Evidence for the direct observation of molecular exchange of a liquid at the solid/liquid interface

Optical Kerr effect spectroscopy has been used to monitor the orientational dynamics of acetonitrile in the bulk and confined in nanoporous silicate glasses. While the orientational decay in the bulk liquid is described by a single exponential, the confined liquid decays are best described by the sum of three exponentials, the fastest of which has a decay time matching the bulk liquid. On the basis of the activation energies determined from the temperature dependence of the three decay times, we suggest that the slowest exponential arises from orientational relaxation at the solid/liquid interface, while the intermediate exponential results from molecular exchange between the surface population and the bulk liquid.