CO-ION EXCLUSION BY CLAY SURFACES .1. EQUATIONS FOR 1 1, 2/1, AND 3/1ELECTROLYTE-SOLUTIONS/

The distribution of co-ions in the vicinity of charged clay surfaces h as been treated theoretically within the context of an electric double layer structure consisting of a Stern layer and Gouy diffuse layer. E quations have been developed, using Gouy-Chapman theory, that describe the depth of exclusion of co-ions in terms of the electrolyte concent ration 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 m ultiplying the depth of exclusion by the appropriate surface area of t he clay. Since, in general, the surface potentials of clays are less n egative than -100 mV, these equations, especially for 1:1 electrolytes , can be used to obtain surface potentials from exclusion volumes. The se volumes can be used to obtain the surface potential across a wide r ange of electrolyte concentrations, if the area of the interface is kn own from crystallographic data or from N-2 adsorption isotherms. The m easured 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 Angstrom.