Electrostatic Forces Between Clay and Cations as Calculated and Inferred from Electrical Conductivity*

I. Shainberg and W. D. Kemper
U.S. Department of Agriculture and Colorado State University Fort Collins, Colorado
* Contribution from the Northern Plains Branch, Soil and Water Conservation Research Division, Agricultural Research Service, USDA, and the Colorado Agricultural Experiment Station. Colorado Agricultural Experiment Station Scientific Journal Series 1063. Portions of this work were supported by the National Science Foundation and a Colorado State University Faculty Improvement Council Research Grant.

Abstract: Equivalent conductivities of adsorbed cations were determined in clays saturated with Na+, Cs+, Ca++ and with mixtures of these cations. Measurements were also made on Ca++ clays which had been forced by previous drying into bundles of platelets or tactoids. The average mobility of adsorbed Ca++ and Cs+ is much lower than that of adsorbed Na+.

It was concluded that the average mobility of adsorbed Ca++ is low because most of this Ca++ is on the internal surfaces of tactoids. Ca++ adsorbed between these internal surfaces appears to have a mobility much lower than Ca++ on the external surfaces which has a mobility of the same order of magnitude as Na+. Polarization of adsorbed Cs+ accounts at least partially for its low mobility in these clays.

Demixing of adsorbed cations (segregation with Na+ dominant between some platelets and Ca++ between others) is suggested as an initial step leading to breakup of a Na+−Ca++ clay mass into tactoids. The tactoid model, with Ca++ and Na+ preferentially on the internal and external surfaces respectively, furnishes an explanation of the instability of clay and soil aggregates with 15% exchangeable sodium.

Clays and Clay Minerals; 1966 v. 14; no. 1; p. 117-132; DOI: 10.1346/CCMN.1966.0140111
© 1966, The Clay Minerals Society
Clay Minerals Society (www.clays.org)