Charge density effects in ion exchange. Part 1.—Heterovalent exchange equilibria

Abstract

A thermodynamic study is made of the Na–Ca heterovalent exchange reaction in 4 montmorillonite clay minerals with complete octahedral isomorphic layer charge but varying in charge density (Otay, three Camp Berteau samples which are 0, 25 and 40 % reduced in charge by the Hofmann–Klemen effect). The free energy of exchange decreases linearly with the logarithm of the charge density, as predicted from double layer calculations. The endothermicity of the reaction almost balances the entropy, both of them decreasing in magnitude as the charge density decreases. Interlamellar hydration changes, correlated with d₀₀₁ spacing measurements, can explain the observed effects.