Ionic liquid-based reverse micelles. Use of hydrogen–deuterium exchange as a tool for surfactant self-assembly characterization

Abstract

We report the study of the rate for hydrogen–deuterium (H-D) exchange in the cation of the ionic liquid-surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) when it forms reverse micelles (RMs). The results were compared with those found for bmim-AOT dissolved in D2O showing an inhibition larger than 500-fold on going from bulk D2O to the reverse micelle. Also, the effect of varying water content and surfactant concentration of the RMs on the deuterium exchange was evaluated by 1H NMR technique. The results demonstrate that the H-D exchange at the C(2) position of bmim+ when it forms RMs significantly reflects the properties of the micellar microenvironment, specifically of the micellar interface due to the localization of bmim+ in the interfacial region, intercalated between the AOT moiety. The rate of deuterium exchange is directly related to the properties of interfacial water, which, in turn, is closely related to the water-surfactant interaction. Thus, the analysis of deuterium exchange in the 2-position of bmim+ is proposed as a useful tool to explore the properties of RMs formed by this cation, with the advantage of being a non-invasive technique since the ionic liquid-surfactant itself acts as the molecular probe.