Measurement of solute partitioning across liquid/liquid interfaces using scanning electrochemical microscopy-double potential step chronoamperometry (SECM-DPSC): Principles, theory, and application to ferrocenium ion transfer across the 1,2-dichloroethane/aqueous interface

SECM-DPSC is extended as an approach for probing the transfer of a target electrogenerated species across the interface between two immiscible phases, by developing a theoretical model for reversible phase transfer. The SECM-DPSC technique involves the electrogeneration of a target species at an UME positioned close to an interface in an initial (forward) potential step. After a defined period, this species is then collected back at the UME by reversing the potential step. The numerical treatment for calculating the UME current-time response during the forward and reverse potential steps is outlined, without restriction on the partition coefficient of the solute between the two phases, thereby building on an earlier model (Slevin, C. J.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 1997, 101, 10851). It is shown that the technique can be used to measure both the partition coefficient and kinetics of phase transfer. The approach is illustrated through experimental studies of the transfer of the oxidized form of ferrocene (Fc) and derivatives across a 1,2-dichloroethane (DCE)/ aqueous interface, with ClO4- present in excess in each phase. The transfer of ferrocenium and dimethyl-ferrocenium, accompanied by ClO4- to maintain charge neutrality, are both found to be diffusion limited (rate constants > 0.5 cm s(-1)), even on the fastest time scales accessible with SECM. In contrast, deca-methylferrocenium ion does not appear to transfer across the interface under the defined conditions. In certain cases, such as slightly elevated temperatures, UME oxidation of ferrocene derivatives in the vicinity of the DCE/aqueous interface, was found to be accompanied by interfacial instabilities akin to a Marangoni phenomenon. SECM may provide a new method for initiating and monitoring such effects.