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Aggregation Behavior of Aqueous Solutions of Ionic Liquids James Bowers,* Craig P. Butts, Pamela J. Martin, and Marcos C. Vergara-Gutierrez Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom
Richard K. Heenan ISIS Facility, CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom Received October 17, 2003. In Final Form: November 21, 2003 The aggregation behavior in aqueous solutions of three ionic liquids based on the 1-alkyl-3methylimidazolium cation has been investigated by means of surface tension, conductivity, and smallangle neutron scattering (SANS) measurements. From analysis of the SANS data, models for the shapes and sizes of aggregates have been proposed: the short-chain 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] system can be best modeled by treating it as a dispersion of polydisperse spherical aggregates that form above a critical aggregation concentration, whereas the 1-octyl-3-methylimidazolium iodide, [C8mim][I], solutions can be modeled as a system of regularly sized near-spherical charged micelles that form above a critical micelle concentration. Solutions of 1-octyl-3-methylimidazolium chloride, [C8mim][Cl], display weak long-range ordering of possibly disklike particles culminating in the formation of structures with distinct long-range order at higher concentrations.
Introduction As a working definition, an ionic liquid (IL) is a salt composed of an organic cation and an inorganic anion that is molten when the temperature is at, or close to, room temperature. The convenient and wide liquid range is just one of many favorable chemical and physical properties of ILs that have driven their rapid development as solvents for a broad range of synthetic applications.1-3 In recent years, ILs based on the 1-alkyl-3-methylimidazolium cation [Cnmim] have received much attention.4 An interesting aspect of such ILs is that the [Cnmim] cations possess an inherent amphiphilicity. It can, therefore, be anticipated that interfacial and aggregation behavior analogous to that exhibited by short-chain cationic surfactants may be displayed by these ILs. Longerchain ILs self-assemble to form thermotropic liquid crystalline mesophase ILs,5-7 and lyotropic mesophases in concentrated aqueous solutions of 1-decyl-3-methylimidazolium bromide,8 an IL with a moderate alkyl chain length, have been reported. Common to many IL-based applications is the inclusion of the IL as a major or minor component of a mixed system, and, accordingly, establishing the structural and thermodynamic properties of such mixed systems is desirable. * Corresponding author. (1) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T. Eds.; Wiley: New York, 2003. (2) Welton, T. Chem. Rev. 1999, 99, 2071. (3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (4) Wilkes, J. S. Green Chem. 2002, 4, 73. (5) Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1996, 1625. (6) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc.: Dalton Trans. 1999, 2133. (7) Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McGrath, S. E. J.; Nieuwenhuyzen, M. Chem. Mater. 2002, 14, 629. (8) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258.
It may be expected that the amphiphilicity of the [Cnmim]-based ILs signifies that interfacial phenomena will play a major role in the behavior of such IL-containing systems. The ability to form self-assembled structures may have consequences in a number of areas such as the extraction of products from IL-containing systems, the synthesis and purification of bulk ILs, the solvation properties of the IL molecules by simple solutes, and the formation of dispersed or phase-separated systems. Some of these points have a bearing on the chemical reactivity of the systems and have also generated interest in the theoretical aspects of the solvation of ILs.9 The role such structures may play in the synthetic chemistry that can be performed in IL-containing solvents remains to be investigated. The interest in ILs is driving a rapid increase in the mapping of the physical properties of these liquids and their mixtures with other common solvents. For example, because ILs are generally hygroscopic, the reactivity in IL media is dependent on the water content and, thus, determining the solvent properties of IL + water systems is relevant. The solvent properties of ILs with small to moderate water content have been examined experimentally10 and theoretically11 by a number of authors, and these examinations have been extended to the surface structure.12 Reports of the interfacial13-16 and mixing17-22 (9) Hanke, C. G.; Atamas, N. A.; Lynden-Bell, R. M. Green Chem. 2002, 4, 107. (10) See, for example, Mele, A.; Tran, C. D.; Lacerda, S. H. de P. Angew. Chem., Int. Ed. 2003, 42, 4364. Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. Tran, C. D.; Lacerda, S. H. de P.; Oliverira, D. Appl. Spectrosc. 2003, 57, 152. Fletcher, K. A.; Pandey, S. Appl. Spectrosc. 2002, 56, 266. (11) Hanke, C. G.; Lynden-Bell, R. M. J. Phys. Chem. B 2003, 107, 10873. (12) Baldelli, S. J. Phys. Chem. B 2003, 107, 6148. Lynden-Bell, R. M. Mol. Phys. 2003, 101, 2625. (13) Carmichael, A. J.; Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Seddon, K. R. Mol. Phys. 2001, 99, 795.
10.1021/la035940m CCC: $27.50 © 2004 American Chemical Society Published on Web 02/07/2004
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Figure 1. Molecular structures of [C4mim][BF4], [C8mim][Cl], and [C8mim][I], denoted A, B, and C, respectively.
