Molecular dynamics simulations of carbon dioxide and water at an ionic liquid interface.

The ionic liquid-carbon dioxide system is of interest because ionic liquids (ILs) have potential to be used for carbon dioxide capture. Because water will be present in a real carbon dioxide capture operation, the interaction between water and the IL is also of interest. Classical molecular dynamics simulations have been used to study the IL 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmim-Tf(2)N) at the interface with vacuum as well as with carbon dioxide, water, or both present in the system. Density and orientational ordering of the ionic liquid molecules at the interface was not significantly altered by the presence of either carbon dioxide or water. The structure of the liquid solutions in the pseudobulk region in the center of the slab was studied using metrics of pairwise association such as radial distribution functions. At the interface, there is an increased density of cation-anion association. When carbon dioxide is present, it forms a dense layer on the surface, and the cation-anion associations at the interface are disrupted, with a corresponding decrease in surface tension. Water slows down the diffusion of the ions as well as carbon dioxide in the bulk. Water has little effect, however, on the interfacial transport dynamics of carbon dioxide.