Unilamellar Vesicle Formation and Microscopic Structure of Ionic Liquids in Aqueous Solutions

Imidazolium-based ionic liquid [C_nmim]­Br (n = 10, 12, 14) without any additives in aqueous solutions could form unilamellar vesicles was first observed by TEM recently. As molecular aggregation is a complex phenomenon which is difficult to study in detail experimentally, we performed molecular simulations for [C_nmim]Br (n = 10, 12, 14) aqueous solutions. The entire process of spontaneous aggregation for [C₁₂mim]Br into small unilamellar vesicle was elucidated. Radial distribution functions reveal that the strong spatial correlation between cation and anion still exists in the presence of a large amount of water. The inner layer of vesicle is packed denser than the outer layer by analyzing the radial density distribution. Furthermore, anions distribution provides the direct evidence for very little anions being dissolved in the water and verifies the experimental speculation. By analyzing hydrogen bond number and coordination number in the solution, it is implied that binding between counterions enhanced with increasing IL concentration, and the distribution density of ions in vesicle is close to the neat system. Moreover, it is observed that aggregation is facilitated with increasing the alkyl chains by comparing three aqueous solution systems. Additionally, spatial correlation between chain terminal C becomes stronger with increasing the alkyl chain length.