Isomerism and Solubility of Benzene Mono- and Dicarboxylic Acid: Its Effect on Alumina Dispersions

Low molecular weight benzenedicarboxylic acid has a very well-defined molecular structure because of its rigid and planar backbone. Therefore, it is hypothesized to have high potential for highly directed bridging between surfaces. However, phthalic acid cannot participate in particle bridging because the two carboxylic acid groups on the benzene ring are located adjacent to each other which prevent the molecule from bridging between two surfaces. Yield stress measurements showed that isophthalic and terephthalic acid failed to cause significant rheological changes to alumina slurries within their solubility limit. However, upon increasing the concentration beyond the solubility limit, terephthalic acid increased the yield stress by a factor of 7 and isophthalic acid by a factor of 2 when compared to the same colloidal alumina system without additive. Benzoic acid, which has low solubility at low pH, also showed an increase in yield stress by a factor of 2 even though it lacks the second carboxylic group to link neighboring surfaces. These observations suggest that highly directed bridging is unlikely to operate when these acids are present in high concentration. Instead, the dominant mechanism is most likely attraction between the negatively charged precipitates and the positively charged alumina particles and/or capillary bridging.