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Adsorption of N-Butane by Kaolinite and Montmorillonite: Phase Changes in the Monolayer Region¹

ABSTRACT

The adsorption of n-butane by Ca-kaolinite and Ca-montmorillonite, using high vacuum technique, was studied to determine the nature of the differences that occur during the adsorption of nonpolar molecules by clay minerals. The adsorption is described in terms of a 2-dimensional mobile film with the spreading pressure, , calculated by the integrated Gibbs equation. At surface coverages of < 4%, the adsorbed films on both surfaces can be described as an ideal 2-dimensional gas, i.e., A = kT, where A is the molecule area, k is Boltzman's constant, and T is the absolute temperature. Throughout the monolayer region, adsorption of n-butane could be described as either a perfect or imperfect gaseous film. The A — curves show that a transition from a liquid expanded to a liquid intermediate phase occurs in the imperfect gas region of both clay systems. The differences noted in the nature of the phase changes on both surfaces coupled with the isosteric heats of adsorption is explained on the basis that kaolinite is essentially nonporous and has a homogeneous surface, whereas the presence of capillary pores dominates the adsorption of n-butane by montmorillonite.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California, Davis. Presented before the Western Society of Soil Science, June 17, 1958, at Logan, Utah.

² Assistant Soil Chemist, Department of Soils and Plant Nutrition and Professor of Chemistry, Department of Chemistry, University of California, Davis, respectively.

Received for publication May 31, 1958. Accepted for publication July 23, 1958.