The Swelling of Clay: III. Dissociation of Exchangeable Cations

Soil Science Society of America Journal

Philip F. Low

Values of ζ, the zeta potential, were determined for 34 different Namontmorillonites by electrophoresis. These values were used in the equations of diffuse double-layer theory to obtain the corresponding values of σ', the surface charge density at the outer limit of the Stern layer. In addition, data on cation exchange capacities and specific surface areas were used to calculate the corresponding values of σ, the surface charge density at the particle surface. From the respective values of σ′/σ, it was found that < 2% of the exchangeable cations were dissociated. This low degree of dissociation was confirmed by using double-layer theory and experimental data from another study. Then a representative value of σ′ was used in the appropriate equations to obtain data from which a theoretical curve of the swelling pressure, Π, vs. interlayer distance, λ, was constructed. Comparison of this curve with the experimental curve of Π vs. λ indicated that double-layer repulsion makes a relatively small contribution to Π. The necessary conclusion is that Π develops primarily because of surface-water interaction.