Hydrogen bonding strength of interfacial water determined with surface sum-frequency generation

We demonstrate that marked variations exist in hydrogen bonding interactions of interfacial water at different aqueous interfaces. The average hydrogen bond strength and its distribution are inferred from surface sum-frequency generation (SFG) spectra through the center frequency and width, respectively, of the O-D stretch vibration of isotopically diluted HDO in H₂O. The use of partially deuterated water prevents complications due to intramolecular vibrational coupling, which we show gives rise to features in the SFG spectra that are unrelated to hydrogen bonding interactions. At the water-air interface, the SFG spectrum in the hydrogen-bonded region strongly resembles the bulk Raman spectrum, indicating that, at this interface, the interfacial hydrogen bonding properties are very similar to those in bulk water. In contrast, for silica-water and lipid-water interfaces, interfacial hydrogen bonding is substantially stronger, with a larger degree of heterogeneity.