Hydrolysis Kinetics at the Air-Water Interface

Reactions at aqueous interfaces are significant in many disciplines and have recently been implicated in explaining the unique chemistry occurring in aqueous microdroplets. However, understanding the mechanisms behind these phenomena remains debated, emphasizing the need for establishing benchmark reactions and examining them under controlled conditions. Toward this goal, we report hydrolysis of a triflate group via nucleophilic substitution at the air-water interface. Triflate is an excellent leaving group and dissociates upon nucleophilic attack by either OH- or water. In addition, possible protonation of the triflate group in acidic solutions further enhances the leaving group ability, providing an acid-catalyzed reaction pathway. Using infrared reflection-absorption spectroscopy (IRRAS), we find that the surface-bound triflate hydrolyzes on a relatively long time scale in the order of several minutes. Surprisingly, we find that the rate of hydrolysis does not change significantly at the extremely basic pH = 13 or acidic pH = 1 conditions, suggesting that the reaction is not rate-limited by pH. The measurements over mixtures of water and ethylene glycol, however, distinctly slow the reaction. Such direct measurement of a textbook organic reaction at the air-water interface and its insensitivity to pH can help promote an understanding of the role of interfacial proton activity and solvation within the broader context of surface and microdroplet reactions.