Interfacial layering of a room-temperature ionic liquid thin film on mica: a computational investigation

The structure of a thin (4 nm) [bmim][Tf2N] film on mica has been studied b y molecular dynamics simulations using an empirical force field. Interfacial layering at T=300 K and at T=350 K is investigated by determining the number- and charge-density profiles of [bmim][Tf2N] as a function of distance from mica, and by computing the normal force Fz opposing the penetration of the ionic liquid film by a spherical nanometric tip interacting with [bmim][Tf2N] atoms by a short-range potential. The results show that layering is important but localised within ~1 nm from the interface. The addition of a surface charge on mica, globally neutralised by an opposite charge on the [bmim][Tf2N] side, gives rise to low-amplitude charge oscillations extending through the entire film. However, outside a narrow interfacial region, the resistance of the [bmim][Tf2N] film to penetration by the mesoscopic tip is only marginally affected by the charge at the interface. The results obtained here for [bmim][Tf2N]/mica are similar to those obtained using the same method for the [bmim][Tf2N]/silica interface, and agree well with experimental force-distance profiles measured on the latter interface at ambient conditions.