The effects of benzoic acid compounds in α-Al2O3 dispersions: Additional attractive forces of particle bridging and precipitate bridging

The effects of salicylic (2-hydroxybenzoic), 3-hydroxybenzoic, 4-hydroxybenzoic, 3,4-dihydroxybenzoic and 2,3,4-trihydroxybenzoic acids adsorbed additives on the zeta potential-pH and yield stress-pH behaviours of α-Al2O3 suspensions were investigated. With careful selection of the type and nature of adsorbed additives, molecular attribute and surface forces relationship has been determined. At low concentrations, all additives performed well as a steric agent which is reflected by a reduction in the maximum yield stress. At high concentrations, precipitate bridging was found responsible for the 2.5-fold increase in the maximum yield stress by salicylic acid. With 4-hydroxybenzoic acids the 1.2-fold increase occurring at high pH (∼9) could be due to particle bridging by the doubly charged species. Precipitate bridging is a new phenomenon that is not well known in suspension rheology. The 3,4-dihydroxybenzoic and 2,3,4-trihydroxybenzoic acids additives produced similar effects on the yield stress-pH and zeta potential-pH behaviours to that observed with very hydrophilic multiple charged additives despite these compounds being monocarboxylate. The phenolic hydroxyl groups in these compounds essentially uncharged in the pH region of interest, pH < 8. The adjacent hydroxyl groups do form intramolecular hydrogen bonds which were believed not to play a major role in determining their strong steric agent credential.