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Co-Ion Exclusion by Clay Surfaces: I. Equations for 1:1, 2:1, and 3:1 Electrolyte Solutions

Dep. of Chemistry, The Faculties, Australian National Univ., Canberra, A.C.T. 0200, Australia

J. P. Quirk

Dep. of Soil Science and Plant Nutrition, School of Agriculture, Univ. of Western Australia, Nedlands, W.A. 6009, Australia

*Corresponding author (richard.pashley{at}anu.edu.au).

ABSTRACT

The distribution of co-ions in the vicinity of charged clay surfaces has been treated theoretically within the context of an electric double layer structure consisting of a Stern layer and Gouy diffuse layer. Equations have been developed, using Gouy-Chapman theory, that describe the depth of exclusion of co-ions in terms of the electrolyte concentration and the Gouy plane potential for 1:1, 2:1, and 3:1 electrolyte solutions. The volume from which co-ions are excluded is obtained by multiplying the depth of exclusion by the appropriate surface area of the clay. Since, in general, the surface potentials of clays are less negative than –100 mV, these equations, especially for 1:1 electrolytes, can be used to obtain surface potentials from exclusion volumes. These volumes can be used to obtain the surface potential across a wide range of electrolyte concentrations, if the area of the interface is known from crystallographic data or from N₂ adsorption isotherms. The measured volume of exclusion for each concentration is, in this study, corrected for the volume of the Stern layer, with an assumed thickness of 5.5 Å.

Received for publication August 14, 1995.