Pseudo-restricted self-diffusion of molecules in biporous structures: Study by pulsed field gradient NMR

The mobility of liquid (n-decane) that fills the system of primary and secondary pores of a biporous sample (granular Vycor porous glass) is studied by the pulsed field gradient NMR technique. The anomalous time dependence of the slow component of diffusion decay is revealed: the self-diffusion coefficient decreases with an increase in diffusion time t d and, at large t d values, this dependence satisfies the feature of completely restricted self-diffusion. It is established that this component is related to the mobility of liquid molecules filling the system of primary pores. By use of computer simulation, it is shown that the effect of "pseudo- restricted" diffusion is explained by the exchange processes between the phases, where the molecules of the liquid that are present in the systems of primary and secondary pores and differ in self-diffusion coefficients are understood as the phases. The effect of interfacial exchange is confirmed by the time dependence of the fraction of molecules with the lowest self-diffusion coefficients. The revealed phenomenon of "pseudo-restricted" diffusion is not related to real spatial constraints and can be observed in any systems with two (or more) phases with different self-diffusion coefficients, provided that these "phases" are bulky and can be subjected to molecular exchange. © 2005 Pleiades Publishing, Inc

Surface self-diffusion of organic molecules adsorbed in porous silicon

The pulsed field gradient nuclear magnetic resonance method has been employed to probe self-diffusion of organic guest molecules adsorbed in porous silicon with a 3.6 nm pore size. The molecular self-diffusion coefficient and intrapore adsorption were simultaneously measured as a function of the external vapor pressure. The latter was varied in a broad range to provide pore loading from less than monolayer surface coverage to full pore saturation. The measured diffusivities are found to be well-correlated with the adsorption isotherms. At low molecular concentrations in the pores, corresponding to surface coverages of less than one monolayer, the self-diffusion coefficient strongly increases with increasing concentration. This observation is attributed to the occurrence of activated diffusion on a heterogeneous surface. Additional experiments in a broad temperature range and using binary mixtures confirm this hypothesis. © 2005 American Chemical Society

Concentration-dependent self-diffusion of adsorbates in mesoporous materials

The pulsed-field gradient NMR method has been applied to study self-diffusion of liquids in mesoporous materials with different pore sizes and morphologies as a function of pore loading. It is found that the effective diffusivities of adsorbate molecules in mesopores at partial loadings are related to two mechanisms, the Knudsen diffusion through the gaseous phase in the pore space and the diffusion within the layer of molecules adsorbed on the pore walls. The relative contributions of these modes, which are determined by the details of the interphase equilibrium, change with variation of the pore loading, leading to a complex behavior of the effective self-diffusion coefficient. The impact of the pore size and the adsorbate-surface interaction on self-diffusion is elucidated. Possible reasons for an experimentally obtained hysteresis in the diffusivities measured on adsorption and desorption in mesopores are discussed. © 2005 Elsevier Inc. All rights reserved

Pseudo-restricted self-diffusion of molecules in biporous structures: Study by pulsed field gradient NMR

The mobility of liquid (n-decane) that fills the system of primary and secondary pores of a biporous sample (granular Vycor porous glass) is studied by the pulsed field gradient NMR technique. The anomalous time dependence of the slow component of diffusion decay is revealed: the self-diffusion coefficient decreases with an increase in diffusion time t d and, at large t d values, this dependence satisfies the feature of completely restricted self-diffusion. It is established that this component is related to the mobility of liquid molecules filling the system of primary pores. By use of computer simulation, it is shown that the effect of "pseudo- restricted" diffusion is explained by the exchange processes between the phases, where the molecules of the liquid that are present in the systems of primary and secondary pores and differ in self-diffusion coefficients are understood as the phases. The effect of interfacial exchange is confirmed by the time dependence of the fraction of molecules with the lowest self-diffusion coefficients. The revealed phenomenon of "pseudo-restricted" diffusion is not related to real spatial constraints and can be observed in any systems with two (or more) phases with different self-diffusion coefficients, provided that these "phases" are bulky and can be subjected to molecular exchange. © 2005 Pleiades Publishing, Inc

Pseudo-restricted self-diffusion of molecules in biporous structures: Study by pulsed field gradient NMR

