Transport of neutral and ionic solutes: the gel/electrode and gel/electrolyte interfaces

Transport through a polysaccharide gel phase has been investigated voltammetrically using both redox voltammetry at a gel covered electrode and ion transfer voltammetry across the gel/liquid interface (Gel/L). The apparent diffusion coefficients, D(app), of a range of neutral and ionic electroactive species have been determined in both uncharged and anionic polysaccharide media, agar, and kappa-carrageenan, respectively. It is shown that the diffusion of electroactive species in agar gel occurs at a rate similar to that of diffusion in aqueous solution for a range of redox couples. In the kappa-carrageenan medium, by contrast, the diffusion coefficient obtained for cationic solutes was found to be approximately an order of magnitude lower than the value in aqueous solution. The difference in D(app) is attributed to two independent processes: electrostatic interactions between the charge of the sulfonate groups of the kappa-carrageenan gel and the charge of the solute, as well a change in hydration of the solute molecules.