THE INFLUENCE OF OXYGEN DEFECT STATES ON THE SURFACE-CHARGE OF ALKALI-HALIDES

The sign of the surface charge for alkali halides in their saturated b rines has been established from nonequilibrium electrophoresis measure ments, and on this basis the flotation response of alkali halides was explained based on the adsorption of oppositely charged collector coll oids by heterocoagulation. These surface charge results for alkali hal ides are generally as expected from simplified lattice ion hydration t heory with some exceptions, specifically LiF, KCl, NaCl, KBr, RbBr, KI , and RbI. The presence of natural oxygen defect states in the crystal lattice and the inaccuracies involved in the gaseous ion hydration fr ee energy values used appear, in the past, to be responsible for these deviations. The presence of natural oxygen defect states in KCI, KBr, RbBr, and RbI, as characterized by UV/Vis absorption, was found to ac count for their anomalous surface charge. Further, other positively ch arged alkali halides, LiF, NaCl, and KI, were found to be free of oxyg en defect states. Nevertheless, oxygen defects can be created in these alkali halides to make them negatively charged, and this has been dem onstrated for NaCl. Such negatively charged NaCl can be readily floate d with positively charged amine collector colloids, thus confirming th e surface charge/collector colloid adsorption theory. (C) 1996 Academi c Press, Inc.