Diffusion of redox probes in hydrated sol-gel-derived glasses

Cyclic voltammetry and chronoamperometry have been used to characterize the translational mobility of eight different redox probes trapped in hydrated silica gel monoliths and evaluate the extent of surface interactions. The redox probes, selected for their size and charge, were introduced into a silica sol prepared by the acid-catalyzed hydrolysis of tetramethoxysilane along with an ultramicroelectrode (r = 13 microns) and a Ag/AgCl reference/counter electrode. Relative changes in the magnitude of the Faradaic current, the half-wave potential, E1/2, and the apparent diffusion coefficient (D) were evaluated for each redox probe as the sol gelled, aged, and dried. Significant variations were observed in the diffusion rates between molecules of similar size and structure but varying ionic charge. Large shifts in the redox potential were also observed, with the direction of shift dependent on the entrapped reagent. These results demonstrate the importance of internal surface interactions versus surface confinement in wet and partially dried sol-gel glasses.