Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This will be shown via Density Functional Theory-Molecular Dynamics (DFT-MD) coupled with vibrational Sum Frequency Generation (SFG) and THz-IR absorption spectroscopies.<br/>We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D- HB-Network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of “vertical” and “horizontal” hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-Network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

Molecular fingerprints of hydrophobicity at aqueous interfaces from theory and vibrational spectroscopies (SFG and THz-IR)

ACS Fall 2021

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