Interfacial Properties of Barium Sulfate Suspensions. Implications in Their Stability

Abstract

A surface characterization of barium sulfate particles in aqueous suspensions was carried out in this work. With the aim of predicting the stability conditions of these widely used suspensions, the electrical surface properties of the particles were first studied by electrophoretic mobility determinations. It was found that both H+ and OH− ions can be considered as potential‐determining ions for the barium sulfate/water interface. The same conclusion was reached concerning the lattice ions, Ba2+ (mainly) and SO42−. It was also found that increasing the concentration of sodium chloride in the dispersion medium can even change the sign of the zeta (ζ) potential: it is suggested that this behavior is an indirect effect provoked by changes in the solubility of barium sulfate with the ionic strength. To compute the van der Waals (LW) attraction between the particles as well as the acid‐base contribution to the total energy of interaction, a thermodynamic characterization of the interface was also carried out by measuring the rate of penetration of selected liquids through plugs of the particles. It was found that barium sulfate particles are essentially monopolar in nature; that is, they show electron‐donor character as demonstrated by the essentially zero value of the electron‐acceptor component of their surface free energy. Pretreatment of the particles with 10−2 M solutions of BaCl2 and CaCl2 significantly reduced the electron‐donor component, whereas both NaCl and Na2SO4 provoked the opposite effect. This result is explained in terms of the acid–base character of the ions added. These data were used to calculate the interaction energy between the particles. The effect of electrolyte concentration on the stability of the suspensions was analyzed on the basis of the dependence with distance of the interaction energy between the particles. Our results suggest that more stable suspensions of barium sulfate are predicted if moderate amounts of SO42− ions are added to the dispersion medium. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89: 1134–1142, 2000