thermodynamics of ILs and IL-containing mixtures are only now beginning to emerge. Consequently, we have recently begun investigations into the interfacial and mixing thermodynamics of ILs with common solvents. The primary objective of the work presented in this article is to establish that aggregation occurs in aqueous solutions of ILs and to determine the shape and size of the particles. With this established, we are then in a position to proceed to further parametric studies identifying more fully the role of the chain length and anion on the aggregation behavior and the solution behavior in other solvents. Here we report the aggregation behavior of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate, [C4mim][BF4], 1-octyl-3-methylimidazolium chloride, [C8mim][Cl], and 1-octyl-3-methylimidazolium iodide, [C8mim][I], as investigated using surface tension, conductivity, and small-angle neutron scattering (SANS) measurements. The molecular structures are shown in Figure 1. All three ILs are completely miscible with water over the entire composition range at the temperature of study (298 K). However, [C4mim][BF4] + water has an upper critical solution temperature of 278 K.23 Experimental Section Materials. The ILs were synthesized according to standard methods by reaction of 1-methylimidazole with an excess of the appropriate haloalkane.1 The reactants were stirred without additional solvent at 70 °C for 72 h. The [Cnmim][X] salt, where X ) I or Cl and n is the carbon number of the alkyl chain, was then purified by repeated washing with ethyl acetate (minimum of six washes) before drying overnight at 70 °C under a vacuum. To produce the [C4mim][BF4] salt, metathesis of the [C4mim][Cl] salt was achieved by stirring a 1:1 mole ratio mixture of [C4mim][Cl] and AgBF4 with a minimum of added water for at least 2 h. The AgCl precipitate was removed by filtration, and the [C4mim][BF4] was extracted from the crude aqueous solution by repeated extractions with dichloromethane (3:1 v/v). The dichloromethane was then removed from the product first by rotary evaporation (>90 min; 70 °C) followed by transfer to a vacuum line (>8 h; 70 °C). The products were flushed with nitrogen and stored in a dry place before use. Purity of the products was assessed by 1H NMR spectroscopy. For this exploratory SANS study, detailed isotopic substitution was not applied and aqueous solutions were prepared by mass in deuterium oxide (Fluorochem >99.9 atom D%). H2O used for surface tension and conductivity (14) Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429. (15) Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Phys. Chem. Chem. Phys. 2001, 3, 2879. (16) Law, G.; Watson, P. R. Langmuir 2001, 17, 3168. (17) Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2002, 4, 81. (18) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2001, 105, 10942. (19) Najdanovic-Visak, V.; Esperanc¸ a, J. M. S. S.; Rebelo, L. P. N.; Nunes da Ponte, M.; Guedes, H. J. R.; Seddon, K. R.; Szydlowski, J. Phys. Chem. Chem. Phys. 2002, 4, 1701. (20) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (21) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. J. Chem. Soc.: Chem. Commun. 2003, 2444. (22) Friberg, S. E.; Yin, Q.; Barber, J. L.; Holbrey, J. D.; Seddon, K. R. J. Dispersion Sci. Technol. 2000, 21, 185. (23) Dullius, J. E. L.; Suarez, P. A. Z.; Einloft, S.; de Souza, R. F.; Dupont, J.; Fischer, J.; De Cian, A. Organometallics 1998, 17, 815.
Table 1. Selected Parameters Used in the SANS Analysis chain lengtha
scattering length densityb (10-6 Å-2)
head/tail volume ratioc
IL
ln (Å)
IL
tail
head
Vhead/Vtail
[C4mim][BF4] [C8mim][Cl] [C8mim][I]
6.6 11.7 11.7
1.2 0.5(4) 0.3(5)
-0.5 -0.4 -0.4
2.0 1.7 0.9
1.8 0.8 1.3
a Fully extended alkyl chain lengths estimated using Tanford’s formula.34 b Estimated using mass densities of 0.8 g cm-3 and 1.1 g cm-3 for the tail- and headgroups, respectively. c Vhead/Vtail calculated assuming zero net charge per headgroup.
measurements was purified using a Barnstead Easypure RF water purification unit. Surface Tension and Conductivity Measurements. The surface tensions of the solutions were measured using a Kru¨ss K9 tensiometer with a Wilhemy plate. Conductivities were measured using a PTI-20 Digital Water Analyzer calibrated using aqueous KCl solutions. Care was taken to ensure that all glassware and probes were thoroughly cleaned before use. SANS. SANS is now a standard technique used to determine the size, shape, and polydispersity of aggregates in solution.24 In a SANS experiment, the intensity of a scattered neutron beam with wavelength λ is measured as a function of the wave-vector transfer, or scattering vector, Q, where Q ) |Q| ) |ks - ki| ) 4π sin(θ/2)/λ, where θ is the angle between the straight-through direction and the scattered direction and ki and ks are the wave vectors of the incident and scattered beams, respectively. After accounting for detector efficiency and pixel solid angles, sample transmission, illuminated volume, and the incidence flux, the differential scattering cross section dΣ/dΩ can be determined. dΣ/dΩ can be calibrated by reference to scattering from a welldefined sample to yield absolute values. SANS measurements were performed in a single day on the LOQ instrument at the ISIS Facility, Rutherford Appleton Laboratory. ISIS is a pulsed neutron source, and, thus, LOQ is a time-of-flight instrument that, operating at 25 Hz, uses neutrons with wavelengths 2.2 < λ < 10 Å to give 0.006 < Q < 0.24 Å-1 on its main detector located 4.1 m from the sample. Solutions of the ILs in D2O were placed in Hellma fused silica spectrophotometry cuvettes with a path length of 2 mm and beam diameter of 12 mm and were thermostated at 298 K. Scattered neutrons are detected using a 3He gas area detector. The time-of-flight data were corrected for the wavelength-dependent monitor spectrum, sample transmission, and detector efficiency. Background scattering was removed by subtraction of data collected from scattering from D2O in a cuvette. Any residual incoherent background scattering from the samples was allowed for by inclusion of a flat background term in all fits. The dΣ/dΩ data were finally normalized to an absolute scale by reference to scattering from a standard sample (a solid blend of protonated and deuterated polystyrene) with a known differential scattering cross section.25 For the purposes of this work, sufficient refractive index (scattering length density) contrast between the particle and the solvent is gained by using protonated ILs in deuterium oxide. Table 1 shows the scattering length densities of the ILs used in this study, and the scattering length density of D2O was taken to be 6.33 × 10-6 Å-2. (24) See King, S. M. In Modern Techniques for Polymer Characterisation; Pethrick, R. A., Dawkins, J. V., Eds.; J. Wiley & Sons, Ltd.: New York, 1999; p 171. (25) Heenan, R. K.; Penfold, J.; King, S. M. J. Appl. Crystallogr. 1997, 1140.
