Water Structure at the Hydrophobic Nanodroplet Surface Revealedby Vibrational Sum Frequency Scattering Using Isotopic Dilution br

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