Surface Potentials Derived from Co-Ion Exclusion Measurements on Homoionic Montmorillonite and Illite

D. Y. C. Chan1, R. M. Pashley1 and J. P. Quirk2
1 Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, Australia
2 Waite Agricultural Research Institute, Glen Osmond South Australia 5064, Australia

Abstract: Simple equations are presented which allow double-layer potentials of clays to be derived from co-ion exclusion measurements in monovalent and divalent electrolyte solutions. These equations have been used to re-interpret earlier results for illite and montmorillonite. The potentials derived follow the lyotropic series for the various homoionic modification of each clay. We have demonstrated that the Schofield equation, which assumes high double-layer potentials, cannot be applied to co-ion exclusion in clay systems. Re-analysis of earlier measurements has shown that for a given homoionic clay the potentials are almost independent of concentration over the range 0.3 to 0.003 molar. Thus, clay surfaces appear to behave more like constant-potential than constant-charge surfaces.

Key Words: Co-ion exclusion • Double-layer potential • Illite • Montmorillonite • Surface charge

Clays and Clay Minerals; April 1984 v. 32; no. 2; p. 131-138; DOI: 10.1346/CCMN.1984.0320207
© 1984, The Clay Minerals Society
Clay Minerals Society (www.clays.org)