pubs.acs.org/Langmuir © 2010 American Chemical Society
Ionic Liquid-Induced Unprecedented Size Enhancement of Aggregates within Aqueous Sodium Dodecylbenzene Sulfonate Rewa Rai,† Gary A. Baker,§ Kamalakanta Behera,† Pravakar Mohanty,‡ Narayanan D. Kurur,† and Siddharth Pandey*,† †
Department of Chemistry, and ‡Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India, and §Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37931, United States Received September 15, 2010. Revised Manuscript Received October 22, 2010
Physicochemical properties of aqueous micellar solutions may change in the presence of ionic liquids (ILs). Micelles help to increase the aqueous solubility of ILs. The average size of the micellar aggregates within aqueous sodium dodecylbenzene sulfonate (SDBS) is observed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) to increase in a sudden and drastic fashion as the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) is added. Similar addition of [bmim][PF6] to aqueous sodium dodecyl sulfate (SDS) results in only a slow gradual increase in average aggregate size. While addition of the IL [bmim][BF4] also gives rise to sudden aggregate size enhancement within aqueous SDBS, the IL 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), and inorganic salts NaPF6 and NaBF4, only gradually increase the assembly size upon their addition. Bulk dynamic viscosity, microviscosity, dipolarity (indicated by the fluorescent reporter pyrene), zeta potential, and electrical conductance measurements were taken to gain insight into this unusual size enhancement. It is proposed that bmimþ cations of the IL undergo Coulombic attractive interactions with anionic headgroups at the micellar surface at all [bmim][PF6] concentrations in aqueous SDS; in aqueous SDBS, beyond a critical IL concentration, bmimþ becomes involved in cation-π interaction with the phenyl moiety of SDBS within micellar aggregates with the butyl group aligned along the alkyl chain of the surfactant. This relocation of bmimþ results in an unprecedented size increase in micellar aggregates. Aromaticity of the IL cation alongside the presence of sufficiently aliphatic (butyl or longer) alkyl chains on the IL appear to be essential for this dramatic critical expansion in self-assembly dimensions within aqueous SDBS.
Introduction Surfactant solutions comprising normal or reverse micelles are used as media for a variety of chemical analyses and syntheses.1 At ambient conditions, physicochemical properties of an aqueous surfactant solution depend, among others, on the identity of the surfactant.1 A common, simple, and effective way to modify the physicochemical properties of a given aqueous surfactant solution is to use external additives, such as cosolvents, cosurfactants, electrolytes, polar organics, nonpolar organics, and so forth.1,2 Ionic liquids (ILs) are receiving increased attention from both academic and industrial research communities due to their unusual and interesting properties.3,4 Combined with the fact that ILs are composed entirely of cations and anions but still exist in the liquid state at ambient conditions, the recent investigations on ILs are partly also due to their potential environmentally benign nature.3 Consequently, the role of ILs as additive in modifying *To whom correspondence should be addressed. Telephone: þ91-1126596503. Fax: þ91-11-26581102. E-mail:
[email protected].
(1) (a) Moroi, Y. Micelles: Theoretical and Applied Aspects; Springer: New York, 1992.(b) Jones, M. J.; Chapman, D. Micelles, Monolayers, and Biomembranes; Wiley-LISS: New York, 1995. (2) (a) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (b) Ohde, H.; Wai, C. M.; Kim, H.; Kim, J.; Ohde, M. J. Am. Chem. Soc. 2002, 124, 4540. (c) Ruiz, C. C.; Aguiar, J. Langmuir 2000, 16, 7946. (d) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958. (3) (a) Welton, T. Chem. Rev. 1999, 99, 2071. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (c) Wasserscheid, P. Nature 2006, 439, 797. (d) Seddon, K. R. Nature 2003, 2, 363. (4) (a) Meli, L.; Lodge, T. P. Macromolecules 2009, 42, 580. (b) Solinas, M.; Pfaltz, A.; Cozzi, P. G.; Leitner, W. J. Am. Chem. Soc. 2004, 126, 16142. (c) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 6731. (d) Seddon, K. R. Green Chem. 2002, 4, G25–G26.
