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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.

Nanoemulsions, nanoscale oil droplets dispersed in an aqueous medium, can be stabilized by polymer−surfactant (PS) mixtures, making them ubiquitous in commercial, industrial, and pharmaceutical applications. It is well-known that the presence of PS layers coadsorbed at the droplet surface plays a significant role in droplet stability and functionality; however, little is understood about the molecular nature of this coadsorption. Such insights are especially important for application in drug delivery where physiological conditions can vary the environmental pH and significantly impact stabilization. Hence, the focus of this study examines the surface properties of ∼300 nm nanoemulsions stabilized by the coadsorption of polyethylenimine (PEI) and charged alkyl surfactants sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB). PEI is a common charge-tunable polymer used in nanocarrier templates. This study employs vibrational sum frequency scattering spectroscopy, coupled with ζ-potential and surface pressure measurements performed as a function of varying concentrations and pH. The surface specific spectroscopic results reported herein reveal that PEI adsorption and molecular ordering is influenced by both electrostatic and hydrophobic interactions. While the degree of PEI adsorption is stronger in the presence of anionic SDS than cationic DTAB, for both surfactants, PEI is molecularly disordered in acidic conditions and adopts a persistent net ordering as the solution pH becomes more basic. Both surfactants also display degrees of interfacial conformational ordering that is altered by the presence of the coadsorbed polymer. These results demonstrate the molecular-level diversity in PEI behavior at the droplet interface and provide insight into how such behavior can be controlled to yield nanocarrier technology with specific functions and enhanced efficacy.

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