The interfacial region of aqueous systems also known as the electrical double layer can be characterized on the molecular level with second harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liquids. Here, we model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. We show that, in the presence of a surface electrostatic field, interference effects, which originate from oriented water molecules on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10 −3 M. For reflection experiments this interference phenomenon leads to a significant reduction of the SHG/SFG intensity. Transmission mode experiments from aqueous interfaces are hardly influenced. For SHG/SFG scattering experiments the same interference leads to an increase in intensity and to modified scattering patterns. The predicted scattering patterns are verified experimentally.

We established the charge and structure of the oil/water interface by combining ζ-potential measurements, sum frequency scattering (SFS) and molecular dynamics simulations. The SFS experiments show that the orientation of water molecules can be followed on the oil droplet/water interface. The average water orientation on a neat oil droplet/water interface is the same as the water orientation on a negatively charged interface. pH dependent experiments show, however, that there is no sign of selective adsorption of hydroxide ions. Molecular dynamics simulations, both with and without intermolecular charge transfer, show that the balance of accepting and donating hydrogen bonds is broken in the interfacial layer, leading to surface charging. This can account for the negative surface charge that is found in experiments.

Surfactants such as sodium dodecylsulfate (SDS) can reduce the interfacial tension between bulk water and bulk n-hexadecane by 42 mN/m. Although reduction of interfacial tension should also take place on the interface of nanoscopic oil droplets in water, vibrational sum frequency scattering experiments indicate otherwise. In these measurements we have directly measured the adsorption of SDS onto hexadecane oil droplets with an average radius of 83 nm. We find that the interfacial density of adsorbed SDS is at least 1 order of magnitude lower than that at a corresponding planar interface. The derived maximum decrease in interfacial tension is only 5 mN/m.

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.

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