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Particle Shape and Sodium Self-diffusion Coefficient in Mixed Sodium-calcium Montmorillonite¹

ABSTRACT

Various parameters related to the shape of clay particles and self-diffusion coefficients of sodium were measured in Camp-Berteau montmorillonite suspensions equilibrated with NaCl-CaCl₂ solutions of 0.01 total normality, and variable ratios of Na to Ca. As the equivalent ionic fraction of exchangeable sodium decreased from 1.0 to 0.0, the anion exclusion volume decreased from 2.9 to 0.5 ml g^-1, the hydration of clay sediment decreased from 8.6 to 4.4 ml g^-1, the average number of plates per tactoid increased from 1.4 to 3.9, and the axial ratio of the clay particles decreased from 190 to 90. At clay content of about 27 g/liter, the self-diffusion coefficient of Na increased from about 0.40 x 10^-5 to 0.76 x 10^-5 cm² sec^-1 at 16°C and from 0.55 x 10^-5 to 1.16 x 10^-5 cm² sec^-1 at 33°C, as the ionic fraction of exchangeable Na decreased from 1.0 to 0.15.

The calculated surface diffusion coefficient of sodium was found to be about 30% of its value in the free solution for a pure sodium clay and thereafter increased up to about 50% of that value for a clay loaded by 15% Na. The activation energy involved in the surface diffusion process was calculated to be in the range of 3 to 6 kcal/mole, and a minimum was found for the clay containing about 70% Na. As evaluated from the data on the number of platelets per tactoid the internal surfaces of which were assumed to be preferentially occupied by Ca, this minimum could perhaps correspond to a completely demixed system. A less refined but nevertheless rather satisfactory approach was also attempted for interpreting the variation of the observed self-diffusion coefficient of Na as a simple result of the change in the distribution of that ion between the surface phase and the free solution.

¹ Contribution to a joint project of the Soil Science Dep. of the Hebrew Univ. of Jerusalem and of the Univ. of Louvain.

² Research-fellow (Aspirant) of the "Fonds National de la Recherche Scientifique" and Professor, respectively; Univ. of Louvain, Place Croix du Sud, 2; B-1348 Louvain-la-Neuve, Belgium.

³ Associate Professor and Research-assistant, respectively; Hebrew Univ. of Jerusalem, PO Box 12, Rehovot, Israel.

Received for publication July 7, 1975. Accepted for publication December 22, 1975.