Microelectrochemical measurements at expanding droplets: Effect of surfactant adsorption on electron transfer kinetics at liquid/liquid interfaces

Adsorption of the nonionic surfactant Triton X-100 at the interface between two immiscible electrolyte solutions (ITIES) and its effect on the electro n transfer (ET) reaction between tetracyanoquinodimethane in 1,2-dichloetha ne and aqueous Fe(CN)(6)(4-) was studied using microelectrochemical measure ments at expanding droplets (MEMED). A numerical model was developed for th is process by assuming that adsorption of the surfactant at the ITIES was L angmuirian and that the ET reaction only occurred at the uncovered portion of the ITIES. Theoretical results show that, for typical MEMED conditions, the surfactant adsorption. process attains the diffusion-controlled limit i f the rate constant is' greater than 1 cm s(-1). The inhibitory effect of s urfactant adsorption on the ET process produces changes in the reactant con centration profile adjacent to the droplet, which depend on the bulk surfac tant concentration, equilibrium constant (K), the maximum surface coverage (Gamma (max)), and the ET kinetics. Methods for determining these parameter s are suggested. The effect of Triton X-100 on the ET reaction was measured over a; wide range of conditions, with bulk aqueous Triton X-100 concentra tions in the range 2.5 x 10(-5) to 2.5 x 10(-4) M, over various time scales . Experimental results were found to be in excellent agreement with theoret ical predictions and yielded an ET rate constant of 0.0020 +/- 0.0001 cm s( -1) for the clean interface, with the potential across the ITIES establishe d with 0.1 M ClO4- in each phase. The diminution in the ET rate with surfac tant present was consistent with diffusion-controlled surfactant adsorption , characterized by K = 2.7 x 10(4) M-1 and a value of Gamma (max) = 3 x 10( -10) mol cm(-2).