Surfactant control of interfacial reaction rates in aqueous microdroplets

Microdroplets are purported to enhance reaction rates and produce spontaneous chemical transformations that are unfavorable in macroscale systems. The gas-liquid interface is widely implicated for the emergence of these chemical anomalies. Experiments conducted on individual microdroplets provide a unique platform for studying interfacial effects while bypassing transport limitations often encountered in macroscopic systems. We investigate such interfacial effects by analyzing the suppression of the I^- + O₃ surface reaction by the nonionic surfactant Triton X-100 in an array of levitated microdroplets. Using a novel kinetic framework describing surface and bulk reactivity, we find that increasing surfactant concentration shifts the locus of reaction from the top-most nanometer of the interface to a subsurface region located <2 nm below the surface. This detailed picture of spatial reactivity is afforded only by considering the timescales for O₃ adsorption and solvation combined with reaction monitoring in single microdroplets. Chemical uniqueness is further illustrated by the I^- + O₃ rate dependence on [Triton X-100], which is quantitatively understood by considering finite size effects in microdroplets. Both size and surface effects are considered to derive an analytical expression for reactive uptake of O₃ which demonstrates the role of competitive adsorption of Triton-X and I^- at the gas-liquid interface. This work establishes a novel approach to understanding interfacial reaction kinetics that successfully links nanometer-scale dynamics from molecular simulations with macroscale measurements of surface tension and microscale measurements of chemical reactivity.