A Study of Water Vapour Adsorption on Surface Hydroxyl Groups

Abstract

Initially in this thesis the author attempts to characterise a silica gel prepared by a special technique utilising ion exchange resins. Hence the emphasis on the silica gel-water vapour system in the first nine chapters. This silica gel appeared to have some unusual properties and an extensive investigation was carried out in an effert to interpret these properties in terms of skeletal and surface structure. The usual technique of studying adsorption and desorption isotherms was employed. In this investigation argon and water vapour were the adsorbates. The uptake of argon was followed volumetrically in a classical B.E.T. apparatus while water vapour adsorption was followed gravimetrically. Differential thermal analysis and thermogravimetric analysis were also employed. Results indicated that an amorphous silica gel had been prepared possessing a microporous structure. This gel was unusual in that all physically adsorbed water vapour could be removed by vacuum drying at 10-5m.m. Hg and at room temperature. Also used in the investigation of silica gel was an istopic exchange technique initially developed by Wilson.(1) The exchange of water vapour in the gas phase with water vapour adsorbed on the silica gel surface was followed using tritiated water vapour and a specially modified gas geiger counter. Mathematical analysis of the exchange curve led to some interesting results. One of the more important was that hydrogen atoms on the SiOH groups were not exchangeable with water vapour at 0ºC. The study of the silica gel-water vapour system by this method has led to new interpretations of results obtained by others who used the isotopic exchange technique. (1) A. T. Wilson and R. J. Furkert – Trans. International Soil Conference, New Zealand (1962) Allophane, a common amorphous hydrous alumino-silicate found in soils, was the next adsorbent to be studied by the isotopic exchange technique. A commercially prepared alumina (Actal) was also studied. This enabled a comparison to be made between the interaction of water vapour with SiOH groups (silicagel), AlOH groups (alumina) and a random mixture of SiOH and AlOH groups (allophone). Most interesting of the results was that under the experimental conditions the hydrogen atoms of the surface hydroxyl groups did not exchange with water vapour. Of the hydroxyl surfaces present in nature the three most common would be Si-OH, Al-OH and C-OH. To complete the comparison it was decided to study the polyvinyl alcohol-water vapour system using the isotopic exchange technique. Kuralon, a modified polyvinyl alcohol, provides a ready source of a C-OH surface. Results indicated that the C-OH surface, when exposed to water vapour, interacted in a distinctly different manner to the Si-OH and Al-OH surfaces. The exchange between tritiated water vapour and water vapour adsorbed on the Si-OH and Al-OH surface groups appeared to involve only one exchange process. The exchange between tritiated water vapour and the C-OH –adsorbed water vapour system involved three exchange processes. Comparison between the isotopic exchange date and the water vapour sorption isotherm showed that one of these processes was due to exchange of the hydrogen of the C-OH group. B.E.T. plots of the amount of water vapour associated with each of the other two exchange processes indicated that twice the amount of adsorbed water vapour was associated with one exchange process than with the other.This suggested the following models of water vapour adsorption: A significant difference between the C-OH –water vapour system and the Si-OH –water vapour and the Al-OH –water vapour systems was that the hydroxyl hydrogen atom exchanged in the case of C-OH but not in the case of SiOH or AlOH. It is postulated that the exchange mechanism is This is possible in the C-OH case but not for Si-OH and Al-OH cases. In a survey of all adsorbents studied by the isotopic exchange technique it was noted that results could be characterised into two distinct groups: (a) results typical of adsorbents with no exchangeable hydrogen atoms on the surface (b) results typical of adsorbents with no exchangeable hydrogen atoms on the surface Examples of group (a) were the silica gel-water vapour, alumina-water vapour, allophane-water vapour, orlen-water vapour and terylene-water vapour systems. Examples in group (b) were polyvinyl alcohol-water vapour, wool-water vapour and cotton-water vapour systems. This work has correlated the data of others who have used the isotopic exchange technique and has enabled a better interpretation of their results. This eventuated because the author was able to compare the results of the isotopic exchange experiments with the water regain curves of the particular adsorbents under study. This had not been done previously and had resulted in misinterpretations. Also, "blank" runs performed by the author gave a better picture of the effect of the geiger counter design on the exchange kinetics.