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The differential scattering cross section26 for neutrons scattered from a uniform particle is commonly written as
dΣ (Q) ≈ NV 2(∆F)2P(Q) S(Q) dΩ
(1)
where V is the volume of one scatterer, ∆F is the difference in the scattering length density between the particle and the solvent, P(Q) is the particle shape factor, and S(Q) the structure factor. Equation 1 is expressed as an approximation because the S(Q) value used here does not account for any preferred orientational alignment among anisotropic particles. Approximate forms of S(Q) can be derived on the basis of a particular choice of interaction potential. A suitable choice for the systems under investigation here is the RMSA one-component macroion, with penetrating background, model of Hayter and Penfold,27 which is a good approximation provided the axial ratio of the particles is not very much greater than unity. This S(Q) has been combined with a core-shell ellipsoid model for a uniform ellipsoid with core radii R, R, and XR and shell of constant thickness t. However, for the [C4mim][BF4] solutions the degree of dissociation is apparently small and good quality fits to the data cannot be obtained using this combination of S(Q) and P(Q). Even for cases when a good quality fit can be obtained, the absolute intensity of the scattering is not consistent with the expected concentration of aggregates. Therefore, a hard-sphere structure factor28 and a form factor for polydisperse spherical particles29 or rods/disks have been applied. The SANS data have been analyzed using the multimodel fitting program FISH30 using the models just mentioned. Fitting was performed using the Marquardt steepestdescent least-squares method, and the best-fit model was reached from several starting parameter configurations. For the core-shell ellipsoidal particles and Hayter-Penfold S(Q), the main fitting parameters were the core radius R; the axial ratio X; the volume fraction of water in the shell of the micelle β; the overall charge on the micelle qtot; the inverse Debye screening length 1/rD; the scattering length densities of the solvent, core, and dry shell materials; and an overall scale factor proportional to the volume fraction φ of micelles in the solution. Further to these, the shell thickness t was constrained to a value computed for the given core size, β, and the expected head-tail volume ratio of the dry amphiphile. The volume fraction obtained from the scale factor, φfit, is then compared with the expected volume fraction, φexp, estimated using the fitted β, estimates of the tail and headgroup volumes of the IL, and the known IL concentration minus the critical aggregation concentration (cac). The Debye length rD has at each stage of the analysis been calculated taking into account all the free monomeric surfactant and unbound counterions present in the solution; that is, rD has been determined by iteration with respect to the fitted qtot for a particular concentration. The effect of changes in the counterion concentration with the micellar charge on the molar volumes and neutron scattering length density of the headgroup region have been accounted for. The solvent scattering length density has been calculated taking into account the reasonably high concentration of IL. Aggregation numbers have been determined from the ratio of the core volume to the presumed volume of the tail group in the core. The models used are not particularly sensitive to the choice of core and shell scattering length densities, and, hence, the estimated mass densities used in the calculation of the scattering length densities (Table 1) are not crucial. In the S(Q) function, the volume fraction of scatterers is fixed at the value of φ. For the [C4mim][BF4] system, the polydisperse sphere model with a hard-sphere S(Q) has been employed in addition to the core-shell ellipsoid with charged-sphere S(Q) model described previously and rod/disk model [with hard-sphere S(Q)]. In the (26) See, for example, Spalla, O. In Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter; Lindner, P., Zemb, Th., Eds.; Elsevier Science Publishing: New York, 2002; pp 49-71. (27) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 64, 651. (28) Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966, 145, 83. (29) Kotlarchyk, M.; Chen, S. H. J. Chem. Phys. 1983, 79, 2461. (30) Heenan, R. K. FISH; Rutherford Appleton Laboratory: Didcot, U.K., 1989.
Figure 2. Surface tension σ versus IL concentration c isotherms measured at 298 K for aqueous solutions of [C4mim][BF4] (O), [C8mim][Cl] (b), and [C8mim][I] (0). The lines are included to indicate the break points associated with the cac. Table 2. Aggregation Concentrations aggregation concentration (mmol dm-3) isotherm break point concentrationa IL
surface tension conductivity 1 conductivity 2 SANS
[C4mim][BF4] [C8mim][Cl] [C8mim][I] a
800 ( 100 100 ( 10 100 ( 10
820 ( 100 90 ( 20 150 ( 40
0.4 ( 0.2 0.3 ( 0.1
e800 e100
See Figures 1 and 2.
rod/disk model, the particles are of radius Rd and length L. For the polydisperse sphere model, in eq 1 the shape factor P(Q) ) 〈|F(Q)|2〉, where F(Q) is the particle form factor, is replaced with P(Q) ) ∫∞0 |F(QR)|2f(R) dR in which f(R) is the particle radius distribution function. For the modified Schultz distribution, this probability is
f(R) )
[ ] Z+1 R h
Z+1
[
]
-(Z + 1)R RZ exp Γ(Z + 1) R h
(2)
where R h is the mean particle radius and Z is a width parameter defined as Z ) (R h /σ)2 - 1, in which σ is the standard deviation of the particle size distribution from R h . The fitted scale factor is used to determine φfit, by further assuming the average water content of the particles, which is then compared with the expectation value, φexp, as for the core-shell ellipsoid model described previously.
