Hydrogen Bonding and Proton Transfer in Aqueous Toluene Microdroplets Studied by Particle Collision Electrochemistry

Abstract This report describes hydrogen bonding and protonation in toluene microdroplets (0.4 to 4.0 μm) dispersed in water. Such effects are revealed by studying the two‐electron reduction of tetrachloro‐1,4‐benzoquinone (TCBQ) trapped inside the toluene droplets using particle collision electrochemistry (PCE). This approach allows interrogation of microscopic particles in Brownian motion colliding with an ultramicroelectrode and can provide information when the collision encounter is coupled to an electrochemical event. Upon addition of acids in homogeneous toluene, the half‐wave potential of TCBQ shifts positively (easier reduction), thus implying hydrogen bonding and/or protonation of the reduction product TCBQ 2− . The shift is commensurate to the proton donating capacity of each acid, so that, acetic acid (pK a =4.8) displays higher shift than oleic acid (pK a =9.9). However, when the reduction takes place inside toluene droplets, the trend is completely reversed and appears to be driven by preferential partition of acetic acid in water, as opposed to the hydrophobic oleic acid. Furthermore, these effects compete with the underlying binding between TCBQ 2− and the tetraalkyl phosphonium cation of the ionic liquid used to enhance the conductivity of toluene. This work aims to illustrate a subtle, yet consequential effect that emerges from micro‐confinement of immiscible phases, as well as the potential of PCE to extract chemical reactivity information at the organic‐water interface.