Adsorption in relation to pore structures of silicas II. Water vapor adsorption on wide-pore and microporous silica gels

Abstract

Adsorption-desorption isotherms of water vapor at 35°C and of nitrogen at −196°C were measured on two silica gels which were precharacterized to contain only wide pores, and on one silica gel which contained only narrow pores. The silica gels were heat treated at 120°, 400°, and 1000°C. Specific surface areas were calculated by various methods, and pore-structure analysis could be performed with water vapor as adsorbate. The results were checked against those obtained from nitrogen. Wide-pore silica gels contained composite surfaces, i.e., partly hydrophilic, partly hydrophobic; but after the exposure to water vapor, particularly at about saturation pressure, they became hydrophilic. In such cases the total surface areas could be obtained from the desorption and not the adsorption isotherms. Reduction in the pore size was shown to increase the hydrophilicity of the surface, and the microporous silica gel proved to be totally hydrophilic. Tentative explanations are offered for these phenomena, and it has been shown that besides the hydroxyl content of the surface, the porous structure influences to a considerable degree the extent of adsorption interaction. In narrow pores it seems that dispersion forces are involved besides the hydrogen bonding.