Small‐angle Scattering Studies of Adsorption and of Capillary Condensation in Porous Solids

Abstract

AbstractSmall‐angle scattering (SAS) studies are reviewed of adsorption and capillary condensation of water, hydrocarbons and halogenated hydrocarbons near room temperature, and of nitrogen at 78 K in some mesoporous solids, mainly silicas. The theory needed for the interpretation of SAS data is briefly covered. Calculations of the scattered intensity I(q) for a model porous medium show that I(q) depends markedly on the film thickness t. Adsorption and capillary condensation of nitrogen at 78 K in mesoporous silicas was studied by use of in situ SANS, and t as function of the relative pressure P/Ps was estimated. Adsorption of N2 in defects within the silica skeleton at P/Ps<0.1 lead to a significant increase in I(q). Isolated vapor bubbles in capillary condensed nitrogen in a Gelsil® appeared on adsorption near saturation of the pore system. The kinetics of capillary condensation and of drainage were followed. Power law scattering at low q indicated the formation of ramified clusters of voids on drainage of liquid nitrogen from the xerogel Gelsil®. Similar clusters were observed on drainage of water from Vycor® glass. Provided the clusters indicate a percolation process, the desorption branch should not be used for the estimation of a pore size distribution for materials with networked pores. The adsorptive smoothing by benzene was observed of a rough interface in a controlled pore glass.