Water Structure at the Hydrophobic Droplet Surface Revealed by Vibrational Sum Frequency Scattering using Isotopic Dilution

Water structure at the hydrophobic/water interface is key towards understanding hydrophobicity at the molecular level. Herein, we characterize the hydrogen bonding network of interfacial water next to sub-micron sized hydrophobic 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. Roughly half of the spectral width of the oil droplet SFS spectrum is determined by inter- and intramolecular coupling of water molecules. Isotopic dilution also revealed a broad distribution (~2640 – 2750 cm-1) of non-hydrogen bonded O-D modes that were red-shifted and broadened compared to similar species observed previously at the air/water interface. This band confirms the presence of charge transfer between water and oil.