Using Molecular Simulation to Understand the Structure of [C<sub>2</sub>C<sub>1</sub>im]<sup>+</sup>–Alkylsulfate Ionic Liquids: Bulk and Liquid–Vapor Interfaces

Using molecular dynamics simulations we have studied the structure of alkylsulfate-based ionic liquids: 1-ethyl-3-methylimidazolium <i>n</i>-alkylsulfate [C<sub>2</sub>C<sub>1</sub>im]­[C<sub><i>n</i></sub>SO<sub>4</sub>] (<i>n</i> = 2, 4, 6 and 8). The structure of the different ionic liquids have been interpreted taking into account radial and spatial distribution functions, and structure factors, that allowed us to characterize the morphology of the polar and nonpolar domains present in this family of liquids. The size of the nonpolar regions depends linearly on the anion alkyl chain length. Furthermore, properties of the surface of ionic liquids, such as surface tension, ordering, and charge and density profiles, were studied using molecular simulation. We were able to reproduce the experimental values of the surface tension with a maximum deviation of 10%, and it was possible to relate the values of the surface tension with the structure of the liquid–vacuum interfaces. Microscopic structural analysis of orientational ordering at the interface and density profiles along the direction normal to the interface suggest that the alkyl chains of the anions tend to protrude toward the vacuum, and the presence of the interface leads to a strong organization of the liquid phase in the region close to the interface, stronger when the side-chain length of the anions increases.