Results and Discussion Surface Tension Measurements. The surface tension data were measured with the intention of establishing the surface activity of the aqueous solutions and to determine the aggregation concentrations, either as a critical micelle concentration (cmc) or, for the shorterchain species, as a critical hydrotrope concentration; both hereafter referred to as cac’s. Figure 2 shows the surface tension σ versus concentration c isotherms for the three solutions studied. There are a number of features to note. First, all three systems display a pronounced decrease in surface tension from the water value to a plateau region (although the surface tension increases after this point for the [C8mim][Cl] system) with distinct break points in the isotherms indicating aggregation. The concentrations at which these break points appear are given in Table 2. The straight lines in Figure 2 are guides for locating the cac values. Upon changing from n ) 4 to n ) 8, that is, from [C4mim][BF4] to [C8mim][Cl] or [C8mim][I], the cac
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decreases by a factor of approximately 8, in line with a factor of 2 per methylene unit expected for ionic surfactants. The cmc of sodium octyl sulfate is 133 mmol dm-3, which is slightly higher than ∼100 mmol dm-3 found for [C8mim][Cl] and [C8mim][I]. Therefore, the cac values for these three systems generally adhere to common rules of thumb31 for cmc’s for established surfactants. However, it is worth noting that the IL volume fractions corresponding to these cac values are not negligible (∼15 vol % for the [C4mim][BF4] system and ∼2 vol % for the [C8mim][Cl] and [C8mim][I] systems) and this has a bearing on the analysis of the SANS data. Second, the values of the surface tensions, σ*, in the plateau region at concentrations greater than the cac are low (∼35 mN m-1), despite the short chain lengths of these amphiphiles, and the surface activity is more similar to that commonly associated with surfactants than weakly surface active hydrotropes. The σ* values are similar for the [C4mim][BF4] and [C8mim][I] systems despite the difference in chain length. Interestingly, the surface tension of the [C8mim][Cl] solutions displays a pronounced minimum near the cac and subsequently increases for concentrations greater than the cac to reach σ* ∼ 42 mN m-1. This observed minimum may arise from impurities. However, the minimum remains upon repurification of the materials and upon repeated measurements, and, furthermore, the procedures for the synthesis and purification are identical to those for the other two ILs, the isotherms for which display no such pronounced minima. A thermodynamic interpretation of the increase in σ for c > cac in terms of the Gibbs adsorption equation (see eq 3 below) is that there is a relative surface excess of water. A limitation of using surface tension measurements to establish cac values arises when such surface phenomena mask the influence of the bulk system. Finally, for concentrations just above 0.1 mmol dm-3, the isotherm for [C8mim][Cl] solutions displays a slight depression from the pure solvent value; note from the error bars that the measurements were subject to moderate fluctuations. A similar feature is also found in the [C8mim][Br] + water system we are in the process of examining32 but not in the [C8mim][I] system. For completeness, the average area per amphiphile molecule residing at the surface, Apm, can be estimated by application of the Gibbs adsorption isotherm to the surface tension versus concentration data. The Gibbs adsorption isotherm may be expressed as
∂σ 1 Γ)2RT ∂ ln c
(
)
(3)
Figure 3. Conductivity isotherms measured at 298 K for aqueous solutions of (a) [C8mim][Cl] (b) and [C8mim][I] (0) and (b) [C4mim][BF4]; the straight lines are guides to the eye to highlight the breakpoints. The insets to both part a and part b show the regions near the break points associated with the cac in more detail.
values of the Apm can be used to estimate aggregate shapes near the cac. An estimate of the shape adopted by an aggregate may be obtained from geometric arguments summarized using Israelachvili’s ratio f ) v/la0, where v is the tail group volume, a0 is the headgroup area, and l is the tail group chain length of the amphiphile.33 If f < 1/3, spherical aggregates are expected. Using the Apm determined from the surface tension data and values of l and v established using Tanford’s formulas,34 we find that the values of f are 0.32(5), 0.28, and 0.18, for respectively the [C4mim][BF4], [C8mim][Cl], and [C8mim][I] systems. Thus, at concentrations close to the cac, spherical particles may be expected for all three systems. Neutron and X-ray reflection experiments are in progress that are investigating the structural implications of the features in the surface tension isotherms and verifying directly the areas per molecule. For our present purposes, we use these data to aid the assignment of cac values. Conductivity Measurements. The measured conductivities κ have been converted to molar conductivities Λm ) κ/c and plotted according to Kohlraush’s empirical law
Λm ) Λm0 - Rc1/2
(4)
where Γ is the relative excess of surfactant with respect to water, σ is the surface tension, c is the IL concentration in the aqueous solution, R is the gas constant, and T is the thermodynamic temperature. The factor of 2 in the denominator arises from the number of solute components. By applying eq 3 to the linear portion of the σ versus ln c data for c e cac, the estimated values of Apm are 63 ( 7 Å2, 75 ( 4 Å2, and 117 ( 5 Å2 for aqueous solutions of [C4mim][BF4], [C8mim][Cl], and [C8mim][I], respectively. Note that activity coefficients have not been applied in these calculations despite the high ionic strengths of the solutions near the cac, and, thus, these Apm values must be considered as crude estimates. Nonetheless, the relative
as shown in Figure 3. In eq 4, Λm0 is the molar conductivity at infinite dilution and R is an empirical constant. The linear portions represent regimes of approximately constant association. The break points between these portions occur when the nature of the charge carriers changes. In Figure 3a, data for the [C8mim][Cl] and [C8mim][I] solutions are shown. For both systems, there are two distinct breaks in the Kohlrausch plots (shown in the main figure), indicating, possibly, two regimes of differing aggregate nature. The concentrations at which the break points occur are given in Table 2. It is the higher of the two concentrations that is associated with the cac from the surface tension measurements, and the regions surrounding the higher concentration breakpoints have been expanded in the inset of Figure 3a. It is notable that
(31) These rules of thumb are concisely presented in Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley: New York, 1998; p 39. (32) Bowers, J.; Butts, C. P.; Lord, J. C. D.; Webster, J. R. P. Unpublished work.
