Aggregation behavior of a chiral long-chain ionic liquid in aqueous solution

Abstract

The synthesis of a chiral long-chain ionic liquid (IL), S-3-hexadecyl-1-(1-hydroxy-propan-2-yl)-imidazolium bromide ([C(16)hpim]Br), is presented. The adsorption and aggregation of this surface active IL in aqueous solution is described. The critical micelle concentration (cmc) measurement suggests that the chiral IL has superior capacity for micelle formation compared to traditional ionic surfactants. The relatively larger hydrophilic head group of the IL results in a larger maximum surface excess concentration (Gamma(max)) and a smaller minimum molecular cross-sectional area (A(min)). Electrical conductivity studies show a small degree of counterion binding to these micelles, which may increase the electrostatic repulsions between the hydrophilic heads of adjacent surfactant molecules. Both factors of the hydrophilic headgroup size and electrostatic repulsion in [C(16)hpim]Br micelles lead to a looser packing of the surfactant molecules in the micelles. As a result, a higher micropolarity and smaller mean aggregation number is observed. Moreover, the looser micellar packing of the [C(16)hpim]Br molecules results in a unusual upfield shift of the proton NMR signals in the hydrophobic chains after micelle formation. (1)H NMR and 2D ROESY spectroscopic analyses confirm a chiral arrangement of the micelles. Chiral IL micelles may have potential applications in the stereochemical recognition of surfaces or of biological structures.