How Low Can You Go? Molecular Details of Low-Charge Nanoemulsion Surfaces

Abstract

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.

Communication: On the Stability and Necessary Electrophoretic Mobility of Bare Oil Nanodroplets in Water

“…B: Electrophoretic mobility values of n-hexadecane nanodroplets in water from Ref. 12 , showing data from the Richmond lab (red data), and data from a study reported by Roger and Cabane 13 (green and blue data), showing the effects of carboxylic acid impurities in the oil phase. The dotted black lines indicate the difference in chemical equilibrium constant between the green and blue curves, and the red curve.…”

“…Finally, two recent studies contributed by Carpenter et al 12,35 concluded that nanodroplets of hexadecane can be produced by standard preparation techniques, but by using a proprietary compound, NoChromix® and a 'stringent multi-step glassware cleaning procedure', resulting in the production of "moderately" or "marginally" stable nanoemulsions with near-zero droplet charge. The ζ-potential value from 2019 35 is shown as the black data point in Fig.…”

“…The ζ-potential value from 2019 35 is shown as the black data point in Fig. 1A, and the electrophoretic mobility recorded as a function of pH 12 is shown in Fig. 1B.…”

“…Finally, based on our data and the data provided in Refs. 12,35 we propose an alternative mechanism for reduced ζ-potential that can result from prolonged acid treatment of the surface of the used glassware.…”

“…To investigate the conclusions drawn in Refs. 12,35 we prepared nanoemulsions according to the following protocol. The glassware used for preparing nanoemulsions were freshly taken out of the manufacturer's packaging and never re-used after the preparation.…”