SLOW CHANGE IN THE ELECTRICAL POTENTIAL AT GLASS AND SILICA SURFACES DUE TO NA+ SORPTION IN THE HYDRATED LAYER

Controlled Pore Glass (CPG) is a high-SiO2 content, Vycor-type glass w hich resembles fused silica. In CPG-Oxine the ligand oxine is covalent ly bound to the CPG surface. The presence of SiO- groups on the CPG ge nerates a negative electrical potential at the surface (psi(o)) and al so in the solution adjacent to the surface (psi(x)), where the immobil ized oxine is to be found. The potential psi(x) influences the extent of complexation of Ca2+ (from solution) by oxine so that the bound oxi ne serves as a probe of electrical potential near the surface. When th e solution pH is raised there is a relatively rapid ionization of SiOH groups to SiO-. Then, slowly (i.e. tens of minutes), Na+ from solutio n diffuses into the hydrated gel layer on CPG. This reduces the negati ve charge, making psi(x) less negative and, consequently, reduces the extent of complexation of Ca2+ by oxine. A linear relationship is pred icted and experimentally observed between the logarithm of the sorbed Ca2+ concentration (mmol/g) and the potential psi(x) (V). The potentia l psi(x) is expected to correlate strongly with the zeta potential whi ch controls the rate of electroosmotic flow at silicious surfaces.