Enhanced self-diffusion of adsorbed methanol in silica aerogel

Abstract

Molecular transport of a two-component system of liquid and vapor in a porous medium can be anomalously increased owing to fast exchange between the two phases. 1 We have investigated this phenomenon measuring the self-diffusion coefficient of methanol adsorbed in a 98% porosity aerogel using nuclear magnetic resonance (NMR) field gradient techniques. We found enhancement of several orders of magnitude from which we determined the ballistic mean-free-path in the vapor phase. We have grown globally uniform anisotropic aerogels and applied the diffusion measurements to characterize the anisotropy. Our results are important for understanding the novel properties of superfluid 3 He confined within an aerogel framework and for application to other physical systems.

Helium-3 gas self-diffusion in a nematically ordered aerogel at low temperatures: enhanced role of adsorption

“…At high pressures the effect of the 4 He adsorbed layer on the 3 He diffusion is not significant and therefore D(P) coincides with that of pure 3 He in the aerogel. Similarly to our result, Lee et al 11 reported the observation of the ''anomalous'' diffusion of methanol in aerogels due to the effect of adsorbed layers in the aerogels at room temperatures. Following ref.…”

“…7) even at very low pressures where diffusion is expected to be strongly anisotropic in the Knudsen regime due to anisotropy of the aerogel. Thus we must conclude that the correctness of the simple models application used by Lee et al 11 and Mueller et al 10 is doubtful. The observed diffusion of 3 He gas in the aerogel is significantly more suppressed by adsorption than expected.…”

“…Nacher and G. Tastevin 8 who measured hyperpolarized gas diffusion in various silica isotropic aerogels for a broad range of pressures and at room temperature with a negligible adsorbed layer effect due to high temperature. Coming back to Lee's article we should say that the described analysis technique 11 is very critical to the determination of the gas and the adsorbed layer densities. The described l anisotropy is highly probable to appear due to the methanol density uncertainty in experiments and is not connected with the properties of the aerogel geometry.…”

“…However in that case the adsorption should be taken into account. 9,10 Lee et al 11 reported diffusion experiments of methanol gas significantly adsorbed in slightly anisotropic aerogel samples at room temperature with the aim to characterize the mfp for subsequent studies of superfluid 3 He in this aerogel. An empirical model based on ''fast exchange '' 4 that accounts for the adsorbed layer was applied to determine the gas diffusion in the aerogel, which was assumed to be in the Knudsen diffusion regime.…”

“…3,7 The aims of our study are to examine empirical diffusion models likewise applied by Lee et al 11 and Mueller et al 10 at various conditions and to assess the ordered aerogel structure. This is the first report on low temperature 3 He gas diffusion measurements in nanostructures to our knowledge.…”