Probing Proton Delivery in Microheterogeneous Water Structures Using a Proton-Dependent Isomerization Reaction of a Merocyanine Photoacid.

Water structure and proton dynamics in complex environments, such as mixed electrolytes, biological environments, and microdroplet surfaces, are often hypothesized to affect reaction thermodynamics, kinetics, and selectivity. Toward better understanding the influence of water microphases in complex mixtures, this study leverages the proton-dependent recovery kinetics of a merocyanine photoacid in acetonitrile (ACN) and dimethyl sulfoxide (DMSO) over a range of water mole fractions χ. We report that the rates of recovery, , do not scale linearly with χ. In DMSO, which is a strong hydrogen bond acceptor, is quite slow until χ ∼ 0.7 and increases linearly beyond that value. This observation implies that the reaction requires the establishment of an extended hydrogen bond network that can only be afforded beyond a threshold. In contrast, the recovery rate in the more weakly hydrogen bond acceptor, ACN, shows three distinct regions as a function of increasing χ, implying isolated water molecules χ < 0.2, water nanopools 0.2 < χ < 0.6, and extended hydrogen bond networks χ > 0.6. Furthermore, when adding a model surfactant, cetyltrimethylammonium bromide (CTAB), to the ACN-HO mixtures, a sharp decline in the recovery rates is observed beyond χ ∼ 0.8. This behavior is consistent with the formation of micelles, likely incorporating the photoacid and limiting their access to the otherwise largely hydrogen-bonded network of water. This study informs the design principles of water delivery and proton access for creating tailored protonic environments for tuning reactivity.