Gas transport in porous Vycor glass subjected to gradual pore narrowing

Abstract

Porous Vycor® glass was modified by deposition of silica on the internal pore surface using consecutive cycles of liquid-phase silylation with silicon tetrachloride, and hydrolysis. Macroscopically uniform deposition was achieved by exploiting the self-limiting nature of the reaction and the extent of deposition was monitored by the weight change of the samples. Weight increases as high as 24% were recorded and the average pore diameter was estimated to decrease from ∼44 Å to ∼20 Å. Permeation measurements were conducted in the Henry’s law region at various levels of deposition for hydrogen, methane, isobutane and nitrogen, at temperatures between 60°C and 180°C. The measurements were compared to values calculated with a model using the effective medium approximation to treat network effects and Clausing’s correction to account for conductances in pores of finite aspect ratio. The calculated values proved to be inaccurate for hydrogen, overestimating the permeance by a factor of 2 at high levels of deposition possibly because of non-ideal pore shapes accentuated by the deposition. For nitrogen and methane the agreement between calculations and measurements was better due to a fortuitous cancellation of deviations caused by the enhanced potential energy well within the pores and the non ideal pore shape. The intrapore potential energy effect was especially strong for isobutane and as a result the calculated flux was always less than the experimental.