Sum frequency spectroscopy of the hydrophobic nanodroplet/water interface: Absence of hydroxyl ion and dangling OH bond signatures

Abstract

Vibrational sum frequency scattering measurements were performed on deuterated hexadecane oil nanodroplets dispersed in aqueous pH neutral and basic solutions. The predominant symmetry of the observed vibrational signatures was derived using nonlinear light scattering theory. The probed spectral region contains information about two distinct phenomena: The surface structure of water, and the spectral signature of surface OD− ions. The spectral data differ from that of the planar alkane/water interface, but can be interpreted consistently by considering differences in chain orientation and molecular surface corrugation. In terms of the second phenomenon, we find that although the electrokinetic mobility increases with pH, the SF spectra do not change with increasing pH, and do not contain any evidence for the presence of interfacial hydroxyl ions. Based on expected surface charge densities, signal strength and recent literature, we conclude that a charge transfer scenario is most in accord with the present data.