Molecular characterization of water and surfactant AOT at nanoemulsion surfaces

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 114; no. 51; pp. 13351 - 13356
• Main Authors: Hensel, Jennifer K., Carpenter, Andrew P., Ciszewski, Regina K., Schabes, Brandon K., Kittredge, Clive T., Moore, Fred G., Richmond, Geraldine L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 19.12.2017; National Academy of Sciences, Washington, DC (United States)
• Series: From the Cover
• Subjects: Chains; Emulsions; Hydrophobicity; Industrial applications; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Interfaces; Microemulsions; Molecular interactions; Molecular structure; Molecules; Mud-water interfaces; Nanoemulsions; Nanoparticles; Oil; Oil-water interface; oil-water interfaces; Ostwald ripening; Physical Sciences; Polar environments; science & technology; Sodium; Spectroscopy; Sulfosuccinates; surface spectroscopy; Surfactants; vibrational sum-frequency scattering spectroscopy; Water chemistry; nanoemulsions; vibrational sum-frequency scattering spectroscopy; surfactants; surface spectroscopy; oil–water interfaces
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1700099114

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.