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.