Ceramics-Silikáty 27, (1) 19 - 29 (1983)

The exchange reaction between the single charged Na and the double charged Ca on the bentonite from Braňany has an endothermic character with ΔG°° = -0.398 kJ/mol eq*. The selectivity coefficients are not constant within the entire occupation of the surface by exchange cations. This property is prevailingly due to the formation of tactoids. It has been found on the basis of the activity coefficients of the ions bound on the surface of montmorillonite that the adsorption energy depends on the character of the surface. The increase of selectivity for Ca²⁺ of mineral in an area where this cation is widely distributed on the surface is probably caused by the presence of active centres with unequal bond energy.