Structure and Dynamics of Benzene Confined in Silica Nanopores

Abstract

The structure and dynamics of benzene confined at 293 K in silica nanopores of different diameters (D = 2.0 nm and D = 3.6 nm) are investigated by means of grand canonical Monte Carlo and molecular dynamics simulations. In order to account in a realistic way for the interactions between benzene and the silica surface, we consider a recent model that accounts for the π-electrons of the aromatic cycle in the benzene molecule. Confined benzene exhibits significant layering and orientational ordering in the vicinity of the silica surface (up to two adsorbed layers) and tends to recover its bulk properties in the pore center. Using suitable order parameters, we show that benzene molecules close to the pore surface tend to have their ring lying flat on the silica surface (and hence perpendicular to the pore axis). Such a preferential parallel orientation with respect to the silica surface suggests that a proper description of the π-electrons of the benzene aromatic ring and its specific Coulombic interaction with the partial charges carried by the silica atoms is crucial. The dynamics of benzene confined in the silica nanopores is always slower than in the bulk. Both the translational and rotational dynamics of confined benzene can be described as a bulklike contribution in the pore center that depends on the pore size and a surface contribution that is nearly insensitive to the pore size. These simulation results are discussed in the light of available experimental data on the structure and dynamics of benzene confined in nanoporous silicas.