits were set at 10 nm and 1.5 nm, respectively. A small amount of pyrene was added into methanol (10 mL) and stirred until the solution was saturated. Approximately 0.1 mL of this stock was diluted into 1 L of water. A DTAB stock solution in water was prepared and diluted to various concentrations in pyrene-containing water and water/IL mixtures, which were transferred into a 3 mL Spectrosil precision cuvette for fluorescent probing. CMCs were detected as the first point of deviation of the I/III intensity ratio of the vibronic band from its value in the solvent itself [49,50]. Small angle X-ray scattering (SAXS) measurements were performed on an Anton Paar point collimated camera (SAXSess, Graz, Austria) with a Cu Ka (0.1541 nm) sealed tube X-ray source. Scattering was collected on reusable image plates that were converted to a 2D image with a Perkin Elmer CycloneÒ Plus fluorescence reader before being averaged to create 1D data and normalised to the direct beam intensity. All spectra were run for 5 min at 25 °C in 1 mm path length, sealed quartz capillaries, under a vacuum of less than 1 mbar. The spectra were background subtracted using the solvent. The CMC is detected as the onset point of increased the low-angle scattering above solvent background. 2.3. Small angle neutron scattering SANS measurements were performed on the Quokka beamline [51] at the Australian Centre for Neutron Scattering (ACNS). DTAB samples were prepared above their CMC in hydrogenous IL-D2O solvent mixtures by mass. Isotropic scattering patterns were reduced to I(Q) using modified NIST routines [52] in IgorTM and fit to various models in SASView [53]. Details of models are provided in Supplementary Information. 3. Results and discussion 3.1. Critical micelle concentrations Fig. 1 summarises the CMC of DTAB as a function of composition in EAN/water, EtAN/water and PAN/water mixtures determined using all techniques. As ILs are added at low concentrations the CMC decreases markedly from its value in water up to a few weight percent ionic liquid. Beyond this the behavior of EtAN differs from
Fig. 1. CMC of DTAB versus composition in mixtures of water with the ILs EAN (red), EtAN (green) and PAN (blue) determined by various techniques; j conductivity, N iTC, d pyrene fluorescence, SAXS. Literature results for CMCs in pure EAN [23], PAN [54] and water [42] are also shown (r). Inset: expanded low IL concentration region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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that in either EAN or PAN (which was only examined over a limited composition range). In EAN/water mixtures, there is overall good agreement between the various techniques across the composition range yielding a CMC curve with a characteristic minimum followed by a strong increase towards the pure IL. However, each technique has its own idiosyncracies, and can only be used under a limited set of circumstances. These are discussed first. The CMC can only be determined by conductivity for modest IL ionic strengths where that the background conductivity does not swamp the signal from the surfactant. In the present systems with no extraordinary precautions this restricts its use to below 5 wt% IL. The onset of scattering by micelles and good contrast (in part due to the bromide counterion) means that SAXS is effective across a broad range of EAN/water compositions, and best in EAN-rich mixtures. This is mainly due to the relatively high CMCs obtained. At lower concentrations the onset of scattering against background is difficult to distinguish, making SAXS inaccurate for EtAN where CMCs are low. Pyrene fluorescence measurements were effective at determining CMCs across a wide solvent composition range. The I/III intensity ratio of pyrene, which is widely used as a measure of the average polarity of its microenvironment [50], decreased from 1.85 in water to 1.52 in 50/50 water/EAN, then remained constant up to pure EAN. This is consistent with pyrene sensing a less polar average solvent environment as EAN is added to water up to 50%. Beyond this the probe is essentially completely solvated by the IL due to the retention of the underlying amphiphilic nanostructure of EAN [30]. Above the CMC, the I/III ratio is 1.4, independent of solvent composition, which suggests that the pyrene is experiencing a similar microenvironment within the micelles. In EtAN/water mixtures the pyrene I/III ratio surprisingly decreases from 1.85 to 1.1 at 80/20 EtAN/water, below the I/III ratio in micelles. This means