Dielectric spectroscopy of confinement effects in polar materials

Abstract

Spatial confinement on a nm scale can affect molecular dynamics changing form, strength, and frequency of relaxation processes. Dielectric spectroscopy can reveal confinement effects, however, an effective medium analysis has to remove the effects of dielectric heterogeneity. Depending on the microstructure they cause remarkable discrepancies between the intrinsic and measured effective properties. Here, the theoretical response of various three-dimensional (3D), 2D, and 1D confinements is analyzed: molecules in dispersed droplets, noncrossing or interconnected channels, and films. Experimental data on the $\alpha$ relaxation of confined propylene glycol reflect both the microstructure and the molecular reorientation, but effects of dielectric heterogeneity can be separated from finite-size and surface effects. While randomly distributed nanodroplets correspond to a well-defined 3D confinement, porous Gelsil glass exhibits a transition from a 2D to a 2D-3D topology with decreasing porosity and pore size. Nonuniform interaction with the surface or surface attached molecules essentially affects only the shape of the $\alpha$ process and results in a broadening. The relaxation of the confined liquid becomes faster and its glass transition temperature is lowered. The diameter above which the finite-size effect vanishes does not depend on geometrical details or chemical nature of the confinement and characterizes the dynamics of the liquid.