Standard Free Energies of Exchange for Alkali Metal Cations on Wyoming Bentonite

Standard free energies of exchange, Δ G o , were determined for the exchange of Na with Li, K, Rb, and Cs and Li with K between 0.001 N chloride solutions and 5% Wyoming bentonite gels at 25C. Values were also determined for Na → Rb, Na → Cs, and Rb → Cs exchanges between 0.0075 N chloride solutions and 0.75% Wyoming bentonite suspensions. Application of the additivity rule indicates that the values are accurate to within ±50 to 100 cal mole ‐1 . Values for the 5% gels range from +48 cal mole ‐1 for Na → Li exchange to −1,081 cal mole ‐1 for Na → Cs exchange. There is a small but consistent increase in the Δ G o values for the 0.75% suspensions compared to those for the 5% gels. With the possible exception of Li, a linear relationship was found between Δ G o values and Debye‐Huckle parameters for the respective cations indicating that the adsorption of monovalent cations from strong electrolyte solutions is governed by their hydrated radii.