Calculation of the Standard Free Energy of Cation Exchange in Clay Systems

The paper deals with methods for calculation of the standard free energy of cation exchange on clays, by assuming certain models for the distribution of the cations between the adsorbed phase and the equilibrium solution. From a comparison of the results calculated with the help of a hypothetical discharging/charging process of the exchanger in the respective homoionic forms, with those found from graphical integration of In K(exchange) over the equivalent fraction adsorbed (method of Gaines and Thomas), the free energy change of the “associated-solvent” appearing in the latter procedure is identified with that of the “salt-free” solvent present in the system. The calculated values of ΔG0 (exchange) corresponding to the chosen models are given for the purpose of comparison with experimental values that are found in, or could be derived from, the relevant literature.