(325k) Probing the Interface of Oil-in-Water Emulsions

Water-hydrophobic interfaces are relevant for many disciplines, including colloidal synthesis and microdroplet chemistry. Unfortunately, characterizing the structure of the interface is challenging due to its nanoscopic length scale. Here, we discuss a new spectroscopic method in which we apply the Raman multivariate curve resolution (Raman-MCR) technique to an oil-in-water emulsion. Importantly, the large surface area of the emulsion droplets vastly improves the signal relative to that of a planar interface. We find that the lower frequency part of the OH-stretch region of the solute-correlated spectra has diminished relative to bulk water, and that a new shoulder has arisen around 3575 cm^-1. Computationally, we use molecular dynamics simulation and the newly developed monomer-field model for Raman spectroscopy to probe the structure of the interface. We attribute the decline at lower frequencies to a decay in the tetrahedral ordering at the interface. Furthermore, we assign the new shoulder as a free OH peak that has been shifted ~95 cm^-1 from its original value. Using the monomer-field model, we attribute this shift to the strong electric field associated with the zeta potential of oil droplets. This strong electric field will have a strong influence on reactivity at such interfaces, and highlights the use of emulsions droplets as stable mediums for microdroplet reactivity.