Rheological and electrokinetic properties of sodium montmorillonite suspensions - I. Rheological properties and interparticle energy of interaction

In this article, we describe the rheology of Na montmorillonite suspensions as a function of pH, at constant ionic strength. The observed behavior is discussed quantitatively in terms of the potential energy of interaction be tween particles, keeping in mind the anisotropic nature of clay particles. The extended DLVO model that includes electrostatic, van der Waals, and pol ar acid-base contributions to the total energy is used. It is found that fa ce-to-face interactions are virtually independent of pH, whereas edge-to-ed ge interactions are most attractive at the isoelectric point of edges (pH s imilar to 7). The most significant variations occur in face-to-edge potenti al energy, with strong attractions at pH < 7. Steady-state viscometry showe d that the yield stress decreases up to an order of magnitude between pH 3 and pH 7, with a much slower rate of decrease in the 7-11 pH interval. Conc erning oscillatory measurements, it is found that both the elastic (G') and viscous (G") moduli are practically independent of frequency. It is also d emonstrated that G' > G", the difference being larger at acid pH values. Th ese results, in addition to potential energy calculations, suggest the exis tence of an elastic, coagulated structure up to pH 7, whereas as the pH is increased such structure is more relaxed because of electrostatic repulsion s. Similar conclusions are reached when creep-recovery data are analyzed. ( C) 2000 Academic Press.