Self-Aggregation of Catanionic Surface Active Ionic Liquids in Aqueous Solutions

The self-aggregation behavior of catanionic surface active ionic liquids (SAIL), 1-alkyl-3-methylimidazolium alkyl sulfates, [C4mim][C16SO4] and [C6mim][C14SO4], in aqueous solutions was explored by several techniques. These results were analyzed together with the concerning data of [C8mim][C12SO4] reported previously. The feature of these SAIL is that the total carbon number of the hydrophobic chains is kept constant, while only the length of the two hydrocarbon chains varies. Because of the different intermolecular and intramolecular interactions, the hydrophobic interaction of asymmetrical SAIL is enhanced, which explains the minimal CMC value of [C4mim][C16SO4]. Thermal motion of hydrocarbon chains at the air–water interface leads to higher A min and lower Γmax values with the increasing asymmetry of two hydrophobic chains. The conductivity measurements reveal that the micellization process is spontaneous and entropy-driven in the studied temperature range. This work indicates that the symmetry of alkyl chains can influence the aggregation behavior of [C n mim][C m SO4] in aqueous solutions.