Effect of Low Molecular Weight charged molecules on the interactions between spherical alumina particles

Adsorbed low molecular weight charged molecules are known to give rise to a range of surface forces that affect the rheological behaviour of oxide dispersions. The behaviour of dicarboxylic acid bolaform compounds in alumina slurry was investigated to determine the influence of the molecular structure on the nanoscale interactions between spherical alumina particles and the macroscopic properties of the slurry. The surface forces in dispersions and between a single particle and a flat surface were characterized by yield stress and atomic force microscopy (AFM) respectively. The surface chemistry was accessed via zeta potential. The bulk adsorption behaviour was determined using total-carbon-content technique. Absorbed muconic acid increased the yield stress of the alumina system which indicates an additional attractive interaction between the particles. However, adsorbed (trans, trans) TT muconic acid resulted in a much higher yield stress than (cis, cis) CC muconic acid. Force-distance data obtained with an AFM displayed features indicating the presence of a capillary force attraction at low pH between the alumina surfaces with both adsorbed TT and CC muconic acids. This force appeared to explain the high yield stress at low pH (pH 3.6) but the absence of a net attractive force at higher pH (pH 5) did not correlate with the yield stress results. At low pH, the muconic acids become even less soluble in the confined space between the interacting surfaces resulting in the formation of an “oily” muconic acid phase located between the interacting surfaces. The nanosized “oil” phase is the source of the capillary force.