The stabilization of nanoemulsions, nanosized oil droplets dispersed in water, is commonly achieved through the addition of surfactants and polymers. However, nanoemulsions in the absence of emulsifiers have been observed to acquire a significant negative charge at their surface, which ultimately contributes to their stability. While the source of this negative charge is disputed to this day, its presence is taken as an inherent property of the aqueous–hydrophobic interface. This report provides a look at the molecular structure and bonding characteristics of bare aqueous–hydrophobic nanoemulsion interfaces. We report the creation of bare nanoemulsions with near zero surface charge, which are marginally stable for several days. The process of creating these low-charge nanoemulsions (LCNEs) required rigorous cleaning procedures and proper solvent storage conditions. Using vibrational sum-frequency scattering spectroscopy, we measure the structure and bonding of the interfacial aqueous and hydrophobic phases. The surfaces of these LCNE samples possess a measurable free OH vibration, not found in previous studies and indicative of a clean interface. Tuning the nanoemulsion charge through addition of anionic surfactants, modeling potential surface-active contaminants, we observe the free OH to disappear and a reorientation of the interfacial hydrophobic molecules at micromolar surfactant concentrations. Notably, the free OH vibration provides evidence for stronger dispersion interactions between water molecules and the hydrophobic phase at the LCNE surface compared with similar planar water–alkane interfaces. We propose the stronger bonding interactions, in addition to an ordered interfacial aqueous layer, contribute to the delayed droplet coalescence and subsequent phase separation.

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 ...

Negative charge accumulation at aqueous–hydrophobic interfaces and its pH-dependent behavior are routinely ascribed to special adsorption properties of hydroxide ions. Mounting experimental and computational evidence, however, indicates that this negative charge accumulation is the result of surface-active impurities. If true, these impurities would obfuscate our fundamental understanding of the molecular structure and bonding environment at aqueous–hydrophobic interfaces. In this work, we describe the preparation and characterization of bare low-charge nanoemulsions (LCNEs), nanosized droplets of oil-absent emulsifiers. Electrophoretic mobility measurements of LCNE droplets in varying pH environments suggest that trace surface-adsorbed impurities are contributing to the lingering negative surface charge that leads to their marginal stability. We then use vibrational sum-frequency scattering spectroscopy to support this claim and to study the molecular structure and bonding environment of the interfacial aqueous and hydrophobic phases on both the LCNE surface and the surface of nanoemulsions with increasing amounts of adsorbed surfactants. For LCNE samples, our results show that interfacial water bonds more strongly to the oil phase on the droplet surface compared to similar planar interfaces. Interfacial oil molecules are found to orient mostly parallel to the bare droplet surface and reorganize upon surfactant adsorption. In summation, the results reported here provide a new look at the molecular structure and bonding of bare nanoemulsion surfaces and contribute to our evolving understanding of bare aqueous–hydrophobic interfaces.

Whether a charged macromolecule remains solvated or adsorbs to the interface between two immiscible fluids depends on a variety of complex factors. Ions, in particular, have the capacity to affect the degree of polyelectrolyte hydrophobicity and conformation through electrostatic screening and site-specific interactions. Here, surface-specific vibrational spectroscopy is utilized to probe the roles of ions in the oil− water interfacial behavior of a model carboxylic acid-containing macromolecule. Specifically, the adsorption time dependence, conformation, and extent of carboxylate−cation interactions of poly(methacrylic acid) at the carbon tetrachloride−water interface are deduced as a function of CaCl 2 and KCl ionic strength. Additionally, the role of the polymer backbone configuration in interfacial behavior is explored through isomer-specific studies. Results show that ion binding and charge screening induce the adsorption of the isotactic isomer of poly(methacrylic acid) to the oil− water interface under solution conditions unfavorable to surface adsorptivity and that the degree of cation−carboxylate interactions strongly affects the adsorption dynamics and interfacial structure of the adsorbed polymer. The backbone structure of syndiotactic poly(methacrylic acid) does not allow for strong interactions with cations, and therefore, it remains water-solvated in the presence of salt. This work has significance for understanding the role of metal ions in the assembly of natural charged macromolecules at biological and environmental interfaces.

Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, ζ-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The ζ-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain–chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.