Sorption-desorption hysteresis in glassy silica gel and fibrous silica gel (Santocel C) with water, carbon tetrachloride, normal aliphatic alcohols

Abstract

By means of the quartz fiber spring technique, sorption-desorption hysteresis has been studied on glassy and fibrous silica gels with water, carbon tetrachloride, methyl, ethyl, n-propyl, n-butyl, and n-amyl alcohols as sorbates at 35°C. Permanent and reproducible hysteresis loops are obtained in every one of these cases. At the end of the first cycle of sorption and desorption, glassy silica gel retains irreversibly certain amounts of the sorbates excepting carbon tetrachloride. The bound alcohols and water are attributed to the polar character of the molecules and the presence of micropores in the gel. The isotherms of the five alcohols on the two gels show clearly defined “knees.” By the application of the BET theory, the specific surface areas are found out, assuming alcohol molecules to be spherical and also linear with its two modes of sorption, perpendicular and parallel to the surface. The results indicate the existence of oriented sorption, perpendicular to the surface, on fibrous silica gel. Cohan's theory of hysteresis has been applied by calculating the molecular diameter D from the point of inception of the hysteresis loop. The values obtained are higher than those of D for the spherical case. The cavity theory can explain in a qualitative way the hysteresis loops obtained in all the cases. Pore-size distribution and the predominant neck and body radii of the cavities in the two gels have been determined. The results indicate the existence mainly of micro- and transitional pores in glassy silica gel and of macropores in fibrous silica gel. The pores in the former gel are all ink bottle shaped, and in the latter they are cylindrical. The shapes of the isotherms of the alcohols on fibrous silica gel indicate an increase in contact angle from methyl to n-amyl alcohol on the gel surface.