The mobility of liquid (n-decane) that fills the system of primary and secondary pores of a biporous sample (granular Vycor porous glass) is studied by the pulsed field gradient NMR technique. The anomalous time dependence of the slow component of diffusion decay is revealed: the self-diffusion coefficient decreases with an increase in diffusion time t d and, at large t d values, this dependence satisfies the feature of completely restricted self-diffusion. It is established that this component is related to the mobility of liquid molecules filling the system of primary pores. By use of computer simulation, it is shown that the effect of "pseudo- restricted" diffusion is explained by the exchange processes between the phases, where the molecules of the liquid that are present in the systems of primary and secondary pores and differ in self-diffusion coefficients are understood as the phases. The effect of interfacial exchange is confirmed by the time dependence of the fraction of molecules with the lowest self-diffusion coefficients. The revealed phenomenon of "pseudo-restricted" diffusion is not related to real spatial constraints and can be observed in any systems with two (or more) phases with different self-diffusion coefficients, provided that these "phases" are bulky and can be subjected to molecular exchange. © 2005 Pleiades Publishing, Inc

Pseudo-restricted self-diffusion of molecules in biporous structures: Study by pulsed field gradient NMR

The mobility of liquid (n-decane) that fills the system of primary and secondary pores of a biporous sample (granular Vycor porous glass) is studied by the pulsed field gradient NMR technique. The anomalous time dependence of the slow component of diffusion decay is revealed: the self-diffusion coefficient decreases with an increase in diffusion time t d and, at large t d values, this dependence satisfies the feature of completely restricted self-diffusion. It is established that this component is related to the mobility of liquid molecules filling the system of primary pores. By use of computer simulation, it is shown that the effect of "pseudo- restricted" diffusion is explained by the exchange processes between the phases, where the molecules of the liquid that are present in the systems of primary and secondary pores and differ in self-diffusion coefficients are understood as the phases. The effect of interfacial exchange is confirmed by the time dependence of the fraction of molecules with the lowest self-diffusion coefficients. The revealed phenomenon of "pseudo-restricted" diffusion is not related to real spatial constraints and can be observed in any systems with two (or more) phases with different self-diffusion coefficients, provided that these "phases" are bulky and can be subjected to molecular exchange. © 2005 Pleiades Publishing, Inc

Concentration-dependent self-diffusion of liquids in nanopores: A nuclear magnetic resonance study

The details of molecular motion of low-molecular-weight polar and nonpolar organic liquids in nanoporous silicon crystals were studied using nuclear magnetic resonance. The effective self-diffusion coefficients were found to pass through a maximum with increasing concentrations for all liquids. A generalized model for the effective self-diffusion coefficient was developed by taking into account the concentration dependent coexistence of capillary condensed, adsorbed, and gaseous phases. The applicability of the mesoporous range in the model was extended by the explicit use of the adsorption isotherm properties, and this highlighted the role of surface interaction for the transport of molecules in small pores

Concentration-dependent self-diffusion of liquids in nanopores: A nuclear magnetic resonance study

The details of molecular motion of low-molecular-weight polar and nonpolar organic liquids in nanoporous silicon crystals were studied using nuclear magnetic resonance. The effective self-diffusion coefficients were found to pass through a maximum with increasing concentrations for all liquids. A generalized model for the effective self-diffusion coefficient was developed by taking into account the concentration dependent coexistence of capillary condensed, adsorbed, and gaseous phases. The applicability of the mesoporous range in the model was extended by the explicit use of the adsorption isotherm properties, and this highlighted the role of surface interaction for the transport of molecules in small pores

Concentration-dependent self-diffusion of adsorbates in mesoporous materials

The pulsed-field gradient NMR method has been applied to study self-diffusion of liquids in mesoporous materials with different pore sizes and morphologies as a function of pore loading. It is found that the effective diffusivities of adsorbate molecules in mesopores at partial loadings are related to two mechanisms, the Knudsen diffusion through the gaseous phase in the pore space and the diffusion within the layer of molecules adsorbed on the pore walls. The relative contributions of these modes, which are determined by the details of the interphase equilibrium, change with variation of the pore loading, leading to a complex behavior of the effective self-diffusion coefficient. The impact of the pore size and the adsorbate-surface interaction on self-diffusion is elucidated. Possible reasons for an experimentally obtained hysteresis in the diffusivities measured on adsorption and desorption in mesopores are discussed. © 2005 Elsevier Inc. All rights reserved