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.