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Demixing of Exchangeable Cations in Free-Swelling Bentonite Clay¹

ABSTRACT

Hypothetical models for the crystalline, interlayer swelling of mixed-ion (Na⁺ and Ca²⁺) saturated montmorillonites in water were developed for two types of cationic distributions on the clay minerals; namely, for complete demixing and for complete random mixing of cations. In addition, the model for demixing was expanded to include both the alternate layering of Ca- and Na-saturated platelets and the complete demixing of cations into separate Na- and Ca-domains. The theoretical development is based on the accepted principles of limited interlayer swelling (10Å maximum) for Ca-montmorillonite, and free-swelling for Na-montmorillonite as the water content of the system is increased beyond approximately 120%. Experimental measurements of interlayer swelling using a low-angle X-ray diffraction technique showed that the relative distribution of Na⁺ and Ca²⁺ ions on the clay exchange complex was a function of the exchangeable sodium percentage (ESP): (i) the two cations were randomly mixed for ESP values greater than approximately 50%; (ii) they were partially demixed for ESP values between approximately 50 and 15%; and (iii) for ESP values less than approximately 10 to 15% no cation demixing in interlayer regions was evident, but the results did suggest that the interlayer exchange complex was predominantly Ca-saturated and the exchange complex on external surfaces was predominantly Na-saturated. Although the experimental techniques could establish that demixing occurred within certain ranges of ESP values, it was not sensitive enough to differentiate between demixing by domains and demixing by alternate layers.

¹ Contribution from the Soil & Water Conserv. Res. Div., ARS, USDA. Presented before Div. S-2, Soil Science Society of America, Tucson, Ariz., Aug. 27, 1970.

² Research Soil Scientist and Research Chemist, US Water Conservation Lab., Phoenix, Ariz.; formerly Research Soil Scientist, US Salinity Lab., Riverside, Calif., presently Associate Professor, Dept. of Agronomy, Washington State Univ., Pullman, Wash.

Received for publication February 15, 1971. Accepted for publication April 5, 1971.