THERMODYNAMICS OF CATION EXCHANGE ON CLAY: Ca‐K‐MONTMORILLONITE

Summary Radioactive elution and batch equilibration techniques have been used to obtain isotherms at 30° C, 50°C, and 70° C for the exchange of Ca for K on K‐montmorillonite. From these the changes in enthalpy and entropy were + 3.87 kcal/mole and +6.7 e.u./mole of Ca respectively. The activity coefficient for K decreased from approximately 1.5 to 1 and for Ca from 1 to approximately 0.5 as the fraction of K on the clay increased from 0 to 1. The results are discussed especially in relation to hydration of the ions and adsorption of water on the clay. The entropy of adsorbed water is inferred, from the results, to be less than that for liquid water.