Investigation of the structure of water at hydrophobic and hydrophilic interfaces by angle-resolved TIR Raman spectroscopy.

To analyse the surface- or interface-specific molecular structure of a condensed molecular system, it is important to measure the spectra of molecules near the surface. Total internal reflection (TIR) Raman spectroscopy is a sensitive technique for surface or interfacial analysis because it retrieves spectra in the region within ca. 100 nm of a surface. However, since the width of the interface itself is often on a molecular scale (one to a few nm), conventional TIR Raman spectroscopy intrinsically lacks surface sensitivity. To overcome this problem, the combination of multiple-angle TIR Raman spectroscopy and principal component analysis (PCA) is expected to enable effective differentiation between the spectra of minute chemical species at the interface and those of dominant species. In the present study, angle-resolved TIR Raman spectroscopy with PCA was applied to SiO2/water and SAM/water interfaces to detect minute species located within a few nm of each interface. This method will likely lead to progress in various surface and interfacial analyses, not only those related to the structure of water, but also those used to determine the interactions among absorbed species.