Dielectric spectroscopy of confinement effects in polar materials

Spatial confinement on a nm scale can affect molecular dynamics changing fo rm, strength, and frequency of relaxation processes. Dielectric spectroscop y can reveal confinement effects, however, an effective medium analysis has to remove the effects of dielectric heterogeneity. Depending on the micros tructure they cause remarkable discrepancies between the intrinsic and meas ured effective properties. Here, the theoretical response of various three- dimensional (3D), 2D, and 1D confinements is analyzed: molecules in dispers ed droplets, noncrossing or interconnected channels, and films. Experimenta l data on the alpha relaxation of confined propylene glycol reflect both th e 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 confinem ent, porous Gelsil glass exhibits a transition from a 2D to a 2D-3D topolog y with decreasing porosity and pore size. Nonuniform interaction with the s urface or surface attached molecules essentially affects only the shape of the alpha process and results in a broadening. The relaxation of the confin ed liquid becomes faster and its glass transition temperature is lowered. T he 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. [S0163-1829(99)04013-8].