We performed a detailed molecular dynamics study of the interfacial structure of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate in order to explain the anomalous dependence of the surface tension on concentration. Both nonpolarizable and polarizable force fields have been employed. The initial decrease of surface tension was confirmed for both systems, while the increase at higher bulk concentrations was observed only for the more hydrophilic tetrafluoroborate anion. Propensity of butyl chains toward vacuum, the preferably parallel orientation of imidazolium rings, and the local balance of cations and anions at the surface layer were observed in accordance with experiment. Most surface phenomena are more pronounced if polarizability is taken into account.