Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface -selective vibrational (chi(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the chi(2) (Im chi(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt -concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 +/- 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Im chi(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

Automated summary

The investigation of the water structure at the silica/water interface is crucial, and in this study, the interface-selective vibrational spectra were measured using vibrational sum-frequency generation spectroscopy. Through analysis, the spectra were separated into the diffuse Gouy-Chapman layer and the compact Stern layer, allowing for a quantitative understanding of each spectral component. The salt concentration dependence of the spectra and the pH-dependent changes were also analyzed, providing valuable insights into the electric double layer at the silica/water interface.