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 electrode-aqueous electrolyte-oil droplet presents a considerable challenge due to the complexity of simultaneous electron transfer between electrode and droplet, ion uptake or expulsion between droplet and aqueous phase, the interaction of redox centers at high concentration, and transport processes accompanying the electrochemical process. For the case of oxidation of para-tetrahexylphenylenediamine (THPD) microdroplet deposits on basal plane pyrolytic graphite electrodes or random arrrays of microelectrodes (RAM) three models may be envisaged which proceed via A) exchange of ions between droplet and aqueous electrolyte with the electrochemical process commencing at the electrode-oil interface, B) rapid electron transport over the oil-aqueous 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 process commencing solely from the triple interface. Numerical simulation procedures for these three models, which allow for interaction of redox centers via a regular solution theory approach, are compared with experimental data. A positive interaction parameter, Z = 1.4, consistent with a dominant ionic liquid-ionic liquid and neutral oil-neutral oil type interaction is determined from experimental data recorded at sufficiently slow scan rates. The overall mechanism, which governs the voltammetric characteristics at higher scan rates, is shown to be apparently consistent with the triple interface model C). However, the rate of diffusional transport determined by comparison of experimental with simulation data is orders of magnitudes too high. Additional convection processes, possibly of the Marangoni type, appear to be responsible for the fast rate observed for the redox process. The significance of the models presented in the context of microdroplet deposits for other related electrochemical systems is discussed.