The ionic liquid-surfactant bmim-AOT and nontoxic lipophilic solvents as components of reverse micelles alternative to the traditional systems. A study by 1H NMR spectroscopy

Abstract

Aqueous reverse micelles (RMs) of the ionic liquid-surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) in the biocompatible nonpolar solvents isopropyl myristate (IPM) and methyl laurate (ML) were formulated and characterized by DLS and 1H NMR techniques; the goal is to develop less toxic and more environmentally friendly reverse micellar systems than traditional RMs formed by organic solvents such as n-heptane (n-Hp). The interfacial composition of these new micellar systems was compared with water/bmim-AOT/n-Hp RMs, analyzing the effect of changing the conventional external nonpolar solvent n-Hp for the biocompatible solvents IPM and ML. Also, they were compared with the RMs of the traditional surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) in IPM and ML to analyze the role of the bmim+ counterion compared to Na+ in the interfacial properties of the generated RMs. The results indicate that both ML and IPM can be used as nonpolar solvents to prepare RMs using bmim-AOT as surfactant. In comparison to n-Hp, IPM and ML penetrate more to the micellar interface, therefore, the ester group of the biocompatible solvent located at the micellar interface interacts with the bmim+ cation. As a consequence of this fact, the bmim+-AOT− interaction decreases when IPM or ML are used as the external solvent instead of n-Hp. In the Na-AOT/IPM and Na-AOT/ML systems, the dissimilar penetration of the external solvent makes the interfacial properties of both RMs clearly different. Interestingly, in the bmim-AOT/biocompatible solvent systems, the micellar interface properties are strongly governed by the bmim+ counterion, finding similar behavior if the external biocompatible solvent is IPM or ML.