Influence of pH, electrolyte, and poly(vinyl alcohol) addition on the rheological characteristics of aqueous dispersions of sodium montmorillonite

Abstract

The rheological characteristics of sodium montmorillonite dispersions were investigated as a function of pH and electrolyte (NaCl) concentration. Steady state and low shear rheology and instantaneous modulus measurements were used to obtain the yield value τ β , the plastic viscosity η PL, the residual viscosity η 0, and the modulus G 0. With the exception of the plastic viscosity, the other rheological parameters showed a minimum at pH = ∼7 increasing rapidly as the pH was increased above or decreased below this value. Moreover, τ β , η 0, and G 0 increased with increase of NaCl concentration (within the range 10 −3 to 10 −1 mole dm −3). These results indicate increase in gelation of aqueous montmorillonite dispersions in acid and alkaline conditions. This gelation becomes more pronounced the higher the NaCl concentration within the range investigated. The various possible interactions between the clay platelets and their dependence on pH and electrolyte concentration were considered assuming that the isoelectric point (iep) of the particle edges was around pH = 7. At pH's below the iep, edge-to-face attraction was considered to dominate the interaction. On the other hand, at pH's above the iep of the edge, the increase in gelation with increase in pH was considered to be due to an increase in double-layer repulsion as the net charges on the edges increase. Preliminary results on the effect of addition of PVA at pH = 7 showed a gradual increase in structure on the addition of polymer, reaching a maximum at an optimum adsorption value (corresponding to ∼1 4 coverage) above which further increase in polymer concentration results in reduction of this structure. This gelation at an optimum PVA concentration was considered to be due to polymer bridging.