Deducing Reaction and Diffusion Depths of Near-Interfacial Solvated Electrons from pH-Dependent Product Evaporation

Near-interfacial electrons in water can be produced by bombarding an aqueous microjet in vacuum with gas-phase sodium atoms. These Na atoms immediately ionize into Na⁺ and e_s^-, which can then react with surface-active molecules that preferentially populate the surface. We carried out these experiments by reacting e_s^- with the surfactant benzyltrimethylammonium (BTMA⁺) in a 6.7 M LiBr/H₂O microjet at 242 K as a function of pH between 1 and 5. The reaction products, trimethylamine (TMA) and benzyl radical, evaporate into the gas phase where they are detected by a mass spectrometer. We find that TMA evaporation sharply diminishes with increasing H⁺ concentration and is barely visible at pH = 1, while benzyl evaporation varies much less. These results indicate that TMA protonation overwhelms TMA evaporation at 0.1 M H⁺. Diffusion-reaction modeling matches the observed trends and predicts that e_s^- reacts with BTMA⁺ within the top 20 Å at all pH values. However, TMA molecules that evaporate and escape protonation diffuse on average only over 20 Å at pH = 1 but over 1000 Å at pH = 5. These observations emphasize that the near-interfacial region provides a controllable reaction environment that is also an escape route for volatile intermediates, a route that is unavailable deep in the bulk. The competition between evaporation and reaction depends on the solubility of the intermediate, the location of its creation, and the propensity for secondary reactions.