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

Abstract

Nuclear magnetic resonance has been applied to study the details of molecular motion of low-molecular-weight polar and nonpolar organic liquids in nanoporous silicon crystals of straight cylindrical pore morphology at different pore loadings. Effective self-diffusion coefficients as obtained using the pulsed field gradient nuclear magnetic resonance method were found to pass through a maximum with increasing concentration for all liquids under study. Taking account of a concentration-dependent coexistence of capillary condensed, adsorbed and gaseous phases a generalized model for the effective self-diffusion coefficient was developed and shown to satisfactorily explain the experimental results. An explicit use of the adsorption isotherm properties within the model extends its applicability to the mesoporous range and highlights the role of surface interaction for the transport of molecules in small pores. The problem of surface diffusion and diffusion of multilayered molecules is also addressed.

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…Our findings compare well with experimental results on the dynamics of "conventional" liquid condensates [8,84,[98][99][100][101][102][103] confined in nanoporous media. In particular, the observation of distinct core and shell molecular mobilities is also in good agreement with experimental studies on pore-confined liquid crystalline systems presented in the past [7, 58-66, 69, 70], most prominently with the work by Aliev et al discussed in the introduction.…”

Diffusion processes in mesoporous adsorbents probed by NMR

“…Being intrinsically related to the different density distributions at equal external conditions, this was expected to show up as well in the patterns of molecular propagation. Indeed, our first experiments with mesoporous silicon have shown that, in parallel to adsorption hysteresis, there is also the phenomenon of "diffusion hysteresis" (Naumov et al 2007;Valiullin et al 2004). The diffusivities measured on the adsorption and desorption branches as a function of the external gas pressure exhibit quite a complex behavior.…”

Diffusion in Porous Media

“…In Ref. 109, by relating p g to the pore filling θ , these dependencies are shown to be readily modeled on the basis of equation (24). It was thus demonstrated that, in line with beds of microporous host materials, 37 the dependency of the effective diffusivities on the degree of pore filling in mesoporous materials is as well determined by the particular shape of the sorption isotherms.…”