Sorption by porous solids

Abstract

A number of isothermals have been determined for the sorption of the vapours of amino compounds on silica and ferric oxide gels. The sorbates studied include the normal aliphatic amines, from ethylamine to n-heptylamine, t-butylamine, ethylene diamine, cyclohexylamine and pyridine. Data for ethyl alcohol and water is also included, for comparative purposes. A standard 'gravimetric' technique has been used in every case, except that of ethylamine, for which a new technique has been developed. The ferric oxide isothermals all contain a large. hysteresis loop, the size of which does not vary with increasing molecular diameter. Silica gel B2 isothermals show a large loop with water, and small loops with ethyl alcohol, n-propylamine and ethylene diamine. Larger molecules give completely reversible isothermals. This data confirms the most important prediction of the 'open-pore' theory of capillary condensation. Reference is made to data in the literature which provides further confirmation of this prediction. Silica gel C and silica gel D systems both contain large hysteresis loops. Only a few results are quoted for the former system, as the isothermals obtained were not reproducible. Where possible, the Kelvin equation has been used to estimate the most frequent occurring pore space available for capillary condensation. In some cases an estimate of the true pore radius has been made, by correcting for the thickness of the adsorbed layer. Evidence is presented for orientation of sorbate molecules in the first layer on ferric oxide gel, and, to some extent, on silica gel D. The volumes adsorbed at saturation obey Gurwitsch Rule in every case. Ethylene diamine has been shown to have a lower freezing point in the adsorbed state than in the bulk state. The vapour pressures and surface tensions of some of the sorbates have been determined, using methods suitable for hygroscopic liquids.