KINETICS OF ALKALI SILICATE AND ALUMINOSILICATE GLASS REACTIONS IN ALKALI CHLORIDE SOLUTIONS - INFLUENCE OF SURFACE-CHARGE

Initial rates of alkali ion exchange from sodium trisilicate, potassiu m trisilicate, lithium disilicate, and several sodium aluminosilicate glasses were measured as a function of electrolyte concentration and p H (concentration scale) using pH-stat methods. For the alkali silicate glasses, rates of ion exchange were depressed in solutions of their r espective alkali chlorides except near the isoelectric point (pH 2.3), where ion exchange rates in deionised water and in electrolytes were indistinguishable. Similar but smaller rate depressions were obtained for sodium aluminosilicate glasses in sodium chloride solutions. Trend s in these initial rates were consistent with predictions of a model r elating glass reaction kinetics to the surface concentration of hydrog en ions, which is a function of the electric surface potential at the glass/solution interface. Although not directly measurable, electric s urface potentials were calculated as a function of electrolyte concent ration and pH using site-dissociation theory.