On the trail of molecular hydrophilicity and hydrophobicity at aqueous interfaces

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.