(33) See Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed., Academic Press: New York, 1992; p 371. (34) Tanford’s formulae for fully extended alkyl chain is ln (Å) ) 1.54 + 1.26n, and the chain volume is vn (Å3) ) 27.4 + 26.9n, where n is the carbon number.
T
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Table 3. Summary of Indicative SANS Data Fitting Parameters and Results for Aqueous Solutions of [C4mim][BF4] Using a Schultz Polydisperse Sphere Model with a Hard-Sphere Structure Factor concn of IL c (mmol dm-3) vol % 400 800 1000 1200 1400 1600 2000 3000 4853
7.4 14.6 18.4 22.2 26.0 29.6 37.8 54.6 91.8
mean radiusa R h (Å) 8.5 ( 1.4 12.1 ( 2.5 13.5 ( 2.7 12.0 ( 2.1 11.3 ( 2.8 13.0 ( 3.2 9.5 ( 0.9
polydispersity σ/R h
mean aggregation no. Naggb
flat background only 0.20 ( 0.10 0.20 ( 0.10 0.20 ( 0.10 0.24 ( 0.10 0.35 ( 0.10 0.35 ( 0.10 0.30 ( 0.10 flat background only
11 11 19 17 17 27 10
aA fully extended [C mim][BF ] molecule has a maximum length 4 4 of ∼9.5 Å.35 bA molecular volume of 338 Å3 has been employed in the calculation of Nagg.
the cac values determined from the surface tension and conductivity measurements are not in entire agreement, and this discrepancy is most significant for the [C8mim][I] system. For the [C8mim][Cl] system, the pronounced minimum in the surface tension isotherm may obscure the precision to which the cac can be located, but the two values agree within experimental error. For the [C8mim][I] system, a different explanation is required. It is plausible that the first (lower concentration) break point arises as a consequence of a change in the aggregate nature rather than the formation of aggregates at this concentration, as we shall discuss shortly. Competing effects such as changes in size and total charge of charge carriers often limit the ability to pinpoint cac values using conductivity measurements. Incidentally, the break point at lower concentration for the [C8mim][Cl] system occurs in the vicinity of the concentration at which a depression is observed in the surface tension isotherm (see Figure 2). However, there is no obvious corresponding feature for the [C8mim][I] system, despite the qualitative similarities in the conductivity data, and at present, there is no evidence supporting a direct correlation between the two features. The origin of the low concentration break points in the isotherms is not yet understood but may possibly arise from ion association or small aggregate formation. As shown in Figure 3b, different conductivity behavior is found for the [C4mim][BF4] solutions. The Kohlrausch plot displays a smoother curve with no distinct break points, although a break point can be assigned at a concentration consistent with the surface-tension-determined cac, as the inset to Figure 3b shows. Because of the curvature of this plot, the precision with which the cac can be determined is limited. For this system, however, no low concentration break point is detected. SANS from Aqueous Solutions of [C4mim][BF4]. SANS data for solutions of [C4mim][BF4] with concentrations as given in Table 3 were collected. The corresponding volume fractions are also given. The SANS data are shown in Figure 4; the lines are model fits using a structural model that is consistent over the concentration regime studied. The model used to obtain these fits is discussed shortly. Samples with concentrations of 400 and 4853 mmol dm-3 displayed no scattering signal above the background, and the SANS data are not included in Figure 4. The SANS increases with increasing concentration up to a maximum in the vicinity of c ) 2000 mmol dm-3. The concentration variation of the SANS intensity allows the cac to be located between c ) 400 and 800 mmol dm-3. Because the SANS intensity is low at c ) 800 mmol dm-3, it is likely that this concentration is very close to the cac.
Figure 4. SANS data and model fits [polydisperse spheres with hard-sphere S(Q)] for aqueous solutions of [C4mim][BF4] with various concentrations. Similar quality model fits can be obtained using a rod/disk model with hard-sphere S(Q). See text for details. The alternating solid and dashed lines are used for clarity.
Figure 5. SANS data and model fits (core-shell ellipsoids with a Hayter-Penfold hard-sphere charged macroion structure factor) for aqueous solutions of [C4mim][BF4] with various concentrations: (solid lines) prolate ellipsoids, (dotted lines) oblate ellipsoids. The axial ratio, X, is given for models involving nonspherical aggregates; X < 1 (> 1) refers to oblate (prolate) ellipsoidal aggregates. See text for details.
