Nuclear magnetic resonance study of the vapor phase contribution to diffusion in nanoporous glasses partially filled with water and cyclohexane

The contribution of the vapor phase to the effective diffusion coefficient of solvents in a partially filled nanoporous silica glass (Vycor) was investigated with the aid of nuclear magnetic resonance diffusometry. Water and cyclohexane as polar and nonpolar adsorbate species show opposite dependencies on the filling factor. The effective diffusion coefficient of water increases with the filling factor, whereas that of cyclohexane decreases. A "two-phase fast-exchange model theory" is presented accounting for these phenomena. In the vapor phase, both Knudsen and ordinary Einstein diffusion were considered. The different dependencies on the filling factor for polar and nonpolar adsorbate species are attributed to different effective tortuosities represented by different exponents in Archie's law anticipated in the model theory. The fast exchange feature stipulates that the pore size is small enough. (C) 2003 American Institute of Physics.