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Pertinent equations of electric double-layer theory have been derived and used with experimental data on ζ, the zeta potential, and Vex, the anion exclusion volume, to determine values of ψδ, the electric potential at the outer Helmholtz plane; σδ, the charge density in this plane; and τ, the distance between this plane and the plane of shear for different clay minerals. At all electrolyte concentrations, ψδ has a constant value of 50–60 mV and τ = 0. Also, σδ increases with electrolyte concentration but remains small relative to the charge density at the clay-water interface. Therefore, the diffuse layer of these minerals is poorly developed, and the plane of shear coincides with the outer Helmholtz plane. forumla, the ratio of the value of any water property, i, in the clay-water system to that in pure bulk water, has been shown to be exponentially related to 1/t, the reciprocal of the average thickness of the water films on the particle surfaces. Inasmuch as this relation does not depend on the nature of the clay, clay-water interaction is believed to be nonspecific in character.

An equation was derived for the osmotic component of II, the swelling pressure of the clay, and has been used to calculate this component at several values of X, the interlayer distance, for samples in which ψδ has a value close to that observed experimentally. A comparison of the calculated and observed results indicates that the osmotic contribution to II is relatively insignificant. The relation between II and λ suggests that II is exponentially related to 1/λ. A similar relation was observed between II and 1/t; hence, II appears to be related to forumla. The latter relation is substantiated experimentally by using data on II and e, the molar absorptivity, for the same systems. The development of II therefore depends on the same factor that causes Ji to differ from forumla, i.e., the non-specific interaction of water with the particle surfaces.

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