that the CMC is more difficult to detect at intermediate EtAN/water compositions by this method, as there is diminished (or at one composition in principle zero) change in the vibronic spectrum at the CMC. iTC detects a CMC by the difference between the heat of dilution of the injected stock solution when micelles are dissociated into monomers (below the CMC) or simply diluted into the sample solution without dissociation (above the CMC). In water, iTC yields a CMC of 14.7 mM and an enthalpy of micellization of 2.9 kJ mol1, which are both consistent with previous measurements [42,55,56]. Upon increasing EAN content to 10 wt% the CMC first decreases, and then increases at higher EAN content. More significantly however, the negative enthalpy of micellization increases (towards zero, less exothermic) above 10 wt% EAN to such an extent that no heat transition at the CMC was detected at all by iTC in pure EAN (DH°mic = 0) at this temperature. This trend is also seen in aqueous octyltrimethylammonium bromide solutions with added NaBr [57], but is the opposite of that seen by iTC in aqueous dodecyltrimethylammonium chloride (DTAC) solutions with added NaCl. At 298 K DTAC micellisation is endothermic, and decreases with addition of salt [58]. There is a qualitative difference between the CMC of DTAB in mixed solvents containing amphiphilic (EAN and PAN) and nonamphiphilic (EtAN) ILs examined here. At low concentrations of ILs the CMCs decrease from 0.015 M in water, but for EAN and PAN they pass through a minimum at around 5–10 wt% before increasing towards their values in the ILs (0.10 ± 0.02 M in EAN, and 0.3 M in PAN [59]). In EtAN the downward trend continues to higher IL content, and the extrapolated CMC would be much lower than in water. This is consistent with other studies showing that the CMCs of 1-alkyl-3-methylimidazolium and N-alkyl-N,N-dimethylethanolammonium chloride were 6–7 times higher in EAN, but four times smaller in EtAN, than in water [60].
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At sufficient dilution, the ILs seem to be simply acting as an added electrolyte. Fig. 2 compares the CMCs of aqueous DTAB with added ILs with those of NaBr and NaNO3 as a Corrin-Harkins plot [10,43] based on the mass-action model of micellization,
lncmc ¼
DGmic blnðcmc þ csalt Þ RT
where b is the fractional counterion association. Increasing ionic strength by addition of ionic liquid (EAN, PAN, or EtAN) or conventional salt (NaNO3 or NaBr) decreases the CMC to an identical extent up to at least 0.5 M electrolyte due to the association of bromide or nitrate counterion, yielding 0.66 fractional counterion association to DTAB micelles. At higher ionic strength the results diverge. For NaNO3 and NaBr the CMC continues to decrease up to their solubility limits. However the CMCs of EAN/water mixtures go through a minimum and then begin to increase. CMCs in PAN/water mixtures (determined by conductivity) follow the same trend. CMCs in EtAN/water mixtures continue to decrease up to much higher IL content due to its higher solubility before going through a weak minimum, and remain far below even the pure water CMC at the highest concentration measured (80 wt% EtAN, cf. Fig. 1). The amphiphilic cation – the co-ion for DTAB – begins to impact on micelle formation of DTAB in these IL/water mixtures at concentrations as low as 5 wt%. Although the underlying nanostructure of EAN is known to remain upon dilution in water to at least 50 wt% [30,61], it is unlikely to persist to such high dilution. Rather, the increase in CMC observed in EAN and PAN is attributed to the incorporation of the amphiphilic IL cation into the micelle. This was originally proposed by Evans et al. [3] to explain small micelle sizes of alkylpyridinium salts in EAN, and we have recently reported similar phenomena in catanionic surfactant systems in EAN [62]. 3.2. Micelle morphology Fig. 3 shows SANS patterns from 0.09 M solutions of DTAB micelles in water with added NaBr, NaNO3, EAN and EtAN. Lines show best fits to core-shell ellipsoids or cylinders, and include inter-micelle interactions through a screened Coulomb interaction [63], at low electrolyte concentrations. Key best-fit micelle dimensions are shown in Table 1, with full fitting data shown in Supplementary Information.
Fig. 2. Mass-action representation of DTAB micellization in water and various ILs and aqueous salt solutions, log(CMC) vs log(CMC + Csalt). Data from all techniques is combined with literature for DTAB + NaBr or NaNO3 and DTANO3 + NaNO3.[10,43].