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properties of aqueous surfactant systems is deemed both interesting and important.5,6 In our recent investigations, we have shown that the effectiveness of an IL in changing the properties of an aqueous surfactant system depends in major part on the kind and extent of interaction(s) between the ions of the IL and the surfactant headgroup.6 At lower concentrations of IL, the electrostatic interactions have turned out to be of utmost importance in deciding the physicochemical properties of an aqueous surfactant system.6 The most noteworthy outcome of our investigation perhaps is that the effect of ILs on properties of aqueous surfactant systems is different from that of the common salts.6 In this Letter, we present an interesting observation pertaining to the aqueous micellar solution of anionic surfactant sodium dodecylbenzene sulfonate (SDBS) as IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) is added (structures are given in Scheme 1). Though the aqueous solubility of “hydrophobic” IL [bmim][PF6] is fairly low (i.e., ∼2 wt % at ambient conditions),7 it increases with the increase in surfactant concentration. We observe a large and sudden increase in the size of the (5) (a) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (b) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (c) He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666. (d) Atkin, R.; Warr, G. G. J. Phys. Chem. B 2007, 111, 9309. (6) (a) Behera, K; Kumar, V.; Pandey, S. ChemPhysChem 2010, 11, 1044. (b) Behera, K.; Om, H.; Pandey, S. J. Phys. Chem. B 2009, 113, 786. (c) Behera, K.; Pandey, S. J. Phys. Chem. B 2007, 111, 13307. (d) Behera, K.; Pandey, S. Langmuir 2008, 24, 6462. (7) (a) Wong, D. S. H.; Chen, J. P.; Chang, J. M.; Chou, C. H. Fluid Phase Equilib. 2002, 194, 1089. (b) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2001, 105, 10942. (c) Liu, J. F.; Jiang, G. B.; Chi, Y. G.; Cai, Y.; Zhou, Q. X.; Hu, J. T. Anal. Chem. 2003, 75, 5870. (d) Alfassi, Z. G.; Huie, R. E.; Milman, B. L.; Neta, B. Anal. Bioanal. Chem. 2003, 377, 159.
Published on Web 11/02/2010
DOI: 10.1021/la103689m
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Letter
Rai et al. Scheme 1
Figure 1. Solubility of [bmim][PF6] as a function of surfactant concentration within aqueous SDBS (4) and SDS (O) solutions at 30 °C.
aggregates within aqueous SDBS as additional [bmim][PF6] is added to the solution. In contrast, addition of [bmim][PF6] to the micellar solution of another anionic surfactant sodium dodecyl sulfate (SDS) fails to induce similar sudden rapid growth in aggregate size. The reason is proposed to be inherent to the interaction between the IL cation and the phenyl moiety of the surfactant SDBS, specifically to the presence of cation-π interaction.
Experimental Section Materials. Surfactants SDBS and SDS of highest purity were purchased from Acros Organics and SISCO Research Laboratories, respectively, and were used as received. ILs [bmim][PF6], [bmim][BF4], [emim][BF4], [bmim][Tf2N], [bmim][OTf], and [hmim][Br] (Merck, highest purity, water content < 10 ppm) were stored under argon atmosphere and were used as received. NaPF6 and NaBF4 of highest purity were purchased from SigmaAldrich and Spectrochem Pvt. Ltd., respectively, and were used as received. Doubly distilled deionized water was obtained from a Millipore, Milli-Q Academic water purification system having g18 MΩ 3 cm resistivity. The following materials were used as received: pyrene from Sigma-Aldrich and 1,3-bis(1-pyrenyl)propane from Molecular Probes. Ethanol (99.9%) was obtained from sd fine-chem. Ltd. Methods. Required amounts of materials were weighed using a Mettler Toledo AB104-S balance with a precision of (0.1 mg. Stock solutions of the fluorescence probes were prepared in ethanol and stored in precleaned amber glass vials at ∼4 °C. Aqueous surfactant solutions of desired concentrations were freshly prepared in doubly distilled deionized water. Aqueous surfactant solutions of the probes were prepared by taking appropriate aliquots of the probes from the stock and evaporating ethanol using a gentle stream of high purity nitrogen gas. An appropriate amount of aqueous surfactant solution was added to achieve the required final probe concentration. A precalculated amount of the additive was directly added to each aqueous surfactant solution to achieve the required additive concentration. The micellar size and zeta potential of aqueous SDBS and SDS solutions in the presence of different concentrations of the additives were measured using a Delsa Nano C particle size analyzer (Beckman Coulter, San Diego, CA). A laser diode (658 nm) with a power of 30 mW was used as a light source. All the measurements were done at a scattering angle of 165° and a temperature of 30 °C, which was controlled by means of a thermostat. Zeta potential 17822 DOI: 10.1021/la103689m
