Mesurement 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-dichloroet

摘要

SECM-DPSC is extended as an approach for probing the transfer of a target electrogenerated species across the interface betwen 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 n an initial (forward) potential step. After a defined period, this species is then collected back at the UME by reversing the potnetial step. The numerical treatment for calculating the UME current-time response during the forward and reverse potnetial steps is outlined, without restriction on the partition coefficient o fthe 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 patition coefficeitn 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)/queous interface, with ClO_4~- present in excess in each phase. The transfer of ferrocenium and dimethyl-ferrocenium, accompanied by ClO_4~- to maintain charge neurality, 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, decamethylferrocenium 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 derivsatives in the vicinity of the DCE/aqueous interface, was found to be accompanied y interfacial instabilities akin to a Marangoni phenomenon. SECM may provide a new method for initiating and monitoring such effects.