Polarizability anisotropy relaxation in nanoconfinement: molecular simulation study of water in cylindrical silica pores.

We report the results of a molecular simulation study of polarizability anisotropy relaxation for water confined in approximately cylindrical silica pores, with diameters in the range from 20 to 40 Å. In our calculations, we use a polarizability model that includes molecular and interaction-induced components. In agreement with optical Kerr effect experimental data, we find strong confinement effects on the relaxation rate of water polarizability anisotropy. Given that water molecular polarizability anisotropy is small, much of the intensity of the polarizability anisotropy response comes from the interaction-induced component. However, we find that, at longer times, the relaxation properties of this component strongly resemble those of collective reorientation, the mechanism by which the molecular polarizability anisotropy relaxes. We also find that the relevant collective orientational relaxation differs considerably from single molecule reorientation and that this difference varies with the extent of confinement. Our investigation of the effects of axial-radial pore anisotropy indicates that these effects play a minor role in water polarizability anisotropy relaxation in this pore diameter range.