measurements were performed using a flow cell with embedded electrodes purchased from Beckman Coulter. All solutions were appropriately and carefully filtered before data acquisition. Fluorescence spectra were acquired on a model FL 3-11, Fluorolog-3 modular spectrofluorometer purchased from HoribaJobin Yvon, Inc. The spectrofluorometer contains single CzernyTurner grating excitation and emission monochromators as wavelength selection devices, 450 W Xe-arc lamp as the excitation source, and PMT as the detector. All the data were acquired using 1 cm2 path length quartz cuvettes. Spectral response from appropriate blanks was subtracted before data analysis. All the measurements were taken in triplicate and averaged. Electrical conductivity measurements were carried out on a CM-183 μp-based EC-TDS analyzer with ATC probe and conductivity cell (CC-03B) purchased from Elico Ltd., India. The solubility of [bmim][PF6] in aqueous surfactant solutions of each surfactant at different concentrations was obtained spectrophotometrically. The bulk viscosities of the aqueous micellar solutions in the presence of additives were measured at 30 °C using an Anton Paar AMVn fallingball automated microviscometer. The size of the aggregates was also established using transmission electron microscopy (TEM). TEM was performed using a PHILIPS CM12 microscope operated at an accelerating voltage of 100 kV. A drop of the micellar solution with or without [bmim][PF6] was dispersed on a carboncoated copper grid and dried before TEM data acquisition. All the measurements were taken at least three times starting from the sample preparation and averaged appropriately.
Results and Discussion Solubility of IL [bmim][PF6] within aqueous SDBS is initially assessed visually followed by a more precise spectrophotometric determination. Solubility of IL [bmim][PF6] in water increases as the surfactant SDBS is added to water (Figure 1). The molar solubility of [bmim][PF6] (S[bmim][PF6]) within aqueous SDBS at 30 °C can be conveniently predicted by a simple linear relation: S½bmim½PF6 ðmMÞ ¼ 0:42ð ( 0:01Þ½SDBS ðmMÞ þ 70:3 ðmMÞ,
r2 ¼ 0:9992
It is interesting to note that though the molar solubility of [bmim][PF6] also increases linearly with increasing [SDS] in aqueous micellar SDS, the increase is significantly more for SDBS [slopes of 0.42((0.01) and 0.36((0.02) for SDBS and SDS, respectively]. This could be attributed to the combined effect of the following. In their aqueous micellar solutions, SDBS and SDS have fairly Langmuir 2010, 26(23), 17821–17826
Rai et al.
Figure 2. (A) DLS data of aqueous SDBS solution in the presence of [bmim][PF6] at 30 °C with 0 mM (4), 71.7 mM (0), and 144.1 mM (b) [bmim][PF6]. (B) Average hydrodynamic diameter versus additive concentration for aqueous SDBS solution. Inset shows average diameter versus [bmim][PF6] concentration for aqueous SDBS (4) and SDS (O) solutions at 30 °C.
similar aggregation numbers (Nagg ∼518a and ∼54,8b respectively); however, the critical micelle concentration (cmc) of SDBS is ca. 2.3 mM,8c which is significantly lower than the cmc of SDS (ca. 8.1 mM).8b Due to the considerably lower cmc of aqueous SDBS, the concentration of micelles in aqueous SDBS would be higher at constant surfactant concentration [S] as [micelle] = {[S] - cmc}/ Nagg. Higher [micelle], in turn, may solubilize more [bmim][PF6].1a The other factor that may contribute to higher solubility of [bmim][PF6] in SDBS could be the presence of “extra” phenyl (in addition to dodecyl chain) close to the surfactant headgroup that may render aqueous micellar SDBS more conducive to [bmim][PF6] solubilization (vide infra). The aqueous solutions having [SDBS] = 300 mM in the presence of [bmim][PF6] up to its solubility limit are investigated further for their properties. The average hydrodynamic diameter of the micellar aggregates within aqueous SDBS obtained using dynamic light scattering (DLS) technique shows A very interesting trend (Figure 2). As the concentration of [bmim][PF6] is increased up to
