Direct Observation of Different Mechanisms for the Inhibition of Molecular Reorientation at a Solid/Liquid Interface

Abstract

Optical Kerr effect spectroscopy has been used to study the collective orientational dynamics of 2-butyne confined in nanoporous glasses at 293 K. The high polarizability anisotropy of this weakly wetting liquid has allowed us to monitor its reorientational dynamics in nanoconfinement with a high signal-to-noise ratio for many tens of picoseconds. In all pore sizes, the decays can be fit to the sum of three exponentialsone with a time constant equal to that of the decay in the bulk liquid, one with twice this time constant, and one with a considerably larger time constant. The fastest decay arises from the liquid molecules in the centers of the pores. The intermediate decay is attributed to surface molecules that rotate off of the pore walls, and the factor of 2 change in the decay time is exactly that expected on the basis of the difference in the hydrodynamic volume required for this reorientation as compared to that of the bulk. The slowest decay is attributed to the inhibition of reorientation along t...