The interactions between water and mineral surfaces are relevant for a wide range of disciplines, including biomolecular physics and geochemistry. For the latter, a molecular-level understanding of water at the mineral interface can provide insights into important dissolution reactions. Therefore, we investigate the silica surface in contact with water at neutral pH. The water/silica interface was probed using phase-resolved and femtosecond time resolved sum frequency generation (SFG) spectroscopy, with special emphasis on the spectral feature around 3660 cm<sup>-1</sup>, which is present after annealing the sample at 900 °C. The phase-resolved data reveal that the O-H groups causing this vibration are oriented towards the silica surface, indicating that the resonance arises from a free OH of water, as opposed to silanol groups. The presence of free OH groups from water at the silica interface suggests the water behaves as if in contact with a hydrophobe. Furthermore, time resolved SFG spectroscopy was utilized to determine the vibrational lifetime and reorientational dynamics of this free OH. The inferred out-of-plane reorientation for the free OH is only slightly slower than the dynamics at the air/water interface. Both phase- and time-resolved SFG experiments thus provide evidence that the spectral feature at 3660 cm<sup>-1</sup> originates from the free OH of water molecules. Surprisingly, there is hydrophobic water at a hydrophilic mineral oxide interface.

Hydrophobic water at a hydrophilic interface

254th National Meeting (2017)

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