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Dissecting the liquid water organization in contact with hydrophobic and hydrophilic surfaces is essential for understanding the chemical and physical properties of aqueous interfaces. Recently developed descriptors for microscopic hydrophobicity/hydrophilicity based either on molecular dynamics (MD) simulations or on surface-sensitive nonlinear optical techniques, such as sum frequency generation (SFG) spectroscopy, manage to capture and quantify the change in local molecular hydrophobicity at heterogeneous surfaces. However, the connections between the theoretical/structural descriptors and spectroscopic fingerprints have not been established yet. Here, we combine density functional theory-based MD simulations (DFT-MD) and both theoretical and experimental SFG spectroscopy to explore how the interfacial water responds in contact with self-assembled monolayers (SAM) of tunable hydrophilicity. We introduce a microscopic metric to track the transition from hydrophobic to hydrophilic interfaces, which combines a structural descriptor based on the preferential orientation within the water network in the topmost binding interfacial layer (BIL) and spectroscopic fingerprints of H-bonded and dangling OH groups of water pointing towards the surface carried by BIL-resolved SFG spectra. This metric builds a bridge between molecular descriptors of hydrophilicity/hydrophobicity and spectroscopically measured quantities, and provides a recipe to quantitatively or qualitatively interpret experimental SFG signals.

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned near the liquid, but out of direct contact. We observe a parabolic dependence of second harmonic intensity on applied potential. Based on standard theory, we associate this dependence with the response of the diffuse layer water molecules to changes in interfacial potential profile. Taking the literature value for this response leads to the unexpected conclusion that the electric fields within the diffuse layer are opposite in sign from those originating from the electrodes. This conclusion implies that the traditional continuum-based models of interfacial screening lack the complexity necessary to properly describe the potential profile of the liquid water-vapor interface. Effects such as overscreening in the topmost interfacial layer and extended correlations in the interfacial hydrogen bonding network may play a role in governing the response of the water interface to external fields.

Glycerol is a major cryoprotective agent and is widely used to promote protein stabilization. Through a combined experimental and theoretical study, we show that global thermodynamic mixing properties of glycerol and water are dictated by local solvation motifs. We identify three water populations, i.e., bulk water, bound water H-bonded to hydrophilic groups of glycerol and wrap water hydrating hydrophobic moieties. Each population provides distinct spectroscopic fingerprints in the THz/FIR spectral range, which allow to quantify their respective abundance and their partial contributions to the mixing enthalpy. We uncover a 1:1 connection between the number of bound waters and the mixing enthalpy, as deduced from experiments as well as from simulations. The balance between local hydrophobic wrap and hydrophilic bound contributions at the molecular level dictates macroscopic thermodynamics of mixing. This offers opportunities to rationally design polyol water mixtures to optimize technological applications by tuning mixing enthalpy and entropy based on spectroscopic screening.

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid out of contact. We observe a parabolic dependence of second harmonic intensity on applied potential. This dependence is reminiscent of bulk-phase electric field induced second harmonic (EFISH) but more complicated because it combines the second-order response of the topmost water layer and the potential dependent response of the interfacial electrical double-layer. Based on the literature values for these contributions, we derive a physical interpretation of our measurements that reveals new insight into the response of the neat water interface to external electric fields. Specifically, we find that the net dipolar orientation of water molecules within the double-layer is primarily responsive to the internal fields generated by the excess surface concentrations of OH- and H3O+ that arise to screen the external potential. Notably, this interpretation implies that the orientational response of water dipoles at the interface can actually oppose the direction of the external field, a subtle effect that is not captured by traditional models.