Surface Area of Homoionic Illite and Montmorillonite Clay Minerals as Measured by the Sorption of Nitrogen and Carbon Dioxide

Abstract

The surface areas obtained by application of the B.E.T. theory to adsorption isotherms of nitrogen and carbon dioxide gases at 77~ and 195~ respectively on homoionic samples of illite and montmorillonite clays have been examined. The isotherms were obtained using a standard volumetric adsorption system and the results are compared with those obtained by Thomas and Bohor (1968) using a dynamic sorption system. Small amounts of residual water have been shown to have a marked influence on the accessibility of the internal surfaces of the montmorillonite clays to nitrogen and carbon dioxide adsorption, in this respect the standard outgassing procedure under high vacuum seems more efficient than that used in dynamic systems. The present data indicate that provided the sample has been satisfactorily outgassed there is little penetration of nitrogen or carbon dioxide gases into the quasi-crystalline regions of montmorillonite clays. With the exception of the caesium saturated montmorillonites the surfaces of the clays are more accessible to the smaller nitrogen molecules than to carbon dioxide assuming the values used for molecular area are correct. IN A RECENT paper Thomas and Bohor (1968) have examined the accessibility of the surfaces of homoionic montmorillonite clays to nitrogen and carbon dioxide adsorbates at 77~ and 195~ respectively using a dynamic measuring system (Nelson and Eggertsen, 1958). From variations in the surface area obtained by application of the B.E.T. (1938) theory to these measurements these authors considered that there was a certain degree of penetration of both nitrogen and carbon dioxide between the unit platelets or lamellae forming the crystals of montmorillonite. Their experimental data indicated that the extent of penetration was time dependent and also a function of the interlayer forces as governed by the size and charge of the replaceable cations. Aylmore and Quirk (1967) have recently pointed out that in the dry state montmorillonite clay forms a complexly interwoven matrix in which one lamella may conceivably pass through several apparently crystalline regions. It is considered that the term "quasi-crystalline" (Aylmore and Quirk, 1969) may be the most appropriate descrip- tion here since in these regions the lamellae are stacked in parallel array but not necessarily in perfect crystalline order. In these circumstances 91 the definition of apparent crystal size must be somewhat arbitrary. It was considered that the area determined by nitrogen adsorption at low temperatures was essentially a measure of the surface area external to these quasi-crystalline regions. The differences in specific surface area observed between montmorillonite saturated with different exchangeable cations was attributed partly to differences in the degree of association of the lamellae in aqueous suspensions (Edwards, Posner and Quirk, 1965) and the subsequent statis- tical arrangement of the units on drying, and partly to variations in accessibility of areas of overlap of quasi-crystalline regions with size of the exchangeable cations. Since the data reported by Thomas and Bohor (1968) was obtained using a dynamic sorption system (Nelson and Eggertsen, 1958) the results of similar sorption measurements obtained using a standard volumetric adsorption system are reported here.