Time-Dependent Molecular Diffusion in Partially Filled Porous Glasses with Heterogeneous Structure

Abstract

Nuclear magnetic resonance (NMR) microscopy of silica glasses with micrometer pores (Vitrapor#5) partially filled with water or cyclohexane reveals a heterogeneous distribution of liquid on a length scale much longer than the pore dimension. This heterogeneity, which is not observed in MR imaging of saturated samples, is attributed to the spatial variation of the granular microstructure visible in scanning electron micrographs. As a consequence of an inhomogeneous filling degree, the effective transverse relaxation time varies, which in turn leads to NMR imaging contrasts. Since the spatial distribution of the transverse relaxation time prevents reliable measurements with a standard pulsed gradient stimulated echo technique, a combination of the fringe field stimulated echo method, on the one hand, and the magnetization grid rotating-frame imaging technique, on the other, was employed. Four decades of the diffusion time from 100 ms to 1 s can be covered on this basis. The data were compared with Monte Carlo simulations of a model structure showing a qualitatively equivalent behavior in the common time window. The self-diffusion in partially filled porous systems is known to be strongly affected by a vapor phase. Here we have shown that the vapor phase contribution to the effective diffusivity is particularly efficient on a diffusion time scale corresponding to mean-squares displacements of the order of the pore dimension.