Perturbation effects due to hydrogen bonding in physical adsorption studied by length-change and infra-red techniques

Abstract

Effects due to hydrogen bonding between physically adsorbed molecules and the hydroxyl groups present on the surface of porous silica glass have been studied. Three methods have been used. As well as the classical isotherms, length changes of the adsorbent have been measured using an interferometer, and infra-red absorption spectra have been obtained both of the surface OH groups and the adsorbed molecules. Contractions of the rigid adsorbent, found under certain conditions at low coverages, are shown to be directly related to the strengths and number of the hydrogen bonds formed between the OH groups and the adsorbed molecules. More than half the surface OH groups were replaced by OCH$_{3}$ groups by methylation. Experiments performed on the glass after this treatment showed that the contractions had almost completely disappeared. It has been shown that two types of adsorption sites exist, one being the OH groups and the other the silicon or oxygen atoms. With acetone and ammonia, it has been shown spectroscopically that the energy of adsorption is lower on the OH sites than on the others. Consequently, as the temperature is raised the distribution of the adsorbed molecules between the two sites changes. Thus the marked decrease in the contractions with increase of temperature reported previously (Folman & Yates 1958) is due to the weakening of the hydrogen bonding with increase in temperature and also to a decrease in the relative numbers of the adsorbed molecules which are hydrogen-bonded. On the basis of all the results, a model of the surface conditions is proposed, which may explain the occurrence of the contractions found when hydrogen bonding is operative.