Solubilization of Benzene, Toluene, and Xylene (BTX) in Aqueous Micellar Solutions of Amphiphilic Imidazolium Ionic Liquids Justyna Luczak,* Christian Jungnickel, Marta Markiewicz, and Jan Hupka Department of Chemical Technology, Chemical Faculty, Gdańsk University of Technology, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland ABSTRACT: Water-soluble ionic liquids may be considered analogues to cationic surfactants with a corresponding surface activity and ability to create organized structures in aqueous solutions. For the first time, the enhanced solubility of the aromatic hydrocarbons, benzene, toluene, and xylene, in aqueous micellar systems of 1-alkyl-3-methylimidazolium chlorides was investigated. Above a critical micelle concentration, a gradual increase in the concentration of aromatic hydrocarbons in the miceller solution was observed. This phenomenon was followed by means of the molar solubilization ratio, the micellar/water partition coefficient, and the number of solubilizate molecules per IL micelle. The molar solubilization ratio for ionic liquid micelles was found to be significantly higher when compared to that of ionic surfactants of similar chain length. The incorporation of the hydrocarbon into the micelle affects also an increase of the aggregation number. ■ INTRODUCTION To exemplify the technological relevance of ionic liquids (ILs), the number of patent applications involving these compounds has risen markedly over the past decade (∼22% increase yearly from patents.google.com by searching for “ionic liquids” each year since 2000). This interest is a result of the low melting point, high electric conductivity, negligible vapor pressure, tunability, among many other properties. Due to the inherent charge on the hydrophilic head group and hydrophobic alkyl substituent in the cation and/or anion, a number of ILs have shown to be surface-active (Figure 1). Thereby, they have been shown to self-assemble into micelles in aqueous solutions. 1-3 Formation of IL micelles in aqueous solutions was recently studied in detail using both experimental and computational methods. 1-7 Cations and anions have been shown to influence bulk solution properties by factors such as hydrophobicity, degree of association between ion and counterion, and type of counterion. 5,8 An important property of aqueous micellar solutions is the ability to increase the solubility of organic solutes with a wide range of polarities and degrees of hydrophobicity. 9 Although the self-organization of imidazolium ILs has been studied for some time, very little is actually known about the solubility enhancement of hydrophobic organic solutes in IL micelles. This enhanced solubility in micellar solutions is a result of the partitioning of the compound between water and the micellar phase. The reason for this phenomenon is that the interior of the micelles acts as a hydrophobic environment in which the hydrophobic molecule may be solubilized. 10 The type of interaction occurring between surfactant and solubilizate determines the locus of the solubilization phenomenon, for example, at the micellar interface, in between the hydrophilic head groups, between the hydrophilic head group and the first few carbon atoms of the hydrophobic fragment (also commonly referred to as the palisade layer), and finally in the core of the micelle. 9 Therefore, generally, solutes with polar functional groups are mainly solubilized in the outer region of the micelle, whereas nonpolar compounds are preferentially located in the inner portions of the micelles. 11 The locus and extent of solubility enhancement are determined by the type and strength of interaction. For example, nonionic surfactants, due to their lack of charge-charge repulsion, can form much lager micelles, which in turn can be more effective solubility enhancers. The solubilization of hydrophobic or partly hydrophobic molecules in aqueous micellar solutions plays an important role in many technological and biotechnological processes. The use of micellar systems in separation science is of increasing importance, for example, in micellar-enhanced ultrafiltration, micellar extraction, micellar liquid chromatography, and Received: November 13, 2012 Revised: April 9, 2013 Published: April 9, 2013 Figure 1. The structure of 1-alkyl-3-methylimidazolium ILs [C n MIM]- [Cl], where R is the alkyl substituent with chain length C 8 ,C 10 ,C 12 , and C 14 . Article pubs.acs.org/JPCB © 2013 American Chemical Society 5653 dx.doi.org/10.1021/jp3112205 | J. Phys. Chem. B 2013, 117, 5653-5658