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 ...

With Benjamin Franklin's oil on water experiments as a historical example, people have long been fascinated with the physical characteristics of the interface between water and an organic liquid and the unique chemistry that can occur at that interface. In this paper, we present our current understanding of the structure, orientation, and bonding characteristics of this fluid and dynamic interfacial region based on the efforts in our laboratory over the past decade and the important research of others in this field. In our studies, in which we have used a combination of surface specific nonlinear vibrational spectroscopy in conjunction with molecular dynamics simulations, we find that a general feature of organic-water interfaces is that of weak bonding interactions between adjacent water molecules and between the water and organic molecules. These weak water-organic interactions, present at all of the interfaces that we have studied, result in significant interfacial structuring and molecular orientation on both sides of the interface. How the structuring of both the interfacial water and organic molecules is dependent on the nature of the organic media is discussed as well as how this interfacial structuring can facilitate molecular and ion transport across the interface. The discussion of the neat organic-water interface is followed by a summary of how this picture is changed with the addition of ions, surfactants, and biomolecules, and how the presence of the organic media plays a role in the adsorption and conformation of adsorbates relative to the vapor-water interface. Examples of recent applications for the oil-water interface to synthesis and future perspectives are discussed as well. † 2008 marked the Centennial of the American Chemical Society's Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem.

We report in-situ spectroscopic measurements of the structure and orientation of molecules in the interfacial region between the semi-soluble ionic solid CaF 2 (fluorite) and an aqueous phase. This paper integrates a series of new experiments with earlier results to give a comprehensive understanding of this very dynamic interface. We employ the surface specific technique, vibrational sum-frequency spectroscopy (VSFS), to study the effect that alterations in the aqueous phase composition, such as pH, surfactant ion concentration, and ion composition have on the bonding interactions, ion exchange behavior, and electrical properties in the interfacial region. These studies demonstrate the complex nature of the interactions of semi-soluble solids with an aqueous phase and the complexity of the surfactant adsorption process. Fundamental studies such as these are essential for understanding the mechanisms involved in surfactant adsorption and interfacial charge reversal; information which is important for industrially relevant processes such as mineral ore flotation, waste processing and petroleum recovery.

Vibrational sum frequency spectroscopy is used to examine the CaF2/H2O interface and the CaF2/D2O interface upon adsorption of the anionic surfactant sodium dodecylsulfate (SDS) onto the CaF2 surface. Proper interpretation of the results from this coherent spectroscopic technique requires a meticulous understanding of the interferences that result between sharp CH stretching modes, broad OH stretching modes, and the nonresonant background. The reported studies demonstrate the appropriate spectral analysis procedure required for a correct interpretation of spectra of this type and the errors that can result from a more simplistic but commonly used analysis procedure.

Vibrational sum-frequency spectroscopy (VSFS) has been used to investigate the adsorption of acetate at the fluorite/water interface in situ. The bulk pH value was varied to control the composition of the liquid phase in contact with a single crystalline fluorite (CaF2) surface. Spectra were recorded in frequency regions corresponding to vibrational modes of the adsorbate and water. Quantitative information on interfacial water molecules in different states of order was obtained from the present data. Furthermore, the effect of background electrolyte concentration was studied, and it was found that surface hydroxyls play a crucial role for the oriented adsorption of acetate ions at the interface.

Visible-infrared sum-frequency spectroscopy is ideally suited to the study of surfaces and interfaces. This paper introduces new sum-frequency spectroscopy instrumentation that we have developed with two novel features: (1) stable and robust infrared generation in the 900-3100 cm(-1) (11-3.2 microm) region using an amplified Ti : sapphire oscillator with a home-built OPG/OPA, and (2) continuous tuning over either 900-2700 cm(-1) (11-3.7 microm) or 1800-3100 cm(-1) (5.5-3.2 microm) in a single experiment. All practical details of baseline correction issues due to the picosecond pulses (including variation in infrared (IR) energy, spatial and temporal overlap, Fresnel coefficients) are addressed while demonstrating signal throughout this region from an amorphous gold surface. A sum-frequency spectrum from an oriented polymer is shown as a complete example of the data treatment, which reveals the vibrational modes accessible in this wavelength region.