The effects of molecular structure and architecture of multiple-functional groups, citric acid-type compounds on the yield stress of 55 wt%α-Al2O3 dispersions were evaluated. They are cis- and trans-aconitic (or dehydrated citric) acids, and propane-1,2,3 tricarboxylic acid. These compounds possessed distinct differences in structure and, the presence and absence of −OH functional group. The (maximum) yield stress at zero zeta potential was used to deduce the effect of molecular forces on the interparticle force. Molecular modeling of these additives with ChemOffice software was used to infer the type of intra- and intermolecular forces arising from the adsorbed additives. Adsorbed citrate reduced the maximum yield stress significantly. In contrast, trans-aconitic acid increased it very significantly at low coverage. Both cis-aconitic and propane-1,2,3 tricarboxylic acids did not increase or decrease it. Adsorbed citric acid acts as a steric layer. Intramolecular hydrogen bonding between the −OH group and the free carboxylic acid group is present in all citric acid ionic species, thus hindering particle-bridging interactions from occurring. Adsorbed trans-aconitic acid bridged the particles in dispersion as the two carboxylic acid groups in the trans-position are in an ideal spatial position for particle bridging. The opposite is true for the cis-aconitic acid; however, some degree of bridging is present and this is due to the third carboxylic acid group that is free but is too flexible for effective bridging. Propane-1,2,3 tricarboxylic acid with an even more flexible backbone possessed a similar inability to bridge particles effectively.
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Document Type: Research Article
Chemical and Process Engineering, School of Mechanical Engineering and Centre for Strategic Nanofabrication, the University of Western Australia, Crawley 6009, Australia
Publication date: 01 September 2010