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Surface Mobilities of Cations in Clays¹

ABSTRACT

The effect of clay content and temperature on the electrica conductivity of two Na-kao inite clays is investigated. The effect of the type of cation is examined in the Camp-Berteau montmorillonite. The ionic surface mobilities (i.e. corrected for tortuosity effects) are calculated from the conductivity results. In Na-kaolinite clays, surface mobility steadily increases with increasing cay content and attains about 40% of its solution value when clay content becomes about 70% by weight. In montmorillonite clays, surface mobility increases until clay content reaches about 35% by weight. Higher clay contents result in nearly constant values for the surface mobilities. In the series of alkali metal cations, surface mobilities increase from Cs⁺ (attains about 7% of its solution value at 55% clay) to Na⁺ (attains about 60% of its solution value at 55% clay); activation energies increase from Na⁺ (4.6 kcal/eq. at 55% clay) to Cs⁺ (5.6 kcal/eq at 55% clay).

The ionic mobilities and activation energies can be correlated with the polarizabilities of the ions. It is suggested that the mobility of an ion along a clay surface is influenced more by crystallographic than by hydrated ionic volume. The large negative entropies of activation, calculated on the basis of the transition theory of rate processes, are interpreted in terms of hydration effects when an ion moves from one site to another.

The agreement between self-diffusion coefficients calculated from the electrical conductivity data and the values which may be found in the literature is very good. The applicability of the electrical conductivity method for investigating purely geometrical effects is discussed.

¹ Contribution of the Dep. of Colloid Chemistry, Inst. of Agronomy, University of Louvain, Heverlee-Louvain, Belgium.

² Research Fellow and Professor, respectively.

Received for publication September 20, 1965. Accepted for publication May 24, 1966.