Nuclear magnetic spin-lattice relaxation experiments have been performed in partially filled porous glasses with wetting and nonwetting fluids. The frequency dependence of the spin-lattice relaxation rate in Vycor (4 nm pores) and VitraPOR #5 (1 microm pores) silica glasses was studied as a function of the filling degree with the aid of field-cycling NMR relaxometry. The species of primary interest were water ("polar") and cyclohexane ("nonpolar"). Spin-lattice relaxation was examined in the frequency range from 1 kHz to 400 MHz with the aid of a field-cycling NMR relaxometer and an ordinary 400 MHz NMR spectrometer. Three different mobility states of the fluid molecules are distinguished: The adsorbed state at the pore walls, the bulklike liquid phase, and the vapor phase. The adsorbate spin-lattice relaxation rate is dominated by the "reorientation mediated by translational displacements" (RMTD) mechanism taking place at the adsorbate/matrix interface at frequencies low enough to neglect rotational diffusion of the molecules. The experimental data are analyzed in terms of molecular exchange between the different mobility states. Judged from the dependence of the spin-lattice relaxation rates on the filling degree, limits for slow and fast exchange (relative to the RMTD time scale) can be distinguished and identified. It is concluded that water always shows the features of slow exchange irrespective of the investigated pore sizes and filling degrees. This is in contrast to cyclohexane which is subject to slow exchange in micrometer pores, whereas fast exchange occurs in nanoscopic pores. The latter case implies that the vapor phase contributes to molecular dynamics in this case at low filling degrees while it is negligible otherwise.
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.
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