The dielectric behaviour of vapours adsorbed on activated silica gel

Abstract

The increase in capacity of a test cell containing a porous activated silica on the admission of adsorbates such as water, ethyl chloride and butane has been measured. The data confirm the existence of a discontinuity in the slope of the capacity against volume adsorbed plots. The electrical evidence suggests that two types of film are established for the adsorbates butane and ethyl chloride. The first type of film in each case has a density close to that of liquid state, while the second is a much more dilute film. The position of the discontinuity, which is independent of temperature within the experimental precision, indicates that the occurrence of the discontinuity may be related to the number of adsorption centres rather than to the area occupied by the adsorbed molecule. From further evidence it appears that water and butane are adsorbed on the same centres, while ethyl chloride and water are not. In the case of water it is possible that the difference in apparent polarization along the two sections of the dielectric data might be accounted for by assuming molecules free to rotate in the plane of the adsorbing surface for the first section, and molecules free to rotate in three dimensions along the second. Certain other differences in the behaviour of polar molecules along the two sections are suggested by the data, but explanations of these differences are not yet clear. The computation of dielectric constants of adsorbed matter in such systems is discussed, and a method based on considerations put forward by Böttcher for powdered dielectrics is developed. It is emphasized that a knowledge of the density of the adsorbed matter is required for such systems, and a method is devised for determining these density values from the dielectric data which is applicable in the case of non-polar adsorbates. The difficulty of interpretation of the data for such systems shows that dielectric measure­ments are less helpful in examining the adsorbed state than had been anticipated from earlier investigations.