A number of models have been used in attempts to fit the data. Core-shell ellipsoid models with a Hayter-Penfold charged-sphere S(Q) have been applied, and the best model fits resulting from this particular analysis are shown in Figure 5. The absence of structure peaks suggests that the aggregates bear a low charge, and for the purposes of fitting using this model, a small arbitrary charge per headgroup of 0.1e, where e is the electronic charge, has been applied. For the lower concentrations (c e 1200 mmol dm-3), the aggregates can be modeled as spheres (with X ) 1.0 ( 0.1) with a core radius of ∼10.5 Å, although the particles are smaller at 800 mmol dm-3 (R ∼ 8 Å), and a shell thickness of 6-7 Å. Moderately better fits are found for slightly prolate particles, as indicated by the magnitude of the sum-of-squares error for the fit and, more reliably, by the closer agreement between the expected volume fraction of scatterers, φexp, and the volume fraction of scatterers determined from the absolute intensity of the SANS data, φfit. For c g 1400 mmol dm-3, the agreement between φexp and φfit and the quality of the fit become
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increasingly poor with increasing concentration, and the modeled aggregates depart from being near-spherical to become increasingly either rodlike or disklike, although the core radius does not alter significantly. For such a departure, the prolate ellipsoid model provides closer agreement between φexp and φfit but with φfit < 0.5 φexp at best. Figure 5 shows some model fits to the data for oblate and prolate ellipsoid models, demonstrating the misfitting for higher concentrations; where the particles depart from a near spherical shape, the values of X are given on the graph. Furthermore, the parameters describing S(Q) are not consistent with the expected values, signifying that this model is unsuitable, especially at higher concentrations and, thus, the S(Q) value used may not represent the interparticle interactions adequately. This is exemplified by the fact that a reasonable volume fraction of scatterers cannot be catered for in S(Q); given that the volume fractions of IL we are dealing with are large, this is a significant discrepancy. In light of this, analyses were then performed using an uncharged hard-sphere S(Q) together with either rod/disk or polydisperse sphere P(Q). The uncharged particle is reasonable because the BF4- ion is hydrophobic and will be less inclined to be solvated than to remain in the proximity of the cation. For both models, good fits to the data were obtained [as shown in Figure 4 for the polydisperse sphere P(Q)] and the agreement between φfit and φexp was significantly improved across the concentration range studied from that found in the preceding model. Although these are cruder models of aggregate structure, these models do nonetheless allow a general overview of the structure and nature of the aggregates as a function of the concentration to be obtained. The rod/disk model reproduces the result obtained from the core-shell model just described at lower concentrations (c e 1200 mmol dm-3), with Rd ≈ 2L, that nearspherical aggregates are present, where Rd corresponds to the overall particle radius (R + t) obtained from the core-shell models. However, disk models with Rd ≈ L also fit the data. At higher concentrations (c ) 2000 mmol dm-3 and 3000 mmol dm-3), the modeled particles are large disks with Rd ≈ L ≈ 30 Å. The transformation from the low to the high concentration structure is unclear, as is the molecular interpretation of this large aggregate structure. Good agreement between φfit and φexp is obtained for both models by allowing the aggregate to contain a mixture of D2O and [C4mim][BF4]. Taking into account the presence of an appreciable concentration of the [C4mim][BF4] in the solvent phase, the maximum water content is ∼45 vol % occurring at c ) 1400 mmol dm-3, but the water composition in the aggregate does vary with particle size. The polydisperse sphere model produces the most reasonable parameters, in terms of quality of fit, agreement between φfit and φexp, sensible progression as a function of concentration, and molecular interpretation. Some indicative parameters are given in Table 3. Approximate values are given in Table 3 owing to the uncertainty in S(Q) and the strong correlation between the mean particle radius R h and the polydispersity σ/R h. The extent of this correlation is indicated by the uncertainties quoted in Table 3. For these reasons, we have not extended the analysis to include other shapes of the polydispersity function. Despite the correlation between R h and σ/R h , a number of trends emerge. At low concentration, a change in the particle size upon changing the concentration from 800 mmol dm-3 to 1000 mmol dm-3 is found. After this initial increase in mean particle size, it appears that the polydispersity increases with increasing
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Figure 6. SANS data and model fits (core-shell ellipsoids with a Hayter-Penfold hard-sphere structure factor for charged macroions) for aqueous solutions of [C8mim][Cl] with various concentrations. The scattering from the solution with c ) 3527 mmol dm-3 could not be fitted using this model. Table 4. Location of the Structure Peaks and Corresponding Characteristic Spacing for Aqueous Solutions of [C8mim][Cl] concn of IL c (mmol dm-3) vol % 56 200 500 747 1393 3527 a
peak position Qmax (Å-1)
1.3 4.7 11.5 17.4 34.7 79.8
fwhh ∆Q (Å-1)
spacing at peaka d (Å)
flat background only flat background only 0.13 0.09 47.0 0.16 0.10 40.0 0.205 0.080 30.5 0.255 0.018 24.6 ( 0.9
Indicative spacing only.
