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.

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.

The synergistic adsorption of polymers with surfactants at the oil/water interface has applications that range from oil remediation to targeted drug delivery. However, the inherent inaccessibility of the buried oil/water interface has challenged the development of a molecular-level understanding of the structure-function relationship of these systems. This study uses vibrational sum frequency spectroscopy to examine the molecular structure, orientation, and electrostatic effects of synergistic adsorption of the surfactant cetrimonium bromide (CTAB) and polymer poly(acrylic acid) (PAA) at a planar oil/water interface. Results demonstrate that coadsorption leads to a high degree of interfacial ordering of both the polymer and the surfactant and a subsequent alteration of the interfacial water bonding and orientation. Complementary zeta potential measurements provide further information about how surface partitioning of a charged polymer and a surfactant relates to their aggregation behavior in a bulk solution. With the CTAB concentration fixed but the PAA concentration variable, hydrophobic interactions result in a modest synergic coadsorption when CTAB is in excess. However, when the PAA carboxylate monomer concentration approaches that of CTAB, the electrostatic interactions between the components change the structure and increase the amount of adsorbed PAA until the interfacial charge is neutralized. This work reveals that the synergic adsorption behavior of this model polyacid/surfactant system arises from a combination of concentration-dependent hydrophobic and electrostatic forces working in tandem.

The vibrational spectrum of water at hydrophobic/hydrophilic interfaces is crucial to understanding complex surface chemistry phenomena. Vibrational sum frequency (VSF) spectroscopy is a valuable nonlinear spectroscopic technique for exploring the details of vibrational spectra of molecules at surfaces. However, spectral assignments and analysis of VSF spectra are often more nuanced than in linear spectroscopy. This study is aimed at understanding the source of the broad VSF signal in the oil–water surface water spectrum at energies slightly higher than traditionally examined, beyond the unbound free OH oscillator of surface water molecules. Analyzing isotopic dilutions of the aqueous solvent with VSF spectroscopy, we demonstrate that this signal is due to a combination band of water stretch and libration motions. The spectral characteristics of this band are found to be highly sensitive to the sign and magnitude of the surface charge induced by adsorption of both anionic and cationic surfactants. The results have implications for VSF measurements of the C–H stretching vibrations of various adsorbates when studied with D2O as the aqueous solvent. Because the vibrational signal from the water combination band is dependent on surface charge, it is imperative to include the presence of the combination band when fitting surface spectra.

Polymer−surfactant mixtures are versatile chemical systems because of their ability to form a variety of complexes both in bulk solution and at surfaces. The adsorption and structure of polymer−surfactant complexes at the oil/water interface define their use surface chemistry applications. Previous studies have investigated the interactions between charged polyelectrolytes and surfactants; however, a similar level of insight into the interfacial behavior of nonionic polymers in mixed systems is lacking. The study herein uses vibrational sum frequency (VSF) spectroscopy to elucidate the molecular details of nonionic polyacrylamide (PAM) adsorption to the oil/water interface in the presence of surfactant. The polymer's adsorption and conformational structure at the interface is investigated as it interacts with cationic and anionic surfactants. Where the polymer will not adsorb to the interface on its own in solution, the presence of either cationic or anionic surfactant causes favorable adsorption of the polymer to the oil/water interface. VSF spectra indicate that the cationic surfactant interacts with PAM at the interface through charge-dipole interactions to induce conformational ordering of the polymer backbone. However, conformational ordering of polymer is not induced at the interface when anionic surfactant is present.