Thermodynamics of heterovalent cation exchange reactions in a montmorillonite clay

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Abstract
Experimental results are presented for the free energy and enthalpy changes pertaining to the exchange process between four divalent cations (Mg2+, Ca2+, Sr2+ and Ba2+) and two monovalent cations (Na+ and NH+ 4) in a montmorillonite clay. The free energy changes have been calculated from the exchange isotherms using a procedure which allows the elimination of incomplete attainment of equilibrium and the need of using activity coefficients in the mixed electrolyte solution. Enthalpy changes have been measured calorimetrically and calculated from the temperature coefficient of the equilibrium constant. Enthalpy changes are correlated with the polarizability of the substituting cation. When comparing the divalent cations studied with each other, the replacement of a particular divalent cation by one of larger polarizability gives rise to an entropy loss. A calculation of the electrostatic free energy exchange between mono- and divalent cations is presented. The result is in good agreement with the experimental value of the free energy of exchange observed for the smaller cations, Na+ and Mg2+.

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