This work reports on the hydrogen bonding effects induced by water solvation in aqueous toluene droplets studied by Particle Collision Electrochemistry (PCE). The latter approach is an emerging methodology employed to characterize microscopic particles that collide with the surface of an ultramicroelectrode as a result of Brownian motion. The ability of PCE to yield in-situ kinetic and thermodynamic information from single collision events has led to fundamental studies of nanoparticle electrochemistry which now is recognized as a sub-field in electrochemistry dubbed as single entity electrochemistry, due to the wide range of probes that can be investigated.

Herein, the reversal of hydrogen bonding affinity between quinone phenolate di-anions (Q^2-) and carboxylic acids, induced by water in micrometer-size droplets of toluene is reported. Such an effect is revealed by the two-electron reduction of the hydrophobic quinone (Q) tetrachoro-1,4-benzoquinone to Q^2- inside the toluene droplets, when performed in the presence of carboxylic acids that partition differently between the aqueous and organic phases. In bulk toluene, the hydrogen bonding affinity of Q^2- for acetic acid (pK_a = 4.8) is higher than for oleic acid (pK_a = 9.9). However, such trend is reversed in emulsified toluene droplets due to preferential solvation of acetic acid by the surrounding water. The goal of this work is two-fold: first, to illustrate a subtle yet consequential water effect that arises from heterogeneous micro-confinement of organic-water phases, and second, to demonstrate how mechanistic information in such interfaces can be extracted from PCE.