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

Abstract

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.