Surface electrical properties of barium sulfate modified by adsorption of poly α, β aspartic acid

The surface electrical properties of precipitated barium sulfate in aqueous solution have been probed in the presence and absence of synthetic poly alpha, beta aspartic acid. The effect of cation type, ion concentration, and pH on the electrokinetic properties of barite with and without adsorbed polyaspartic acid has been analyzed to afford insights into the mechanism by which polyaspartic acid acts as an inhibitor of barium sulfate precipitation. In addition, vibrational spectroscopy has been employed to monitor bond changes within polyaspartic acid molecules adsorbed on precipitated barium sulfate. These studies have indicated that the surface electrical charge of "pure" barite differs significantly from that of barite precipitated in the presence of calcium ions. Moreover, the presence of the cations Ca2+ and Mg2+ in the suspending liquor has a marked influence of the measured electrokinetic properties. The infrared spectral studies strongly suggest that polyaspartic acid is bound to the barite via the backbone nitrogen atoms. This proposal in conjunction with the electrokinetic and adsorption data has allowed a tentative model of barium sulfate inhibition to be proposed; viz, calcium ions are needed to complex with the polyaspartic acid, first, to reduce the molecules electrical charge in order to promote adsorption onto a charged surface; second, to induce a conformational change in the molecule in order to allow optimal anchoring via the backbone nitrogen atoms; and, third, to reduce the solubility of the molecule in order to increase its surface activity. (C) 1999 Academic Press.