Rheological Behavior of Na-Montmorillonite Suspensions at Low Electrolyte Concentration

The prevailing concept that positive-edge to negative-face attraction accounts for the rheological behavior of montmorillonite suspensions at low electrolyte concentration was investigated. In one experiment, Mg 2+ released from Na-montmorillonite was measured at several NaCl concentrations; in a second experiment, the viscosity, η , and the extrapolated shear stress, θ , were measured at several clay concentrations, pHs, and NaCl concentrations; and in a third experiment, the absorbance, A, was measured at two wavelengths (450 and 760 nm) at different clay and electrolyte concentrations. The released Mg 2+ decreased with increasing NaCl concentration until it became zero at a NaCl concentration between 0.01 and 0.02 M, depending on pH. Thereafter, it increased with increasing NaCl concentration. Both θ and η were highly correlated with the amount of released Mg 2+ . Also, A remained constant until the NaCl concentration corresponded to that at the minimum of θ . Thereafter, it increased and became linearly related to θ . These results suggest: (1) positive-edge to negative-face interaction cannot solely account for the rheological properties of montmorillonite at low electrolyte concentration, and (2) the release of octahedral Mg 2+ from montmorillonite affects θ , because it reduces the negative charge on the particles and, thereby, the repulsive force between them.