Scanning Electrochemical Microscopy (SECM) as a Probe of Transfer Processes in Two-Phase Systems: Theory and Experimental Applications of SECM-Induced Transfer with Arbitrary Partition Coefficients, Diffusion Coefficients, and Interfacial Kinetics

Abstract

Scanning electrochemical microscopy induced transfer (SECMIT) is introduced as a new approach for probing the dynamics of partitioning of electroactive solutes between two immiscible phases. The partitioning process, initially at equilibrium, is perturbed by electrolysis of the target solute at an ultramicroelectrode (UME) tip positioned in one phase, close to the interface with a second phase. For a particular separation between the tip and interface, the flux of soluteand hence current flowingat the UME is governed primarily by the partition coefficient of the solute, Ke, the relative diffusion coefficients of the solute in the two phases, γ, and the interfacial transfer kinetics. A numerical model is developed for SECMIT under potential step chronoamperometric conditions, where the target solute is electrolyzed at the UME at a diffusion-controlled rate. When the interfacial kinetics are nonlimiting, the steady-state current is strongly dependent on the product Keγ, particularly for Keγ > 1, while for constant Keγ the corresponding chronoamperometric response depends on the individual values of Ke and γ. In principle, this methodology provides a route to determining the diffusion coefficient and/or concentration of a target solute in a medium, without contact from the UME. This aspect of SECMIT is illustrated, under steady-state conditions, through studies on the transfer of (i) several types of electroactive ions between hydrogels and aqueous solutions and (ii) oxygen transfer across the interface formed between water and either 1,2-dichloroethane (DCE) or nitrobenzene (NB). The application of SECMIT to probe interfacial kinetics is illustrated through studies of cupric ion extraction and stripping between an aqueous phase and DCE containing the oxime ligand, Acorga P50 (LH).