Charged Barite−Aqueous Solution Interface:  Surface Potential and Atomically Resolved Visualization

Abstract

The charged barite−aqueous solution interface has been studied in water and electrolyte solution using laser Doppler electrophoresis to obtain ζ-potentials. High-resolution atomic force microscopy (AFM) was used to gain a local description of the distribution of the Ba 2+ and SO 4 2 - ions that act as the potential-determining ions at the barite (001)−aqueous solution interface. The ζ-potential of natural barium sulfate particles in water, against its saturated solution, was measured to be ca. −20 mV. The addition of soluble barium and sulfate salts significantly influenced the surface potential through specific ion adsorption from solution. Over a small concentration range, for very low Ba 2+ or SO 4 2 - concentrations, the ζ-potential of barite is a logarithmic function of the activity of the electrolyte solution and the slope roughly follows the Nernst relation for a thermodynamically reversible electrode. AFM imaging, using a low net attractive loading force, showed that in addition to the surface lattice, step edges and pointlike defects were visible with true lateral atomic resolution. In the Nernstian region, no surface rearrangement or evidence for adsorbed species was obtained. However, at higher concentrations of the potential-determining ions, islandlike agglomerations of ions (or counterions) were observed with lateral atomic resolution. This observation is explained in terms of changes in both the Debye length and electrophoretic charge densities.