Surface structure of sodium dodecyl sulfate surfactant and oil at the oil-in-water droplet liquid/liquid interface: a manifestation of a nonequilibrium surface state.

We present sum frequency scattering spectra on kinetically stabilized emulsions consisting of nanoscopic oil droplets in water, stabilized with sodium dodecyl sulfate (SDS). We have measured the interfacial structure of the alkyl chains of the surfactant molecules, the alkyl chain of the oil molecules, the weakly dispersive D(2)O response, and the interference between SDS and the oil. We find a big difference in chain conformation: SDS has many chain defects, whereas the oil has very few. Our spectra are interpreted to originate from a surface structure with oil molecules predominantly oriented parallel with respect to the plane of the interface. The SDS headgroup is surrounded by water molecules. The SDS alkyl tail is in a disordered state and partially in contact with water. Such a conformation of surfactant occupies a surface area of several hundreds of squared angstroms.