Direct, real-time observation of a photochemical reaction at the air–water interface
Femtosecond interface-selective nonlinear vibrational spectroscopy revealed that the photochemical reaction of phenol proceeds 10,000 times faster at the water surface than in the bulk aqueous phase.
Chemistry at the interface between water and other phases has been considered to be different from that in bulk. However, there had been no direct, real-time observation that tracks chemical reactions taking place at the interface because of experimental difficulty. In 2016, we developed a new method that provides femtosecond time-resolved vibrational spectra of the interface where chemical reactions taking place: UV-excited time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy.[Ref.1]
Using this method, we first tried to track some photochemical reactions at the water surface by detecting vibrational signals of the products. However, it ended in failure, likely because the amount of the products generated by UV excitation is very small at the water surface. Therefore, we decided to observe vibrational signals due to plenty of water molecules that hydrate the products. Charged products are a key to succeed the measurement because many water molecules can interact strongly with them. Since phenol produces an electron and a proton through UV excitation, we targeted the photochemical reaction of phenol. As a result, we found out that the kinetics of the reaction is very different between at the interface and in bulk.
This study is the first example directly demonstrating that chemistry at water interface can be very different from that in bulk. This fact will be very important to understand how chemical reactions at the water surface has an impact on the global environment because water surface is ubiquitous in nature in the form of bubbles and aerosols in the atmosphere.