THE RHEOLOGICAL AND COLLOIDAL PROPERTIES OF BENTONITE DISPERSIONS IN THE PRESENCE OF ORGANIC-COMPOUNDS .3. THE EFFECT OF ALCOHOLS ON THE COAGULATION OF SODIUM MONTMORILLONITE

The critical coagulation concentration, c(K), of sodium chloride for sodium montmorillonite dispersed in water (solid content 0.025 percent) is 8 mmol/L. It remains virtually constant (7.5-8.5 mmol/L) in water-rich alcohol mixtures (below 50% (v/v) methanol and 40% ethanol and propanol). At higher alcohol contents the c(K) decreases to 3.6 mmol/L (70 percent methanol), 1.2 mmol/L (70 percent ethanol), and 0.8 mmol/L (60 percent propanol). In the presence of 10(-4) M sodium diphosphate the c(K) in water rises to 195 mmol/L. In contrast to the behavior in the absence of diphosphate, even small amounts of alcohol reduce the critical coagulation concentration. In 70% methanol the c(K) is 7.5 mmol/L, in 70 percent ethanol 2.5 mmol/L and in 60% propanol 5 mmol/L. The main mechanism is coagulation by contacts between negatively charged edges and faces. At high alcohol contents montmorillonite-alcohol complexes (interlayer solvates) are formed and the colloidal dispersions become unstable even in the absence of salt. Transition from the state with diffuse ionic layers into the quasi-crystalline structure of the interlayer solvates is also evident from the sediment volume which changes with the alcohol content. Maxima are observed which are indicative of band-type structures as intermediate states between the colloidally dispersed particles with repulsive interaction and the discrete particles of the montmorillonite-alcohol complexes.