ADDITIONAL ATTRACTIVE FORCE BETWEEN ALUMINA PARTICLES DUE TO LOW SOLUBILITY OF DICARBOXYLIC ACIDS

摘要

Low molecular weight charged molecules are known to change the rheological behaviour of oxidedispersions dramatically. Here the isomers of muconic acid were used to investigate the effect ofmolecular structure and solubility on the bulk properties of alumina dispersions and the nanoscaleinteractions between alumina surfaces. The surface forces in dispersions were characterised by yieldstress while atomic force microscopy (AFM) was used to directly measure the force between a singlealumina particle and an alumina substrate. Both (trans, trans) TT and (cis, cis) CC muconic acidwere found to increase the yield stress of alumina slurries significantly at low pH when comparedto that of the pure alumina. TT muconic acid achieved a much higher yield stress than that of CCat high additive concentration. AFM measurements revealed force-distance features that indicate acapillary-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 anelectrostatic 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 sourceof the capillary force. The capillary force at high concentration of TT and CC muconic acid observedduring AFM measurements can account for the large increase of yield stress at low pH. The bridgingmechanism applicable in other cases is not found to be operating at high concentration of adsorbedmuconic acids. This study revealed that not only the molecular structure of these low molecularweight molecules plays a role in increasing the interparticle strength between metal oxide surfacesbut also their solubility and concentration are linked to an attraction between the surfaces.