Additional attractive force between alumina particles due to low solubility of Dicarboxylic acids

Low molecular weight charged molecules are known to change the rheological behaviour of oxide dispersions dramatically. Here the isomers of muconic acid were used to investigate the effect of molecular structure and solubility on the bulk properties of alumina dispersions and the nanoscale interactions between alumina surfaces. The surface forces in dispersions were characterised by yield stress while atomic force microscopy (AFM) was used to directly measure the force between a single alumina particle and an alumina substrate. Both (trans, trans) TT and (cis, cis) CC muconic acid were found to increase the yield stress of alumina slurries signifi cantly at low pH when compared to that of the pure alumina. TT muconic acid achieved a much higher yield stress than that of CC at high additive concentration. AFM measurements revealed force-distance features that indicate a capillary-type attraction between the adsorbed layers of TT muconic acid at high surface coverage. The force-distance curve for the CC muconic acid system displayed a capillary force and an electrostatic force. At low pH, the muconic acids become less soluble resulting in the formation of an 'oily' muconic acid phase between the interacting surfaces. This nanosized 'oil' phase is the source of the capillary force. The capillary force at high concentration of TT and CC muconic acid observed during AFM measurements can account for the large increase of yield stress at low pH. The bridging mechanism applicable in other cases is not found to be operating at high concentration of adsorbed muconic acids. This study revealed that not only the molecular structure of these low molecular weight molecules plays a role in increasing the interparticle strength between metal oxide surfaces but also their solubility and concentration are linked to an attraction between the surfaces.