In pure water a best-fit is obtained for a model of slightly oblate micelles, with axial and equatorial radii of 14.2 and 22.5 Å, respectively. As has been shown previously, discrimination between slightly eccentric spheroids or polydisperse spheres is difficult on the basis of goodness-of-fit alone; [64,65]. A slightly worse bestfit to a prolate spheroid yields axial and equatorial radii of 26.4 and 16.6 Å, respectively. Both are plausible structures, with each having one dimension less than the expected fully-extended dodecyl chain length of 16.7 Å. For the following discussion we will characterize such micelles, with radius ratios in the range 0.5–2.4 as ‘‘globular.” Addition of NaBr induces a globule-to-rod transition and micelle elongation near 1 M. This can be seen in the increase in scattered intensity at low wave-vector, Q [11,65], yielding scattering patterns that cannot be fit by either sphere or oblate spheroid micelle models above 0.1 M. Best fits correspond first to highly eccentric prolate spheroids, then polydisperse cylinders above 1 M. DTAB with added NaNO3 behaves similarly, but the transition starts at higher concentration after which micelle elongation is more pronounced, and continues to higher concentration due to the higher solubility of the salt. This is probably due to competition between bromide and nitrate ions [66] at the micelle surface affecting micelle growth horizons [67]. Strikingly, there is no significant structure change in DTAB micelles when EAN is added. At all EAN-water mixture compositions, micelles remain globular at all compositions. The reduced intensity relative to background arises from the lower deuterium content in the IL-rich mixtures. The best-fit micelle shape at low EAN concentrations is at first oblate, then transitioning to prolate at 1 M, but above 1 M it is not possible to distinguish between oblate and prolate spheroids with axial ratios in the range 0.55– 1.8, or essentially globular. This is also consistent with previous studies of DTAB and other cationic surfactant micelle structure in pure EAN, which are consistently found to be globular, and smaller than in water [3,23,24,62]. In EtAN-water mixtures there is a clear transition from globular (oblate) to highly eccentric prolate or short cylindrical micelles with an axial ratio somewhat > 2 at around 1 M electrolyte, but never proceeds to form longer rods. At higher EtAN content the micelles instead return to being more globular. The evolution of micelle shape with composition can be understood alongside the CMC behavior. At low electrolyte or IL concentrations in water, the counterion is the dominant influence in all cases. It reduces the CMC by altering the binding equilibrium to the micelles, which screens repulsions between cationic headgroups within the micelle. This lowers the area per molecule and increases the surfactant packing parameter, v/a0lc [9], which favours a less-curved, elongated structure – a globule-cylinder or sphere-rod transition. At higher concentrations the co-ion comes into play to different extents. Whereas sodium is a simple, strongly-hydrated cation that has little impact, ethylammonium is an amphiphilic cation that can be incorporated into micelles. In addition to the bulk amphiphilic nanostructure of EAN itself [17,68], it is known to act as a stand-alone surfactant and to also stabilise bicontinuous structures of cosurfactants like alkanols [34]. This gives rise to the minimum in the CMC, as association of the alkylammonium cation is akin to the formation of mixed cationic micelles of DTAB with a less hydrophobic species. Incorporation of such a small, amphiphilic co-ion opposes the screening of head-group repulsion that underlies the globule-to-cylinder transition. Although EtAN does not itself exhibit bulk amphiphilic nanostructure, previous studies of the air-IL interface have revealed that it preferentially orients itself with cation methylene groups towards the non-polar phase [69]. Thus, like conventional amphiphilic IL cations EAN and PAN, which also preferentially orient their alkyl moieties towards air [70,71], EtAN is thus able to act
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Fig. 3. SANS patterns of 0.09 M DTAB in D2O with added (a) NaNO3, (b) NaBr, (c) EAN, (d) EtAN, at various concentrations.
Table 1 Dimensions of hydrocarbon cores of micelles of 0.09 M DTAB in water/salt and water/IL solutions obtained from best-fit to either core-shell spheroids or coreshell cylinders. Grey shading denotes elongated micelles with an axial ratio exceeding 2.4, and dark grey denotes fits to core-shell cylinders rather than spheroids.
