Density functional theory of confined ionic liquids: A survey of the effects of ion type, molecular charge distribution, and surface adsorption

Ionic liquids confined between two planar charged walls are explored using density functional theory. The effort represents a study of the effects of the molecular structure, molecular charge distribution, and degree of surface adsorption on forces between the surfaces and on the inhomogeneous atom density profiles. Surface adsorption was found to significantly affect both the magnitude and sign of the surface forces, while differences in the distribution of molecular charge did not. On the other hand, different bulk densities were found to produce dramatically different surface forces indicating a difference in the degree of molecular packing at and near surfaces. No long-range forces were found in any of the cases considered. We conclude that in the absence of any specific cation-anion pairing, surface charges are effectively screened, and the surface forces are dominated by short ranged steric and dispersion interactions between adsorbed molecular layers. In many cases, very similar surface forces correspond to very different molecular arrangements, suggesting that unambiguous interpretation of measured surface forces in ionic liquids, in terms of molecular behavior, may be difficult to guarantee.