Currently, sum frequency generation (SFG) spectroscopy is widely recognized as a surface and interface specific vibrational spectroscopic technique. Various physical and chemical properties at the liquid interfaces are strongly correlated with the molecular behavior at the surfaces and interfaces, and it is essential to observe their behavior in-situ. In this paper, we present (1) the analysis of molecular orientation of aqueous solution surfaces of the natural surfactants, (2) the relationship between molecular orientation and the foam stability on beer surfaces, and (3) the relationship between molecular conformation and liquid repellent performance on alkylsilane surfaces exhibiting water repellent properties.

The pulmonary surfactant monolayer plays an important role in the respiratory system due to its amphipathic nature. Recently, it was reported that there is a concern about the oxidation of unsaturated lipids in the pulmonary surfactant even by low-level ozone in the ambient air. However, the molecular-level understanding of the reaction mechanism is still limited due to technical difficulties. Sum frequency generation (SFG) spectroscopy is based on a second-order nonlinear optical process and is very sensitive to interfacial molecular structures. Furthermore, heterodyne-detected SFG (HD-SFG) spectroscopy, measuring the interference signal of SFG light, enables us to obtain both the phase and amplitude of SFG electric field and provides more valuable information compared to intensity measurements of SFG light. This review introduces the principle of HD-SFG spectroscopy and its applications to in situ monitoring of the oxidation of phospholipid monolayers, which are major lipids in the pulmonary surfactant.

Polymers in contact with other materials such as air, water and inorganic materials exhibit peculiar properties, which is closely related to the functions of various industrial polymeric materials. SFG (Sum-Frequency Generation) spectroscopy is a powerful technique for analyzing the functional groups anisotropically oriented at the interface. This paper introduces studies with SFG spectroscopy for polymers/quartz interface, which is a model system for polymer/filler nanocomposite materials and polymer adhesion.