Monovalent and Divalent Cations at the α-Al2O3(0001)/Water Interface: How Cation Identity Affects Interfacial Ordering and Vibrational Dynamics

Mineral oxide/water interfaces are important for a wide range of industrial, geochemical, and biological processes. The reactivity of these interfaces is strongly impacted by the presence of ions. Thus, it is critical to understand how ions alter the interfacial environment. This can be achieved by measuring the changes in the structure and vibrational dynamics of interfacial water induced by the presence of ions in close vicinity to the mineral surface. The alpha-Al2O3(0001) surface represents a flexible platform to study the effect of ions on interfacial aqueous environments at positive, neutral, and negative surface charges. By using vibrational sum frequency generation (vSFG) in the frequency and time domains, we investigate how monovalent and divalent cations affect the hydrogen bonding environment of the first few layers of interfacial water next to alpha-Al2O3(0001). Our results indicate that monovalent cations, such as Li+, Na+, K+, and Cs+, appear to have lower binding affinities at the interface compared to Ca2+, Sr2+, and Ba2+. This leads to an interfacial region that is structured in a cation valence dependent manner. The addition of divalent cations at the negatively charged interface (pH 10) increases the spectral intensity in the 3400 cm(-1) region compared to neat pH 10 H2O, in contrast to monovalent cations that only attenuate the vSFG signal. Time-resolved vSFG measurements reveal that the O-H vibrational lifetime (T-1) of interfacial species at pH 10 in the presence of NaCl and BaCl2 remains similar. The restructuring of the interface seen in steady-state vSFG is manifested in the degree to which strongly hydrogen-bonded species recover to their original populations post excitation. By tracking the accumulation of ions at the interface via the vSFG response, we can characterize the unique surface arrangements of interfacial water molecules induced by monovalent and divalent cations at the alpha-Al2O3(0001)/water interface.