Atomistic simulations of a solid/liquid interface: a combined force field and first principles approach to the structure and dynamics of acetonitrile near an anatase surface

The acetonitrile/anatase(101) interface can be considered a prototypical interface between an oxide and a polar aprotic liquid, and is common in dye sensitized solar cells. Using first principles molecular dynamics simulations of a slab of TiO2 in contact with neat acetonitrile (MeCN), the liquid structure near this interface has been characterized. Furthermore, in order to investigate properties that require extensive sampling, a classical force field to describe the MeCN/TiO2 interaction has been optimized, and we show that this force field accurately describes the structure near the interface. We find a surprisingly strong interaction of MeCN with TiO2, which leads to an ordered first MeCN layer displaying a significantly enhanced molecular dipole. The strong dipolar interactions between solvent molecules lead to pronounced layering further away from the interface, each successive layer having an alternate orientation of the molecular dipoles. At least seven distinct solvent layers (approximately 12 angstrom) can be discerned in the orientational distribution function. The observed structure also strongly suppresses diffusion parallel to the interface in the first nanometer of the liquid. These results show that the properties of the liquid near the interface differ from those in the bulk, which suggests that solvation near the interface will be distinctly different from solvation in the bulk.