Adsorption on measured surfaces of vitreous silica

Abstract

Abstract The theoretical treatment of the phenomena of adsorption has been very largely based upon the results of experiments with charcoal, a material which is not chemically homogeneous, and about whose undoubtedly very complex physical structure little is known. The uncertainties thus arising are not rendered less formidable by the various empirical methods of "activation" to which the charcoal is usually subjected. Vitreous silica, on the contrary, has properties which seem to provide a nearly ideal foundation for adsorption experiments of theoretical significance. It is chemically inert and homogeneous, and its structure has been the subject of detailed and successful investigation. In common with that of charcoal the surface of powdered silica undergoes little change by sintering even on prolonged exposure to red heat. The low retentivity (adsorption/pressure) of this form of silica, while destroying its value as a practical adsorbent, proves a great advantage for theoretical development in that the early portions of isothermals are accessible at external pressures easily measurable by simple manometric means. Also a low retentivity obviously minimizes the difficulty of preparing a bare surface. There is lastly the advantage, by no means the least important, that an apparently trustworthy method has been devised for evaluating the surface of the powdered material independently of the adsorption experiments. The Preparation of Powdered Fused Silica for Adsorption Experiments A coarsely crushed fused quartz was narrowly graded by sieving until the material consisted of particles of average diameter (calculated as spheres) of 0·8 mm. After thorough cleaning in acid bichromate and washing in distilled water the silica was dried and then formed the starting point for all subsequent preparations of adsorbent. The superficial area of 1 gm of the silica at this stage was estimated by the method described later as very nearly 100 cm2.