Capillary Condensation and Surface Flow in Microporous Vycor Glass

Depending on the size of the pores and on the temperature and pressure of the fluid, flow in microporous materials can be through gas diffusion, surface diffusion, and viscous liquid flow driven by capillary forces or by hydraulic pressure. We have studied these flow regimes with toluene in untreated Vycor glass with a pore radius of 31 Å and Vycor glass with a pore radius of 21 Å whose pores were derivatized with C₁₈H₃₈. The observed increase in viscosity with decreasing pore size is ascribed to the attractive interaction of the monolayer adsorbed on the pore walls. The surface diffusion coefficient at 22°C is 5 × 10⁻⁵ cm² s⁻¹ for both the derivatized and underivatized Vycor, with an activation energy of 3 kcal in the temperature range 22–100°C.