concentration, reaching a maximum close to c ) 2000 mmol dm-3 before decreasing (accompanied with a decrease in mean particle size) at higher concentrations. Good agreement between φfit and φexp can be obtained by allowing the water content to vary with composition. The higher water content (maximum ∼45 vol %) can be associated with the higher polydispersity or, allowing for coupling between R h and the σ/R h , larger mean particle size. All three models suggest that at lower concentrations near-spherical aggregates form and that there is an increase in particle size between c ) 800 mmol dm-3 and c ) 1000 mmol dm-3. For c g 1200 mmol dm-3, the most appealing and consistent model is one in which the aggregates are modeled as polydisperse spheres. The mean particle radius for c g 1000 mmol dm-3 is greater than the length of a fully extended [C4mim][BF4] molecule. However, the aggregates possess a significant water content. SANS from Aqueous Solutions of [C8mim][Cl]. A very different scattering behavior is found for the [C8mim][Cl] system as a function of concentration as is shown in Figure 6. Table 4 lists the concentrations studied. In addition to the data shown in Figure 6, the sample with c ) 56 mmol dm-3 displayed background scatter only. Illustrative model fits are also shown in Figure 6. As reflected by the overall scattered intensity and the form of the scattering pattern, the SANS is clearly dominated by the contribution from S(Q), although to fit the data oblate ellipsoids (typically with X ∼ 0.15) are required, and small total charges (∼ 3e) are present on the surfaces of the micelle. In the analysis we have performed, the absolute intensities are in agreement with the expected volume fraction of [C8mim][Cl]. Because the Hayter-
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Table 5. Summary of Indicative SANS Data Fitting Parameters for Aqueous Solutions of [C8mim][I] Using a Core-Shell Ellipsoid Model with a Hayter-Penfold Charged Sphere Structure Factor concn of IL c (mmol dm-3)
vol %
100 250 500 750 1000 1973 3193e
2.3 6.5 12.9 19.1 26.2 50.0 84.4
axial ratioa X
core radiusa R (Å)
1.1 ( 0.1 1.1 ( 0.1 1.1 ( 0.1 1.0 ( 0.1 1.0 ( 0.1
11.3 ( 0.4 13.2 ( 0.5 13.0 ( 0.5 13.2 ( 0.5 11.8 ( 0.4
0.15 ( 0.05
shell thicknessb,c t (Å)
mean aggregation no. Nagg
5.3 6.3 6.1 5.6 5.7 unusual scattering pattern
28 45 41 41 29
charge per headd q/e 0.36 0.40 0.48 0.36 0.34
R and X are correlated parameters, but typical uncertainties are ∆R ) ( 0.5 Å and ∆X ) (0.1. b Because the neutron scattering length density of the hydrated shell is intermediate between that of the core and that of the D2O solvent, the SANS patterns from small micelles are not particularly sensitive to the thickness and composition of the shell. The water content in the shell can be estimated to within (5 vol %. c The thickness is constrained with respect to the core radius R and the ratio Vhead/Vtail given in Table 1. d Precision with which charge per headgroup can be determined is limited and the quoted values are indicative; q/e is treated here as a fitting parameter. e The axial ratio is included at this concentration to demonstrate that disklike particles can be modeled. a
Penfold S(Q) is valid only for near-spherical particles, we do not pursue the shape of the micelles here but comment on the qualitative features of S(Q). To demonstrate S(Q) dominance, for c ) 200 mmol dm-3, which is in excess of the cac determined from surface tension and conductivity measurements, there is no contribution above the background arising from the P(Q). Unfortunately, owing to the absence of an appreciable scattering signal at concentrations immediately above the nominal cac, the disappearance of the signal cannot be monitored, and SANS cannot be easily used to aid the determination of the cac for this system. Figure 6 shows the clear progression of how a sharp structure peak develops in the scattering pattern. The location of these peaks in Q and the corresponding real-space separation are given in Table 4 as a function of the solution concentration. The structure peak is broad and centered at lower Q at lower concentrations and increases in intensity, sharpens, and shifts to higher Q as the concentration is increased. At c ) 3527 mmol dm-3, the structure peak sharpens into a distinct Bragg peak, with a corresponding lattice spacing of 24.6 ( 0.9 Å. This corresponds to roughly two molecular lengths because the fully extended length of a [C8mim][Cl] molecule is ∼14.5 Å.35 After allowing for solvation of the headgroups, this spacing suggests some interdigitation of the alkyl chains. Without further studies, we are unable to speculate whether this is more consistent with the ordering of micellar rods or the formation of sheets of bilayers. At this concentration, the solution is visibly in a gel-like state and polarization microscopy has provided evidence for the formation of lyotropic mesophases.36 This result bears strong similarities with the findings of Firestone et al. for the aqueous [C10mim][Br] system.8 SANS from Aqueous Solutions of [C8mim][I]. The scattering from the [C8mim][I] system is shown in Figure 7. The concentrations studied are given in Table 5. The scattered intensity increases with increasing concentration for c e 1000 mmol dm-3, but for c > 1000 mmol dm-3 (35) Butts, C. P.; Lord, J. C. D.; Coles, S. J. Unpublished work. Dimensions based on X-ray-determined crystal structures of N-methylN ′-alkylbenzimidazolium salts. (36) Vergara-Gutierrez, M. C.; Bowers, J.; Amos, K. E.; Bruce, D. W. Unpublished work. Following the visual observation and the evidence from SANS of long-ranged structure, aqueous solutions of [C8mim][Cl] have been investigated by polarization microscopy. Temperature scans of three concentrations were performed in the range 0-40 °C. For c ) 3357 mmol dm-3, a mesophase (possibly cubic) exists at low temperatures and there is a transition from the ordered to an isotropic phase at ∼35 °C. For c ) 2858 mmol dm-3, a mesophase exists only between 8 and 12 °C. For c ) 3794 mmol dm-3, there is no evidence for mesophase formation in the temperature range studied. A more complete phase diagram is under construction.
