The discontinuous nature of the process of sorption of gases and vapours by porous solids

Abstract

During the last five years, a mass of evidence has accumulated in this laboratory pointing to the conclusion that the adsorption of vapours by charcoal is an essentially discontinuous process, and that, for example, an ordinary adsorption isothermal is composed of a series of loops, cutting one another at more or less well-defined pressures. We were for long inclined to attribute such discontinuities to casual experimental error, or to defects inherent in the technique used. As, however, improvements in the latter have so far not led to the elimination of these "breaks," but rather to their more certain definition, and as similar results are beginning to appear from other laboratories, we think it well to present an account of our conclusions, and of the evidence on which they are based. 1. Adsorption of Vapours by Charcoal at Low Pressures (Pirani Gauge Measurements ). The first indication of the existence of discontinuities was obtained in these experiments, of which the technique has been described by Chaplin. A definite break was found on the 25° C. carbon tetrachloride isothermal of charcoal F at 0·096 mm., and breaks were discovered in all isosteres, whatever the charcoal used, at about the same pressure. Later work showed breaks to exist in the 25° C. cabon tetrachloride isothermal of charcoal B at about 0·1 mm. and at adjacent somewhat higher pressures. More recently, further (unpublished) isostere measurements have been made by Dr. E. G. V. Barrett, using carbon tetrachloride, and by Mr. J. L. Lizius, using carbon bisulphide, in both cases with several charcoals. A break has invariably been found at 0·1 mm. or at a slightly higher pressure. In addition, determinations of the adsorption isothermals of mixtures of carbon bisulphide and of water vapour also give evidence of these breaks (unpublished work of Mr. Lizius).