Charge transfer across C-H⋅⋅⋅O hydrogen bonds stabilizes oil droplets in water.

The hydrophobic–water interface plays a key role in biological interactions. However, both the hydrophobic–water interfacial molecular structure and the origin of the negative zeta potential of hydrophobic droplets in water are heavily contested. We report polarimetric vibrational sum-frequency scattering of the O–D and C-H stretch modes of 200-nanometer hexadecane oil droplets dispersed in water. An unusually broad spectral distribution (2550 to 2750 per centimeter) of interfacial water molecules that were not hydrogen bonded to other water molecules was observed, as well as a blue shift in the vibrational frequency of the interfacial hexadecane C-H stretch modes. Oil and water frequency shifts correlated with the negative electrostatic charge. Molecular dynamics simulations demonstrated that the unexpected strong charge-transfer interactions arose from interfacial C–H∙∙∙O hydrogen bonds.