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Acetonitrile on silica surfaces and at its liquid-vapor interface: structural correlations and collective dynamics

Solvent structure and dynamics of acetonitrile at its liquid-vapor (LV) interface and at the acetonitrile-silica (LS) interface are studied by means of molecular dynamics simulations. We set up the interfacial system and treat the long-ranged electrostatics carefully to obtain both stable LV and LS interfaces within the same system. Single molecule (singlet) and correlated density orientational profiles and singlet and collective reorientational dynamics are reported for both interfaces. At the LS interface acetonitrile forms layers. The closest sublayer is dominated by nitrogen atoms bonding to the hydrogen sites of the silica surface. The singlet molecular reorientation is strongly hindered when close to the silica surface, but at the LV interface it relaxes much faster than in the bulk. Antiparallel correlations between acetonitrile molecules at the LV interface are even stronger than in the bulk liquid phase. This strong antiparallel correlation disappears at the LS interface. The collective reorientational relaxation of the first layer acetonitrile is much faster than the singlet reorientational relaxation but it is still slower than in the bulk. These results are interpreted with reference to a variety of recent experiments. We found that defining interface properties based on the distribution of positions of different choices of atoms or sites within the molecule leads to apparently different orientational profiles, especially at the LV interface. We provide a general formulation showing that this ambiguity arises when the size of the molecule is comparable to the interfacial width and is particularly significant when there is a large difference in density at the upper and lower boundaries of the interface. We finally analyze the effect of electrostatics to show the necessity of properly treating long-ranged electrostatics for simulations of interfacial systems.