| This dissertation describes mass transfer in complex geometries encountered in electrochemistry. The effects of space geometry and electric field on the mass transfer are investigated by electrochemical measurements and finite-element simulations.; In the first part of this dissertation, finite-element simulations and linear sweep voltammetry of Au opal modified electrodes in aqueous solutions containing an electroactive molecule and a supporting electrolyte are used to determine molecular diffusion coefficients, Dfcc, within the face-centered cubic opal. Dfcc is related to the diffusion coefficient of the molecule in free solution, D sol, by the relationship, Dfcc = (epsilon/tau)Dsol, where epsilon is the interstitial volume fraction of a fcc opal and tau is the tortuosity. An interesting finding of this study is the fact that diffusion in close-packed structure is anisotropic.; In part two of this work, the voltammetric behavior of nanopore electrodes have been investigated by finite-element simulations and theory. The steady-state diffusive flux of molecules into a deep pore is limited by the restriction near the pore orifice and, thus, the steady-state current is independent of the pore depth. This unique transport property is potentially useful in studying mass transport through nanometer-scale orifices. In addition, molecular transport through the nanopore orifice in low ionic strength solutions was investigated. Steady-state voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and are thus strongly dependent on the charge of the redox molecule, and the relative concentrations of the supporting electrolyte and redox molecule. The limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for oxidation of the neutral species ferrocene is unaffected by the ionic strength.; In the final part of this dissertation, the effects of permeant concentration upon transscleral iontophoretic transport are also reported. A constant direct current (DC) iontophoresis was conducted with rabbit sclera in vitro over a wide range of permeant (salicylate and tetraethylammonium) concentrations. Model simulations of iontophoretic concentration profiles and transport rates were carried out and compared with the experimental data. It was found that the fluxes of the ionic permeants increased linearly with the electric current but were relatively independent of their donor concentrations. Under the asymmetric donor and receiver conditions, the transference of the permeants could not be predicted by the concentrations of the ions in the donor and receiver chambers. |