Salt Conc. (M) 0 0.1 0.5 1.0 2.0a 4.0b 6.0c 7.5d a
Core Equatorial/Axial radii or cylinder half-length (Å) NaNO3 NaBr EAN EtAN 22.5/14.2 23.9/14.5 14.0/14.7 24.6/14.6 24.2/14.6 23.6/14.5 16.7/32.6 24.6/14.4 24.9/14.0 16.7/33.8 16.7/44.6 16.7/36.1 16.7/41.3 16.7/60.0 15.4/29.9 24.7/14.0 16.7/45.2 16.1/68.0 16.4/49.5 21.0/14.2 16.7/42.0 16.3/79.9 16.5/61.6 18.7/12.6 16.7/34.0 16.3/114 insoluble 16.5/11.4 16.0/36.1
2.1 M in EtAN. 4.1 M in EtAN. 6.1 M in EAN and EtAN. d 7.6, 7.7 M in EAN, EtAN, respectively. b c
as a small bolaform cosurfactant or hydrotrope at the micelle surface and to some extent inhibit the globule-to-cylinder transition. This is also reflected in its impact on the CMC of DTAB. Although the CMC in concentrated (or even pure) EtAN is lower than in water, there is still an identifiable minimum (Fig. 2) in the CMC as a function of solvent composition.
4. Conclusions Although consistent values for CMCs in ILs and their mixtures were determined by multiple techniques, no single technique was found to be optimal. Pyrene fluorescence and iTC were the most versatile, but both were unable to detect CMCs over certain (different) solvent composition ranges due to vanishing signals. Conductivity is only applicable at high water content, and SAXS requires a sufficiently high CMC that a reliable sub-CMC baseline can be determined.
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The evolution of CMC and micelle shape of DTAB with composition of IL-water mixed solvents can be broken into three broad domains. At low IL or salt concentrations (<2 wt%, or approx. 0.2 M), counterion binding is the dominant effect. This lowers the CMC by reducing the electrostatic repulsion between charged surfactant head groups. This lowers the area per molecule at the micelle surface and increases the surfactant packing parameter, enabling a sphere-rod transition. At intermediate IL concentrations (2–10 wt%, or approx. 0.2 – 1 M), the structure and particularly the amphiphilicity of the cation comes into play. Although not themselves able to form micelles in aqueous solution, the short alkyl tails on amphiphilic cations such as ethyl- or propyl-ammonium enables their association with cationic DTAB micelles. This co-ion uptake offsets the effect of counterion binding on head-group repulsions and both opposes further reduction in CMC and interrupts the sphere-rod transition. Ethanolammonium is less effective than either alkylammonium examined. At still higher concentrations of IL (>10 wt%, or approx. 1 M), its effect as a component of the solvent becomes dominant. Even though the ionic strengths and degrees of counterion binding are similar in EAN, PAN and EtAN, the CMCs and hence the free energies of micellization have diverged. In this region the long alkyl chains of DTAB experience an average solvent structure very different from water, which corresponds to a solvophobic driving force for micellization that is different in water and between different ILs [21]. In the ILs examined here, amphiphilic nanostructure in EAN and PAN enhances alkyl chain solubility [33] (chemical potential), raising the CMC more than the ionic strength effect decreases it. In EtAN-rich mixtures, the CMC is much lower than in water primarily due to this lack of this nanostructure [17]. Although not a direct consequence of IL nanostructure, the stabilisation of globular micelles in ILs and IL-water mixtures has the same origin: amphiphilicity of the cation. Changing IL cation or anion structure thus enables CMC and micelle morphology in these solvents to be controlled independently. This will have broad implications for performance of various complex fluids, from solubilization capacity to rheology [72]. We would also expect the effect on anionic surfactants in IL-based solvents to depend differently on cation type [62]. Acknowledgments This work was supported by funding from the Australian Research Council. The authors acknowledge the support of ANSTO in providing the neutron beamline facilities used in this work, and also thank Dr Paul FitzGerald and Dr Kathleen Wood for assistance with the SANS experiments, and Dr R.J. Clarke for the use of his spectrofluorimeter. This work benefited from the use of the SasView application, originally developed under NSF award DMR0520547. SasView contains code developed with funding from the European Union’s Horizon 2020 research and innovation programme under the SINE2020 project, grant agreement No 654000. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.082. References [1] D.F. Evans, E.W. Kaler, W.J. Benton, Liquid crystals in a fused salt: b,cdistearoylphosphatidylcholine in N-ethylammonium nitrate, J. Phys. Chem. 87 (4) (1983) 533–535.
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