SPECTROSCOPIC STUDY OF IONIC ADJUSTMENTS IN THE ELECTRIC DOUBLE-LAYER OF MONTMORILLONITE

Prior studies with clays have shown that σδ, the charge density in the Outer Helmholtz Plane (OHP), becomes increasingly negative as the electrolyte concentration is increased. This change in σδ could be due to (1) movement of cations out of the Stern layer, (2) movement of anions into the Stern layer, or (3) the inward displacement of the OHP. Ions in the Stern layer of a 2:1 clay mineral should be close enough to its surface oxygens, which are the basal oxygens of silica tetrahedra, to affect the Si-O vibrations in these tetrahedra. Therefore, infrared spectroscopy was used to determine how the Si-O vibrations in montmorillonite were affected by increasing the concentration of LiCl, NaCl, and KCl when the adsorbed cations were Li+, Na+, and K+, respectively, and by increasing the concentration of NaCl, NaBr, and NaI when the adsorbed cation was Na+. The results showed that the molar absorptivity, ε, at the frequency of Si-O stretching, decreased with increasing electrolyte concentration in all cases, that the values of ε at any electrolyte concentration varied with the species of adsorbed cation in the order Li+ > Na+ > K+, and that the value of ε at any electrolyte concentration was the same when the anions were Cl−, Br−, and I−. Reference to the literature shows that the adsorption intensities of the three cations on montmorillonite are in the order K+ > Na+ > Li+. Consequently, the increasing negativity of σδ with increasing electrolyte concentration could not have been due to either the movement of cations out of the Stern layer or the movement of anions into the Stern layer. It must have been due to the inward displacement of the OHP. © 1993 by Academic Press, Inc.