Voltammetric Analysis of Redox Reactions and Ion Transfer in Water Microdroplets

We report a set of voltammetric experiments for studying redox reactions and ion transfer in water microdroplets emulsified in 1,2-dichloroethane (DCE). The electrochemistry of microdroplets (r_drop ∼ 700 nm) loaded with either ferrocyanide ([Fe(CN)₆]^4-) or ferricyanide ([Fe(CN)₆]^3-), chosen due to their hydrophilic nature, was tracked using cyclic voltammetry. These heterogeneous reactions necessitated ion transfer at the droplet interface to maintain charge balance in the two liquid phases during oxidation or reduction, which was facilitated by the tetrabutylammonium perchlorate ([TBA][ClO₄]) salt in the DCE phase. Experiments were performed with (1) a single macrodroplet (10^-7 L) on a macroelectrode (r ∼ 1.5 mm), (2) millions of microdroplets (10^-15 L) adsorbed on to a macroelectrode (r ∼ 1.5 mm), and (3) at the single microdroplet level via observing individual microdroplet collisions at an ultramicroelectrode (r ∼ 5 μm). We demonstrate that when millions of microdroplets are adsorbed onto a macroelectrode, there are two surprising observations: (1) the half-wave potential (E_1/2) for the [Fe(CN)₆]^3-/4- redox couple shifts by +100 mV, which is shown to depend on the number of droplets on the electrode surface. (2) The reduction of [Fe(CN)₆]^3-, which is assisted by the transfer of TBA⁺ into the water droplet, displays two waves in the voltammogram. This dual-wave behavior can be explained by the formation of TBAK_3-Fe(CN)₆, which is soluble in DCE. Additionally, we demonstrate that the adsorption of microdroplets onto an electrode surface offers significant amplification (×10³) of the water/oil/electrode three-phase boundary when compared to the adsorption of larger macrodroplets, permitting a rigorous evaluation of heterogeneous chemistry at this distinct interface. In combination, these experiments provide new energetic and mechanistic insights for droplet systems, as well as reactivity differences between microscale and bulk multiphase systems.