Voltammetry of electroactive oil droplets. Part II: Comparison of experimental and simulation data for coupled ion and electron insertion processes and evidence for microscale convection

Modelling electrochemical processes at the three phase junction between ele ctrode-aqueous electrolyte-oil droplet presents a considerable challenge du e to the complexity of simultaneous electron transfer between electrode and droplet, ion uptake or expulsion between droplet and aqueous phase, the in teraction of redox centers at high concentration, and transport processes a ccompanying the electrochemical process. For the case of oxidation of para- tetrahexylphenylenediamine (THPD) microdroplet deposits on basal plane pyro lytic graphite electrodes or random arrrays of microelectrodes (RAM) three models may be envisaged which proceed via A) exchange of ions between dropl et and aqueous electrolyte with the electrochemical process commencing at t he electrode-oil interface, B) rapid electron transport over the oil-aqueou s electrolyte interface and the electrochemical process commencing from the oil-aqueous electrolyte interface inwards, and C) slow electron transport across the oil-aqueous electrolyte interface and the electrochemical proces s commencing solely from the triple interface. Numerical simulation procedu res for these three models, which allow for interaction of redox centers vi a a regular solution theory approach, are compared with experimental data. A positive interaction parameter, Z=1.4, consistent with a dominant ionic l iquid-ionic liquid and neutral oil-neutral oil type interaction is determin ed from experimental data recorded at sufficiently slow scan rates. The ove rall mechanism, which governs the voltammetric characteristics at higher sc an rates, is shown to be apparently consistent with the triple interface mo del C). However, the rate of diffusional transport determined by comparison of experimental with simulation data is orders of magnitudes too high. Add itional convection processes, possibly of the Marangoni type, appear to be responsible for the fast rate observed for the redox process. The significa nce of the models presented in the context of microdroplet deposits for oth er related electrochemical systems is discussed.