FORCES BETWEEN ALUMINA SURFACES IN SALT-SOLUTIONS - NON-DLVO FORCES AND THE IMPLICATIONS FOR COLLOIDAL PROCESSING

Direct measurements of the forces between basal (0001) surfaces of sapphire in salt solutions are presented. The measurements reveal the presence of forces in addition to those described by classical DLVO theory. At pH 7.2 in 0.01M NaBr solution, we find an additional short-range oscillatory force with periodicity approximately equal to twice the diameter of a water molecule. At pH 3 we find an additional strong, short-range, monotonic, repulsive force and a long-range attractive force over a range of NaBr concentrations from 0.001M to 0.1M. Both monotonic forces are approximately exponential, with decay lengths of 0.55 and 12 nm, respectively. The short-range force is analogous to hydration forces previously measured on negatively charged surfaces. This force would provide plasticity to alumina slurries and is suggested to be the force responsible for the anomalous viscosity and consolidation behavior of alumina slurries at high salt concentrations.