Effect of spacer chain length on the liquid structure of aqueous dicationic ionic liquid solutions: molecular dynamics studies

The liquid structure of aqueous solutions of five different imidazolium based gemini dicationic ionic liquids 1,n-bis(3-methylimidazolium-1-yl) alkane bromide (n being the length of the spacer alkyl chain), with propyl, pentyl, octyl, decyl and hexadecyl spacer chains has been studied using atomistic molecular dynamics simulations. While solutions with propyl and pentyl spacers are homogeneous, those with octyl and decyl spacers show spatial heterogeneity. Microscopic inhomogeneity in the bulk solution phase increases with an increase in the length of the spacer chain leading to polydisperse aggregates in the solution with a hexadecyl spacer. Organization of the cations at the solution-vapor interface also depends upon the length of the spacer chain with the most organized interfacial layer observed in the solution with a hexadecyl spacer chain.