Ion and water uptake from solution by the sodium and hydrogen forms of montmoril-lonite and illite were measured. The clays first were converted to homoionic form by treatment with synthetic ion exchange resins. The hydrogen clays were used to determine the acid strength of the exchange groups by titration with sodium hydroxide solutions. The sodium clays were used to obtain the ion and water distribution. After attainment of equilibrium with sodium chloride solutions, the sodium clays were centri-fuged to constant weight and the equilibrating solutions analyzed for NaCl. Ion and water uptake and fixed charge concentration in the clays were then measured.

The clay minerals were found to behave as weak acids and the exchange sites are not appreciably dissociated until the pH of the external solution becomes moderately high. In the sodium form, the exchange sites are fully dissociated and the clays, particularly montmorillonite, become efficient Donnan membranes. The partially neutralized clays exhibit intermediate membrane behavior.

At low external phase salinities, the leakage of anions into the clay solution phase is abnormally large, but the membrane activity remains high because of the low activity coefficients of the diffusible ions in the clay phase. At high external solution salinities, the deswelling of the clays and the decrease in the anion to cation mobility ratio partially compensate for the increased anion leakage.

The abnormally low activities of the diffusible ions are directly related to the effect of the internal phase double layer. The concept of ion retardation in the double layer is used to explain the fact that ion transference numbers computed from internal phase ion concentrations are lower than experimental transference numbers.

The relationship of the electrochemical properties of clays to oil-well log interpretation is briefly discussed.