Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 116, Issue 19, Pages 9214-9219Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1900802116
Keywords
nanoemulsions; surface spectroscopy; interfacial water; vibrational sum-frequency scattering spectroscopy; oil-water interfaces
Categories
Funding
- US Department of Energy, Office of Science, Condensed Phase and Interfacial Molecular Science Division [DE-SC0014278]
- U.S. Department of Energy (DOE) [DE-SC0014278] Funding Source: U.S. Department of Energy (DOE)
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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.
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