Vibrational sum frequency (VSF) spectra calculated using molecular dynamics (MD) simulations are compared with VSF experimental spectra to gain a clearer picture of water structure and bonding at the carbon tetrachloride-water (CCl 4 -H 2 O) and the 1,2-dichloroethane-water (DCE-H 2 O) liquid-liquid interfaces. The VSF spectral response from interfacial water at the CCl 4 -H 2 O interface contains spectral features similar to the resonant VSF response of the vapor-water interface and alkane-water interfaces, while the VSF spectrum from the DCE-H 2 O interface has a low signal with no distinguishing OH stretch spectral features. These MD based spectral calculations show how different bonding interactions at the DCE-H 2 O interface lead to spectral broadening, frequency shifting, and spectral interferences that are responsible for the difference in the experimentally measured DCE-H 2 O and CCl 4 -H 2 O spectra. The computational results show that weak H 2 O-H 2 O interactions are perturbed by the presence of DCE, leading to increased water penetration into the more organic-rich portion of the interfacial region and strong orientation of these penetrating water molecules relative to the CCl 4 -H 2 O interface. Strong H 2 O-H 2 O interactions at the interface are not significantly impacted by the presence of DCE.

We report in situ spectroscopic measurements of charge reversal behavior and surfactant bilayer formation at the salt/aqueous solution interface as the aqueous surfactant concentration is varied. The studies, which employ vibrational sum frequency spectroscopy to measure the vibrational response of sodium dodecyl sulfate and water at the CaF 2 /H 2 O interface, demonstrate the complex nature of the adsorption process which includes monomer adsorption, surfactant bilayer formation, surfactant restructuring, surface charge reversal, and water reorientation. These effects have been monitored directly for the first time by taking advantage of the spectroscopy and the nonlinear phase relationships between the CH and OH vibrational modes. The results provide important insight into the adsorption mechanism that is central to processes such as mineral ore flotation and separation, waste processing, and petroleum recovery.

Nanoemulsions and microemulsions are environments where oil and water can be solubilized in one another to provide a unique platform for many different biological and industrial applications. Nanoemulsions, unlike microemulsions, have seen little work done to characterize molecular interactions at their surfaces. This study provides a detailed investigation of the near-surface molecular structure of regular (oil in water) and reverse (water in oil) nanoemulsions stabilized with the surfactant dioctyl sodium sulfosuccinate (AOT). Vibrational sum-frequency scattering spectroscopy (VSFSS) is used to measure the vibrational spectroscopy of these AOT stabilized regular and reverse nanoemulsions. Complementary studies of AOT adsorbed at the planar oil-water interface are conducted with vibrational sum-frequency spectroscopy (VSFS). Jointly, these give comparative insights into the orientation of interfacial water and the molecular characterization of the hydrophobic and hydrophilic regions of AOT at the different oil-water interfaces. Whereas the polar region of AOT and surrounding interfacial water molecules display nearly identical behavior at both the planar and droplet interface, there is a clear difference in hydrophobic chain ordering even when possible surface concentration differences are taken into account. This chain ordering is found to be invariant as the nanodroplets grow by Ostwald ripening and also with substitution of different counterions (Na:AOT, K:AOT, and Mg:AOT) that consequently also result in different sized nanoparticles. The results paint a compelling picture of surfactant assembly at these relatively large nanoemulsion surfaces and allow for an important comparison of AOT at smaller micellar (curved) and planar oil-water interfaces.nanoemulsions | oil-water interfaces | vibrational sum-frequency scattering spectroscopy | surface spectroscopy | surfactants W e are all familiar with the adage that "oil and water do not mix," but of course, it depends upon the definition of "mix." Emulsions are an important special case, where the oil is dispersed as tiny droplets in the aqueous phase, taxonomically called a regular emulsion, or where the water is dispersed as tiny droplets throughout the oil phase, called a reverse emulsion. Because both emulsions are thermodynamically unstable, overcoming this requires an emulsifying agent such as a surfactant. Recently, there has been interest in surfactant-stabilized emulsions with droplet diameters in the nanoscale range for unique applications in drug delivery (1, 2) and oil recovery (3, 4) and as nanoreactors to produce materials ranging from polymers to quantum dots (5). Regular or reverse emulsions with droplet diameters in the range of 10-1,000 nm are called nanoemulsions. Little is known about the processes or molecular structures that result in their stability via surfactants. Even less is known about the structure-function relationship, which is crucial to determine the best surfactant for a given nanoemulsion application. Their utility hinges on a ...