The structure and ultrafast dynamics of the electric double layer (EDL) are central to chemical reactivity and physical properties at solid/aqueous interfaces. While the Gouy–Chapman–Stern model is widely used to describe EDLs, it is solely based on the macroscopic electrostatic attraction of electrolytes for the charged surfaces. Structure and dynamics in the Stern layer are, however, more complex because of competing effects due to the localized surface charge distribution, surface–solvent–ion correlations, and the interfacial hydrogen bonding environment. Here, we report combined time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD) to get direct access to the molecular-level understanding of how ions change the structure and dynamics of the EDL. We show that innersphere adsorbed ions tune the hydrophobicity of the silica–aqueous interface by shifting the structural makeup in the Stern layer from dominant water–surface interactions to water–water interactions. This drives an initially inhomogeneous interfacial water coordination landscape observed at the neat interface toward a homogeneous, highly interconnected in-plane 2D hydrogen bonding (2D-HB) network at the ionic interface, reminiscent of the canonical, hydrophobic air–water interface. This ion-induced transformation results in a characteristic decrease of the vibrational lifetime (T 1) of excited interfacial O–H stretching modes from T 1 ∼ 600 fs to T 1 ∼ 250 fs. Hence, we propose that the T 1 determined by TR-vSFG in combination with DFT-MD simulations can be widely used for a quantitative spectroscopic probe of the ion kosmotropic/chaotropic effect at aqueous interfaces as well as of the ion-induced surface hydrophobicity.

Steady-state and time-resolved vibrational sum frequency generation (vSFG) were used to investigate the structure and dynamics of water at the α-Al 2 O 3 (0001) surface. The vSFG spectra of the OH stretch of water next to the Al 2 O 3 (0001) surface are blue-shifted compared to the Al 2 O 3 (112̅ 0) surface, indicating its weaker hydrogen bonding network. Consequently, the vibrational dynamics of the OH stretch of the neutral Al 2 O 3 ( 0001) surface is two times slower than the neutral Al 2 O 3 (112̅ 0) surface. Furthermore, the vibrational dynamics of the OH stretch of water next to charged Al 2 O 3 surfaces is observed to be faster than that in bulk water and at charged SiO 2 surfaces, which could be due to (a) fast proton transfer dominating the vibrational relaxation and/or, (b) efficient coupling between the OH stretch and the bend overtone via the presence of low frequency (∼3000 cm −1 ) OH stretching modes. Lastly, the addition of excess ions (0.1 M NaCl) seems to have little to no effect on the time scale of vibrational dynamics, which is in contrast with the behavior observed at the silica surface, where addition of excess ions was observed to change the time scale of vibrational relaxation of interfacial water.

Frequency and time-resolved vibrational sum frequency generation (vSFG) are used to investigate the behavior of water at the α-Al 2 O 3 (112̅ 0) surface. In addition to the typical water OH peaks (∼3200 and ∼3400 cm −1 ), the α-Al 2 O 3 (112̅ 0)/H 2 O interface shows an additional red-shifted feature at ∼3000 cm −1 . Addition of ions (0.1 M NaCl) largely attenuates the water OH peaks but has little effect on the 3000 cm −1 peak. The 3000 cm −1 feature is assigned to the O−H stretch of surface aluminol groups and/or interfacial water molecules that are strongly hydrogen bonded to the alumina surface. Density functional theory calculations were performed to test this assignment, revealing the presence of both associated and dissociated H 2 O configurations (chemisorbed surface OH group) with frequencies at 3155 and 3190 cm −1 , respectively, at a hydrated α-Al 2 O 3 (112̅ 0) surface. IR pump−vSFG probe measurements reveal that the interfacial OH species show very fast (<200 fs; bulk waterlike) vibrational relaxation dynamics, which is insensitive to surface charge and ionic strength, thus suggesting that the interfacial OH species at the α-Al 2 O 3 (112̅ 0)/H 2 O interface are in a highly ordered and strongly hydrogen-bonded environments. The observed fast vibrational relaxation of the interfacial OH species could be due to strong coupling between the 3000 cm −1 species and the interfacial water OH groups (3175 and 3450 cm −1 ) via strong hydrogen bonds, dipole−dipole interaction between several interfacial OH groups (Forster type energy transfer), and/or ultrafast photoinduced proton transfer.

While ions are known to perturb hydrogen bonding networks in bulk water, our understanding of such effects is less developed for interfaces. Alumina/water interfaces are highly ordered due to strong hydrogen bonding interactions between interfacial water molecules and adjacent aluminol groups. However, how ions alter this interaction is not yet known. Herein, to address the effect of sodium halide salts on the hydrogen bonding environment of interfacial water, we investigated charged alumina (0001) surfaces using steady-state and time-resolved vibrational sum frequency generation (vSFG) spectroscopy. Our results indicate that the effect of halide anions on the attenuation of the vSFG signal next to positively and negatively charged alumina surfaces followed the sequence F– ≫ Br– > Cl– > I– (slightly varied direct Hofmeister series) and Br– > I– ≈ Cl– > F– (slightly varied indirect Hofmeister series), respectively. Additionally, time-resolved vSFG reveals that only F– perturbs the vibrational lifetime of water next to a positively charged alumina surface by presumably breaking the strong hydrogen bonding interaction between the surface aluminol groups and the nearby water molecules.

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 α-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 α-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 α-Al2O3(0001)/water interface.

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.

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