Surface diffusion in liquid‐filled pores

Abstract

Abstract Surface diffusivities of benzaldehyde in liquid‐filled pores of Amberlite particles (polystyrene) were measured at 25°C using water as a solvent. For particles of different surface areas but chemically similar pore surfaces, the intrinsic surface diffusivity D s ' was about the same, but the relative importance of surface to pore‐volume diffusion increased with surface area. For a single type of particle, the adsorption capacity was decreased about twenty‐fold by adding up to 19 mole % methanol to the solvent. This was accompanied by an increase in D s ' from 1.2 × 10 −8 to 1.2 × 10 −7 cm 2 /s. These results were interpreted in terms of a two‐step theory for surface migration: (1) formation of a vacant site on the adsorbent surface followed by (2) movement of the adsorbate molecule into the site by breaking the surface‐adsorbate bond. The theory predicts that surface transport will be large when the surface area is high and that the D s ' will be large when the heat of adsorption is low, and when the bond between solvent molecules and the surface is weak. In our studies the surface contribution to intraparticle transport was as much as 20 times the contribution due to pore‐volume diffusion. This ratio increases as the concentration of adsorbate in the liquid decreases.