Further observations on the sorption of oxygen and nitrogen by calcium sulphate

Abstract

Results are reported of a study of the sorption of oxygen and nitrogen by calcium sulphate dihydrate in various stages of dehydration and rehydration by water vapour. Oxygen, the sorption of which was undoubtedly physical, gave hysteresis loops with some samples that gave reversible isotherms with nitrogen. Once the dehydrated sample had come into contact with water vapour, the sorption of oxygen became slower. Moderate outgassing produced a material yielding with oxygen a hysteresis loop that extended down to low pressures. As dehydration was made more rigorous the oxygen loop contracted from the low pressure region and sorption of nitrogen began to give a loop. The explanation is that oxygen molecules can penetrate smaller openings than nitrogen molecules on account of their smaller size. Further, water vapour which promotes sintering leads to the adsorption of molecules or hydroxyl groups which hinder the movement of adsorbate through the openings of fine pores. Outgassing at 200°C yields a product in which the micropore system is empty and the obstruction to the entry of adsorbate into the pore system is not serious.