Adsorption of Quaternary Ammonium Compounds on the Surface of Sodium Oxalate: FTIR/ATR Investigation under High-Ionic-Strength, Highly Alkaline Conditions

Using a recently developed method for the in situ investigation of the surface of sodium oxalate a Bayer process solid in highly alkaline, high-ionic-strength, aqueous media, “dose-response” curves for a series of alkyltrimethylammonium bromides on the surface of sodium oxalate have been obtained. These curves show the extent of adsorption increasing with alkyl chain length until the point is reached where the alkyl chain of the adsorbed compound adopts a coil-like orientation at the oxalate surface, thereby reducing the amount adsorbed compared with those for the shorter chain homologues. These shorter chain compounds are adsorbed to the oxalate surface in a predominantly perpendicular manner, while the longer chain compound may adopt a more tilted surface orientation. Results obtained for a synthetic oxalate stabilizer formulation suggest that the compound is adsorbed in a manner analogous to that of the shorter chain compounds. Decreased adsorption observed at higher initial concentrations is due to the formation of large cylindrical or lamellar micelles, which exhibit increased stability relative to that of the adsorbed form. The formation of these large micellar aggregates is a result of the high ionic strength of the adsorption matrix. We believe this to be the first instance in which FTIR/ATR has been used to obtain both qualitative and quantitative information as to the nature of the interfacial layer of dispersed particulate matter.