Water Structure at the Hydrophobic Nanodroplet Surface Revealed by Vibrational Sum Frequency Scattering Using Isotopic Dilution.

The water structure at the hydrophobic/water interface is key toward understanding hydrophobicity at the molecular level. Herein, we characterize the hydrogen-bonding network of interfacial water next to sub-micron-sized hydrophobic oil droplets dispersed in water using isotopic dilution vibrational sum frequency scattering (SFS) spectroscopy. The relative intensity of different modes, the frequency shift of the uncoupled O-D spectrum, and a low-frequency shoulder (2395 cm -1 ) reveal that water forms an overall stronger hydrogen-bonding network next to hydrophobic droplets compared to bulk water and the air/water interface. Half of the spectral width of the oil droplet SFS spectrum is determined by inter- and intramolecular coupling of water molecules. Isotopic dilution also confirms the presence of a broad distribution (ca. 2640-2745 cm -1 ) of non-water-hydrogen-bonded O-D modes that are red-shifted and broadened compared to similar species at the air/water interface. This band corroborates the presence of charge transfer between water and oil.