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Current-time recordings of toluene microdroplets emulsified in water and containing 20 mM Ferrocene (Fc), show multiple electrochemical peaks from oxidation of Fc on disk microelectrodes (5μm-diameter). The average droplet diameter (~0.7 μm) determined from area integration of the peaks was close to Dynamic Light Scattering measurements (~1 μm). Random walk simulations were performed deriving equations to simulate droplet electrolysis using the diffusion and thermal velocity expressions established by Einstein. The simulations show that multiple droplet-electrode collisions, lasting ~0.11 μs each, occur before a droplet wanders away. Updating the Fc-concentration at every collision shows that a droplet only oxidizes ~0.58 % of its content in one collisional journey. In fact, it would take ~5.45 x 106 collisions and ~1.26 h to electrolyze the Fc in one droplet with the collision frequency derived from the thermal velocity (~0.52 cm/s) of a 1μm-droplet. To simulate adsorption, the droplet was immobilized at first contact with the electrode while the electrolysis current continued to be iteratively computed until the end of the simulation. This approach along with modeling of instrumental filtering produced the best match of experimental peaks, which were attributed to electrolysis from single adsorption events instead of elastic collisions. These results point to a heightened sensitivity and speed when relying on adsorption instead of elastic collisions. The electrochemical current for the former is limited by the probability of adsorption per collision, whereas for the latter, the current depends on the collision frequency and the probability of electron transfer per collision.
Single entity electrochemistry, Emulsion, Random walk, Sensor, Bulk electrolysis, Droplet electrochemistry, ultramicroelectrode, Ferrocene
Julio C. Alvarez
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VCU Graduate Research Posters