INTERFEROMETRIC MEASUREMENT OF DEPLETION LAYER STRUCTURE AND VOLTAMMETRIC DATA IN CONCENTRATED ORGANIC REDOX SOLUTIONS

Phase-measurement interferometric microscopy (PMIM) of the depletion layer surrounding 15-mu m-radius Pt disk electrodes is described. PMIM images record spatial variations in the optical height (Delta h) resulting from refractive index gradients generated by an electrochemical reaction. Images obtained during the voltammetric reduction of 0.1-1.5 M nitrobenzene in benzonitrile solutions containing 0.2 M tetra-n-butylammonium hexafluorophosphate are compared to predictions based on the Lorentz-Lorenz formula and Saito's analysis of transport to a disk-shaped electrode. Good agreement between theory and experiment is obtained at distances within +/-10 radii of the electrode; at larger distances, density-driven fluid convection reduces the refractive index (and concentration) gradients to negligible values, Optical voltammograms, i.e., plots of Delta h vs electrode potential (E), are shown to faithfully mimic the true i-E response, allowing a purely optical measurement of the electrochemical response of a microdisk electrode.