Molecular dynamics simulations of aqueous solutions of ionic liquids
We performed a molecular dynamics study of aqueous solutions of ionic liquids 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-octyl-3-methylimidazolium chloride ([OMIM]Cl) in order to elucidate the anomalous dependence of the surface tension on concentration. We found that cations are placed preferably at the surface with alkyl chains pointing towards vacuum. Anions compensate the surplus of a positive charge on the surface by forming a layer below the cation layer. At lower concentrations the surface tension decreases with concentration. At higher concentrations the surface becomes saturated by cations and the decrease slows down. The decrease of surface tension with concentration is a consequence of the structural changes, although the experimentally observed anomalous increase at even higher concentrations was not reproduced.