Figure 7. SANS data and model fits (core-shell ellipsoids with a Hayter-Penfold hard-sphere structure factor for charged macroions) for aqueous solutions of [C8mim][I] with various concentrations. The scattering from the solution with c ) 1973 mmol dm-3 could not be analyzed and probably arises from scattering from coexisting structures. The fits shown correspond to a model in which the aggregates are near-spherical. See text for details.
the nature of the scattering pattern changes. No solutions with c < 100 mmol dm-3 were studied, but the scattered intensity for the 100 mmol dm-3 sample is very low and suggests that the cac is not significantly lower than this value. At low to moderate concentrations, the scattering displays a structure peak arising from charge ordering effects on S(Q) and the particles bear substantial charge. Accordingly, the core-shell ellipsoid model with HayterPenfold S(Q) has been employed in the analysis. For c e 1000 mmol dm-3, the aggregates can be modeled either as near-spherical particles or as oblate ellipsoidal particles. For the oblate ellipsoid model, the average core radius is R ) 15.5 ( 1.0 Å, the shell thickness is 5-6 Å, and the particles become more disklike at higher concentrations. However, the agreement between φfit and φexp and the quality of fit become increasingly worse as the concentration is increased, resulting in clear misfitting at c ) 1000 mmol dm-3. Figure 8 shows the model fits corresponding to the oblate ellipsoidal model and demonstrates the misfitting at higher concentrations. The values of X corresponding to the fits are given for X * 1. Significantly better agreement between φfit and φexp is found over this concentration regime (c e 1000 mmol dm-3), with φfit ≈ φexp, for the (near-)spherical model. The fits for this model (for c e 1000 mmol dm-3) are shown in Figure 7. Indicative
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Figure 8. SANS data and model fits (core-shell ellipsoids with Hayter-Penfold hard-sphere structure factor for charged macroions) for aqueous solutions of [C8mim][I] with various concentrations. The model fits shown correspond to a model with oblate ellipsoidal aggregates. Values of the axial ratio, X, are given where the aggregates are nonspherical. See text for details.
model parameters are given in Table 5. For all fits shown, the water content in the shell was ∼50 vol %. The particle radius is 16.5-19.5 Å. Because the fully extended length of a [C8mim][I] molecule is ∼14.5 Å, this suggests that these aggregates are loosely packed. It is worth noting that the particle radius increases when the concentration is increased from 100 to 250 mmol dm-3. This change in the particle size may provide a possible, but not definitive, explanation for the location of the higher concentration breakpoint in the conductivity isotherm for this system. The scattering pattern measured from the sample with c ) 1973 mmol dm-3 is bizarre and cannot be fitted using a single model. It appears that there is a structural transition occurring in the range 1000 < c < 1973 mmol dm-3. The bizarre scattering possibly arises from coexisting microphases, although visual inspection revealed the sample to be isotropic. At the high concentration of c ) 3193 mmol dm-3, the scattering is dominated by S(Q), as was found for the [C8mim][Cl] system. Oblate ellipsoids are required to represent the P(Q) contribution to the overall scattering at this concentration. General Remarks. The magnitudes of the cac values for these three systems were discussed earlier, and the determined values using surface tension, conductivity, and SANS measurements are summarized in Table 2. Within the bounds of uncertainty, a consistent cac value is determined by all three methods for the [C4mim][BF4] system. For the [C8mim][Cl] system, although the SANS measurements did not permit a cac value to be established, the values obtained from surface tension and conductivity agree within error. However, for the [C8mim][I] system, although the cac values determined from SANS and surface tension measurements are in agreement, the conductivity measurements yield an inconsistent cac value. We tentatively ascribe the inability to determine a consistent cac using conductivity to a change in the nature of the aggregates in this system as a function of concentration. The low-concentration SANS data add some
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support to this speculation. The conductivity data for the [C8mim][Cl] and [C8mim][I] systems indicate that there may exist possible aggregation behavior at concentrations lower than the cac. However, there is no evidence supporting this conjecture here because we have not examined the concentration ranges for c < cac in sufficient detail using SANS. Further experiments are required. For the [C8mim][Cl] system, because the P(Q) does not contribute significantly to the scattering even for c > cac, it may not be possible to resolve any sub-cac bulk structures. The values of Israelachvili’s ratio, f, calculated using headgroup areas estimated from surface tension data near the cac, indicated that spherical aggregates may form at concentrations close to the cac. This is certainly consistent with the findings for the [C4mim][BF4] and [C8mim][I] systems. Analysis of the SANS data for the [C8mim][I] system results in a consistent model for c e 1000 mmol dm-3 in which the aggregates are near-spherical. For the [C4mim][BF4], the model is less clear-cut, but a consistent model is one in which polydisperse spherical aggregates are formed and these aggregates contain a significant water content. All three models used for the SANS analysis for this system support spherical particles at low concentrations. From inspection of the structure factor contribution to the SANS, it appears that the aggregates of the [C8mim][I] system bear a larger charge per headgroup than in the [C4mim][BF4] system. This is consistent with the I- being a soft anion that can be readily dissociated from the cation and the hydrophobic BF4despite its size having a preference to remain associated with the cation. We have been unable to examine the nature of the aggregates formed in the [C8mim][Cl] system in any great detail because the SANS is dominated by the interparticle structure factor. At moderate concentrations, the long-range order between aggregates in this system is more pronounced than in the [C8mim][I] system. In the absence of a clear idea of the nature of the aggregates, we cannot speculate on the origin of the more ordered structure. Summary The work reported here has demonstrated that ILs based on 1-alkyl-3-methylimidazolium salts act as shortchain cationic surfactants in aqueous solution and form aggregates above a cac. Clearly further parametric studies are required to define the role of chain length and counterion on the shape and size of the aggregates formed by these ILs. The issue regarding the bulk structure at concentrations below the cac values for the [C8mim][Cl] and [C8mim][I] systems also requires clarification. Knowledge of the aggregation behavior is a vital part of understanding how ILs participate as components in a mixed solvent system. Furthermore, the potential range of ordered structures formed in various solvents, such as the gel phases formed by [C10mim][Br]8 and [C8mim][Cl]36 in aqueous solution, may be able to provide materials for synthetic applications. Acknowledgment. The authors would like to thank the EPSRC for funding this project (GR/R56129) and the CCLRC of the U.K. for the provision of beamtime on LOQ. LA035940M
