Electrochemical properties of illites

Abstract

Electrochemical analyses of two illitic clay minerals (Fithian and Muloorina illite) are presented and show these minerals to have similar surface chemistries. Aqueous acid/base potentiometric titration of suspensions of these clays, both prior to and after conversion to fully homoionic forms, indicates that analytic or titratable surface charge density is particularly sensitive to surface preparation. Although hydrogen and hydroxyl ion adsorption/desorption are mechanisms responsible for the development of some surface charge on the particles, the electrolyte cation also exerts a significant influence. If surface sites are assumed to be evenly distributed over the particles then the changes in surface charge densities, over the pH range 3–10, are small (<0.2 C m −2; especially so for the Muloorina mineral) compared with those found for homogeneous oxides such as silica or alumina (>0.5 C m −2), over a similar range in pH. Even if this titratable charge were localized on the edges of these clay particles, estimates of changes in the local surface charge density for the Muloorina illite are still comparatively small. Titration of suspensions at various ionic strengths produced families of titration curves which show points of inflection within the range 3 < pH < 10. Curves for incompletely prepared homoionic sodium Fithian illite possessed a common intersection point which disappeared after further preparation. No common intersection points were observed in the titration curves of Muloorina illite. No point of zero charge, independent of electrolyte concentration, can therefore be unequivocally identified for the particle edges. Interference from cation exchange reactions involving exposed interlayer sites located at or near the particle edges is probably responsible for these effects.