Investigation into the existence of edge—face coagulated structures in Na-montmorillonite suspensions

Abstract

The rheological and coagulation characteristics of Na-montmorillonite suspensions have been investigated as a function of pH and NaCl concentration to ascertain whether edge—face coagulation occurs. Contrary to the current hypothesis of particle interactions in this system, dilute suspensions ( < 1 % w/w) were Newtonian liquids and showed no tendency to coagulate over the pH range 4–11 at NaCl concentrations below 5 × 10^– mol dm^–3. At higher NaCl concentrations, however, coagulation and Bingham flow behaviour were observed. There is no evidence for edge—face interaction in this pH range; the coagulation produced by NaCl additions is considered to be initially a result of the edge—edge type of particle interactions with face—face coagulation occurring at NaCl concentrations > 0.3 mol dm^–3. The extrapolated shear stresses of the salt contaminated systems exhibited a minimum in the pH range 7–9. When the concentration of Na-montmorillonite was increased above 1 % w/w, at NaCl concentrations below 10^–3 mol dm^–3, the suspensions became pseudoplastic and a shear modulus became measurable. The shear modulus and the extrapolated shear stress both increased exponentially with clay concentration. These effects are attributed to the interpenetration of electrical double layers because the addition of small amounts of NaCl, by compressing the electrical double layers, markedly decreased the magnitude of both parameters. However, increasing the NaCl concentration above 5 × 10^–3 mol dm^–3 brought about edge—edge coagulation demonstrated by an increase in the shear modulus, the appearance of a yield stress in the flow curves and the onset of thixotropic character.