New approaches for the study of the kinetics of reactions at immiscible liquid/liquid and air/liquid interfaces
This thesis describes the development of new techniques and new approaches for studying the kinetics and mechanisms of reactions which occur at the interface between two immiscible liquids. New approaches for studying the kinetics of transfer processes at the air/water interface are also described. Scanning electrochemical microscopy (SECM) is employed in the equilibrium perturbation (BP) mode for studying reversible transfer processes at liquid/liquid and air/water interfaces. In this application, an ultramicroelectrode (UME) located in an aqueous phase, at micro metre distances from the interface of interest, is employed to drive a transfer process, initially at equilibrium, in the direction of the aqueous phase by depleting the local aqueous concentration of a target species by electrolysis. The UME current flow depends on the transfer kinetics at the interface. The development of a submarine UME allows SECM to be conducted at the air/water interface, or at a liquid/liquid interface with the electrode in the more dense phase. The kinetics of the extraction/stripping reactions of aqueous copper (II) with an oxime ligand (Acorga PSO) in 1,2-dichloroethane (DCE) and heptane are first investigated. Subsequently, measurements of the kinetics of oxygen transfer across a condensed monolayer of l-octadecanol, as a function of surface area, demonstrate that the accessible free area of the interface primarily governs the rate of oxygen transfer. SECM double potential step chronoamperometry is developed to study irreversible transfer processes at interfaces. Theoretical modelling is applied and tested through experimental measurements on model interfaces. Subsequently, the rate of transfer of bromine from aqueous solutions to DCE and to air is shown to be above the upper limit measurable by the technique, however, a lower limit on the first order transfer rate constant ofO.S cm S-l is assigned. A new technique, termed microelectrochemical measurements at expanding droplets (MEMED) is developed for studying spontaneous reactions at liquid/liquid interfaces. In MEMED, the two liquids are contacted by flowing one (feeder) through a capillary submerged in the second (receptor), resulting in the growth of drops at the capillary tip. The interfacial reaction generates a product or reactant concentration profile, which extends into the receptor phase. This is probed directly using a UME positioned opposite the capillary in the solution, operated in either a potentiometric or an amperometric mode, as a local concentration probe. A numerical model for mass transport in this configuration is developed, and the technique and model are assessed by measuring both bromine transfer from aqueous sulfuric acid solutions to drops of DCE, and bimolecular electron transfer between iridium (IV) chloride in the aqueous solution and ferrocene in the organic phase, which exhibit transport-controlled transfer rates under the conditions employed. The MEMED technique is applied to measure the kinetics of the hydrolysis of triphenylmethyl chloride (TPMCl), at the DCE/water interface, through potentiometric measurement of the chloride ion concentration profile. The reaction is shown to be first-order in TPMCI, occurring interfacially, with a rate constant of 6.5 x 10-5 cm s". Subsequently, the oxidation of methylanisole (MA) (feeder) by aqueous solutions of cerium (IV) (Ce(IV» (receptor), is probed. Under the conditions of this study, the reaction at the interface dominates, with a negligible contribution from the aqueous phase reaction between dissolved MA and Ce(IV).