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Thermodynamics of Ion Exchange¹

ABSTRACT

The local formulation of nonequilibrium thermodynamics is used to define the electrochemical potentials in the ion-clay-water system, thus enabling development of an improved nonempirical approach to cation-exchange processes. The presence of the electrical field in the system is expressed by a proportional increase of the pressure in the double layer. The effect of the electrical field on the chemical potentials of the exchangeable cations is divided into the common electrostatic term and a term dependent on their polarizabilities. For systems containing a mixture of monovalent ions it can be assumed that the electrostatic effect of the electrical field is the same for each of the exchangeable cations. Using values of the polarizabilities and intrinsic volumes of the alkali metal cations, the free energies of exchange with sodium in montmorillonite were calculated. The calculated free energies were found to be in good agreement with experimental values.

¹ Contribution from the Soil Sci. Lab., Faculty of Agricultural Engineering, Technion, Haifa, Israel. This work was supported in part by a grant from the USDA under P.L. 480.

² Senior Lecturer and Research Assistant of Soil Physical-Chemistry, respectively.

Received for publication March 30, 1973. Accepted for publication September 24, 1973.