Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR 97401.
Department of Physics, Whitman College, Walla Walla, WA 99362.
Proc Natl Acad Sci U S A. 2017 Dec 19;114(51):13351-13356. doi: 10.1073/pnas.1700099114. Epub 2017 Jul 31.
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.
纳米乳液和微乳液是油和水可以相互溶解的环境，为许多不同的生物和工业应用提供了独特的平台。与微乳液不同，纳米乳液在其表面的分子相互作用的特征化方面几乎没有得到什么研究。本研究详细研究了用表面活性剂二辛基磺基琥珀酸钠（AOT）稳定的规则（油包水）和反相（水包油）纳米乳液的近表面分子结构。振动和频散射光谱（VSFSS）用于测量这些 AOT 稳定的规则和反相纳米乳液的振动光谱。通过振动和频光谱（VSFS）对 AOT 在平面油水界面上的吸附进行了补充研究。这些研究共同深入了解了界面水的取向以及在不同油水界面上 AOT 的疏水区和亲水区的分子特征。尽管 AOT 的极性区域和周围的界面水分子在平面和液滴界面上的行为几乎相同，但即使考虑到可能的表面浓度差异，疏水链的有序性也存在明显差异。发现这种链有序性在纳米液滴通过奥斯特瓦尔德熟化生长时是不变的，并且与不同抗衡离子（Na:AOT、K:AOT 和 Mg:AOT）的取代也是如此，这也导致了不同尺寸的纳米颗粒。这些结果描绘了相对较大的纳米乳液表面上的表面活性剂组装的引人入胜的图景，并允许对较小的胶束（弯曲）和平面油水界面上的 AOT 进行重要比较。
