Transport coefficients of an inorganic Brownian particle in solution: The tungstosilicate anion

Self-diffusion coefficients of the tungstosilicate anion SiW12O404- have be en determined in an HClO4 medium of various concentrations by voltammetric measurements at a glassy carbon electrode. The results have been extrapolat ed to zero ionic strength using an analytical transport theory (mean spheri cal approximation (MSA) transport calculations). The electrical conductivit y of aqueous sodium tungstosilicate solutions with and without added salt h as been then calculated using both the MSA transport method and Brownian dy namics simulations. Calculations have been compared to our experimental dat a. It has been shown that the extrapolation at infinite dilution of the con ductivity of sodium tungstosilicate solutions gives a value for the self-di ffusion coefficient of the polyion consistent with the voltammetric determi nation. Moreover, the sodium tungstosilicate electrolyte exhibits a behavio r similar to a simple electrolyte in aqueous solution, whereas the addition of a salt yields an ionic association phenomenon.