Analysis of Electrosteric Interaction of Polymer Dispersant in Dense Alumina Suspensions with Different Counter-Ion Densities Using an Atomic Force Microscope.

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The mechanism of anionic polymer dispersant acting on dense alumina suspension behavior was analyzed under different solid-volume fractions (ranging from 40 to 55vol%) and counter-ion densities by comparing microscopic solid/liquid interface interaction to suspension viscosity. The solid/liquid interface interaction and adsorbed structure of polymer dispersant on particles were determined by an atomic force microscope (AFM). The dependency of counter-ion density on the suspension viscosity appeared at a relatively low solidvolume fraction (40vol%). However, at a high solid-volume fraction (50vol%), the suspension viscosity was independent of the counter-ion density. To analyze these phenomena, the force curves between a polished surface of alumina and the tip of AFM were measured in solution with polymer dispersant and different counter-ion densities. The increase of counter-ion density reduced the long-range overlapping repulsive force of an electrical double layer ranging over 10nm in surface distance. A short-range steric repulsion force ranging below about 5nm in surface distance slightly depended on counter-ion density. The estimated mean surface distance between particles in dense suspension decreased from about 8 to 4nm with increasing the solid fraction from 40 to 50vol%. Based on both results, it is concluded that the main mechanism for the aggregation/dispersion of alumina powder in dense suspension changed from electrostatic repulsive force to steric force with increasing the solid-volume fraction in suspension.