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Ion Mobilities and Electric Charge of External Clay Surfaces Inferred from Potential Differences and Osmotic Flow¹

ABSTRACT

Bentonite, illite, and kaolinite clays were compacted, made homoionic with various cations, and placed between chloride solutions of the respective cations of different concentrations. Rates of osmotic flow, electric potentials in the solutions and streaming potentials were measured across these compacted clay membranes. The osmotic flow was often from the high salt to low salt concentration side and was generally in the direction of more negative potential, indicating electroosmosis as the mechanism involved in osmotic flow. Diffuse layer theory was used to estimate the concentration ranges in which the measured external potential differences were not appreciably different from the respective potentials in solution inside the clay. Assuming the osmotic movement to be electroosmosis, the potentials were calculated from the observed potential gradients and the osmotic flux. The portion of adsorbed cations participating in the diffuse portion of the double layer was then calculated from . This "mobile" fraction of the adsorbed cations appears to decrease for multivalent cations as the equilibrium solution concentration decreased. La³⁺ kaolinite and Willalooka illites were positively charged and adsorbed Cl^- as diffuse layers. The mobile fractions estimated by this method for bentonite are in reasonably good agreement with mobile fractions estimated from diffusion and conductance measurements.

¹ Contribution from the Dept. of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, and the US Soils Laboratory, Soil & Water Conservation Research Division, ARS, USDA, Beltsville, Md. 20705.

² Director, US Soils Laboratory, Beltsville, Md., and Professor, Dept. of Soil Science and Plant Nutrition, University of Western Australia, respectively.

Received for publication July 16, 1971. Accepted for publication February 9, 1972.