Structure at the air/water interface in the presence of phenol: a study using heterodyne-detected vibrational sum frequency generation and molecular dynamics simulation

Many kinds of organic compounds pollute the aquatic environment, and they change the properties of the water surface due to their high surface affinity. Chemical reactions at the water surface are key in environmental chemistry because, for instance, reactions occurring at the surface of aqueous aerosols play essential roles in the atmosphere. Therefore, it is very important to elucidate how organic compounds affect the properties of water surfaces. Here, we choose phenol as an organic pollutant prototype and report how phenol affects the molecular-level structure of the air/water interface. Interface-selective vibrational spectra, i.e., the imaginary part of second-order nonlinear susceptibility (Im chi((2))), of the air/water-phenol mixture interface in the OH stretch region were collected using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy, and the observed Im chi((2)) spectra were interpreted with the aid of molecular dynamics (MD) simulation. The Im chi((2)) spectra observed via HD-VSFG drastically change as a function of phenol concentration in water, and exhibit two isosbestic points. In the spectra, a positive OH band appears at 3620 cm(-1), which is assigned to an OH group of water that forms an OH center dot center dot center dot pi hydrogen-bond (H-bond) with the aromatic ring of phenol, and a strong negative OH band appears around 3200 cm(-1), which is attributed to a water that accepts a H-bond from the phenol OH, while pointing its OH groups toward the bulk water side. It was concluded that two types of unique water molecules hydrate a phenol molecule: (1) water that forms an OH center dot center dot center dot pi H-bond; and (2) water that accepts a H-bond from a phenol OH group. Each phenol molecule adsorbed at the air/water forms a specific hydration structure, which causes a large change in the interfacial water structure. The present study provides a clear example demonstrating that even such a simple organic pollutant as phenol can drastically alter the interfacial water structure.