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Activation Energy for Ion Movement in Thin Water Films on Montmorillonite¹

ABSTRACT

Desorption isotherms were determined for Li-, Na- and K- montmorillonite at several temperatures. Relative partial molar entropies of the adsorbed water were calculated from these isotherms.

The specific conductance of each clay was determined as a function of temperature at water contents corresponding to one layer, two layers and several layers of adsorbed water. Also, the d₀₀₁ spacings were determined at the two lower water contents by X-ray analysis. For the determination of specific conductance, the clays were packed into porous glass conductance cells which allowed them to equilibrate with an atmosphere of controlled relative humidity. The relative humidity was changed with the temperature in order to maintain the desired water content. Then the specific conductance of each clay was plotted against the reciprocal of the absolute temperature to obtain the activation energy for the movement of the adsorbed ion. The resulting activation energies were interpreted in terms of ion-oxygen interaction at the mineral surfaces and in terms of the orderliness of the adsorbed water as indicated by the entropy data.

¹ Journal paper 1944, Purdue University Agr. Exp. Sta., Lafayette, Ind.; Contribution from the Department of Agronomy. This research was supported in part by a grant from the National Science Foundation.

² Graduate Assistant and Professor of Agronomy, respectively.

Received for publication September 11, 1962. Accepted for publication December 5, 1962.