The oxidation of ferrocene in sessile toluene macro- and microdroplets: An opto-electrochemical study

Obtaining mechanistic insight into interfacial electron and ion transfer processes remains imperative to developing a comprehensive understanding of synthetic and biological processes. There is fundamental interest in the coupled electron-ion transfer processes characteristic of heterogeneous reactions at three-phase boundaries comprised of two immiscible liquids and a solid electrode surface. To probe reactivity at the three-phase boundary, we report a multifaceted analysis of ferrocene (Fc) oxidation in toluene droplets of diverse sizes. In one experiment, we study Fc oxidation in a large toluene macrodroplet. In a second experiment, we study Fc oxidation in an array of toluene microdroplets. We performed each experiment with and without an ionic liquid, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide. Upon Fc oxidation in the oil phase without supporting electrolyte, the ferrocenium cation (Fc(+)), which is soluble in water, is expelled from the oil phase to maintain charge neutrality. Because Fc is only slightly soluble in water (similar to 40 mu M), reversing the scan to reduce Fc(+) in the aqueous phase generates a Fc precipitate adjacent to the three-phase boundary. This observation was confirmed by correlated optical microscopy. Increasing the voltammetric scan rate elucidated sequential versus concerted ion transfer mechanisms, which are shown to be highly dependent on the droplet size and the presence of trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide in the toluene phase. Differences in voltammetric responses are supported by finite element simulations.