Letter pubs.acs.org/Langmuir
Evidence of Water-in-Ionic Liquid Microemulsion Formation by Nonionic Surfactant Brij-35 Rewa Rai and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi − 110016, India S Supporting Information *
ABSTRACT: Brij-35, a common and popular nonionic surfactant, is shown to form water-in-ionic liquid (w/IL) microemulsions with IL 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF6]) as the bulk phase. The presence of w/[bmim][PF6] microemulsions is hinted by the significantly increased solubility of water in Brij-35 solution of [bmim][PF6]. The formation of w/[bmim][PF6] microemulsions by Brij-35 is confirmed using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. Brij-35 forms reverse micelle-type aggregates within [bmim][PF6] in the absence of added-water. These reverse micelles become w/ [bmim][PF6] microemulsions as the water is added to the system. As the water loading (w0) is increased, the average diameter of the aggregates increases. Fourier transform infrared (FTIR) absorbance data reveal the presence of both “bound” and “free” water within the system. The “bound” water is associated with the water pools of the w/[bmim][PF6] microemulsions. Excited-state proton transfer (ESPT) involving probe pyranine shows deprotonation of pyranine within the water pools of the w/[bmim][PF6] microemulsions.
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INTRODUCTION Microemulsions are transparent, optically isotropic, and thermodynamically stable dispersions of two or more immiscible liquids that are stabilized by an adsorbed surfactant (or emulsifier), frequently in combination with a cosurfactant, at the liquid−liquid interface.1,2 Traditionally, depending on the nature and/or the amount of the components, these nanoscale aggregates may be oil-in-water (o/w) or water-in-oil (w/o) microemulsions. The o/w microemulsions have features similar to that of normal micelles in water and thus have similar applications.3 However, the w/o microemulsions contain a nanometer-sized water pool within a hydrophobic oil phase, allowing the effective usage of such compartmentalized systems in biochemistry, nanochemistry, syntheses, extractions, separations, oil recovery, polymerization and in pharmaceutical, cosmetic, agrochemical, and food industries.2−8 The shape and size of the water pools in w/o microemulsions are controlled by the ratio of the water-to-surfactant concentrations (w0) and, of course, by the nature of the surfactant and the continuous oil phase. As a major drawback, the conventional w/o microemulsions contain a huge amount of organic solvents, which not only cause damaging effects to the environment but also produce significant waste disposal issues. Further, the organic solvents usually used as the oil phase in w/o microemulsions have fairly similar properties that restrict the overall applications of such systems. Due to their proposed environmentally-benign nature along with many unique and attractive physicochemical properties, water-immiscible room temperature ionic liquids (ILs) have emerged as an obvious choice as substitutes for traditional organic solvents as the oil phase to form microemulsions.9,10 © 2014 American Chemical Society
The w/IL microemulsions may form highly versatile reaction media that have the potential to be used in many applications because the combination of the inimitable properties of nano/ micrometer-sized water domains in an IL continuous phase with the fascinating properties of ILs may result in unusual properties and outcomes that cannot be obtained using conventional w/o microemulsions. The formation of IL microemulsions is generally hindered by the low miscibility of most conventional surfactants, particularly the ionic and the zwitterionic ones, in ILs. 11,12 Few investigations related to the formation of w/IL microemulsions using nonionic and very few using ionic and zwitterionic surfactants in the absence or presence of a cosurfactant/ cosolvent/additive have been reported in the current literature.13−19 In most such reports, authors have presented the w/IL microemulsion formation within IL 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF6 ]) using nonionic surfactant Triton X-100.13,16,17 Surfactants of the Brij series belong to an important class of nonionic surfactants that are extensively used in cosmetics, pharmaceuticals, paints, and cleaning agents due to the fact that they do not possess a phenyl group. Their considerably less toxicity, high biodegradability, and insignificant absorption in the UV region translate into their widespread use in chromatography.20,21 Their tendency to solubilize membrane proteins render them useful in several biotechnological and biomedical applications.22 These assets make Brij series surfactants superior over Triton Received: June 4, 2014 Revised: August 13, 2014 Published: August 14, 2014 10156
dx.doi.org/10.1021/la502174a | Langmuir 2014, 30, 10156−10160
Langmuir
Letter
and Tween series nonionic surfactants.21 Subsequently, reverse micelles or microemulsions formed by Brij series surfactants may offer superior and interesting properties as far as applications of such systems are concerned. Although a few investigations have been carried out on the aggregation behavior of Brij series surfactants within neat ILs and IL/w swollen micelles, w/IL microemulsions using Brij series surfactants, to the best of our knowledge, have not been reported yet. Recently, Wang et al. studied the hexagonal liquid crystalline phases formed by the ternary systems of Brij-97/ water/ILs.15 In this Letter, we report the formation of w/IL microemulsions in IL [bmim][PF6] using a common Brij series nonionic surfactant Brij-35 [polyoxyethylene(23)laurylether] (structures are given in Figure 1). The evidence of micro-
scattering curve. Attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) absorbance data were acquired from 4000 to 400 cm−1 on an Agilent Technologies Cary 660 ATR double-beam spectrophotometer. Raman spectra were acquired at λex = 532 nm using a model No. X/01/220 XploRA PLUS Confocal Raman spectrometer, and fluorescence spectra were acquired on a model FL 3-11, Fluorolog-3 modular spectrofluorometer, both purchased from Horiba-Jobin Yvon, Inc. The spectrofluorometer contains single Czerny−Turner grating excitation and emission monochromators as wavelength selection devices, a 450 W Xe-arc lamp as the excitation source, and a photomultiplier (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 starting from the sample preparation and averaged.
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RESULTS AND DISCUSSION Brij-35 is soluble in neat imidazolium ILs and undergoes selfaggregation with a reported critical micelle concentration (cmc) of 115 mM in IL [bmim][PF6].11,12 The maximum solubility of water in [bmim][PF6] in the absence of Brij-35 is found to be 1.9 wt % (equal to 1.4 M) (the reported solubility of water in [bmim][PF6] is 1.2−2.3 wt %).23 We prepared 0.5 M Brij-35 solution in [bmim][PF6] and assessed the water solubility within this system. The water miscibility within 0.5 M Brij-35 in [bmim][PF6] was found to be ∼8.55 M under ambient conditions, which corresponds to a water loading (w0) of ∼17.1 (where w0 = [water]/[Brij-35]. The water inherent to Brij-35 can be up to a maximum of ∼3% by weight, which corresponds to a w0 ∼ 2. However, the water contents in our Brij-35 samples were found to be
