Elucidation of the pH-Dependent Electric Double Layer Structure at the Positively Charged Amorphous Alumina/Water Interface Using Heterodyne-Detected Vibrational Sum-Frequency Generation

Alumina is one of the most abundant oxides in nature. Hence, the molecular-level elucidation of the alumina/water interface is important not only for a fundamental understanding of the oxide surface, but also for applications of oxide-related materials in extensive research fields. In the present study, the water structure in the electrical double layer (EDL) at the alumina–water interface is investigated through the interface-selective vibrational spectra (χ⁽²⁾) in the OH stretch region, measured with heterodyne-detected vibrational sum-frequency generation spectroscopy. The imaginary part of χ⁽²⁾ (Imχ⁽²⁾) spectra is collected while salt concentrations are varied (0.01–5.0 M) in the acidic and neutral pH range (pH 2.0–8.0), where the alumina surface is positively charged. The spectral additivity of the Imχ⁽²⁾ spectra enables the decomposition of the spectra into the contributions of the diffuse layer (DL) and Stern layer (SL) to discuss them separately. The surface charge density at each pH is evaluated from the salt-concentration dependence of the amplitude of the DL spectra using the modified Gouy–Chapman theory, providing a single pK_a value of ∼3.7 ± 0.2 due to the Al₂OH₂⁺/Al₂OH equilibrium at the alumina surface. Moreover, it is found that the spectra of the DL and SL are very similar to each other, indicating that the water in the SL is bulk-like water with highly concentrated salt, without specific interaction with the alumina surface. This implies that the water structure of the EDL at the alumina/water interface is much simpler than that at the silica/water interface, where the water molecule has specific interactions with the silica surface. These findings reveal that the alumina/water interface provides another distinct prototype of the EDL structure at oxide/water interfaces.