Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR 97403.
Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR 97403
Proc Natl Acad Sci U S A. 2019 May 7;116(19):9214-9219. doi: 10.1073/pnas.1900802116. Epub 2019 Apr 24.
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.
纳米乳液的稳定化，即将纳米级油滴分散在水中，通常通过添加表面活性剂和聚合物来实现。然而，人们已经观察到，在没有乳化剂的情况下，纳米乳液的表面会获得显著的负电荷，这最终有助于其稳定性。尽管这种负电荷的来源至今仍存在争议，但它的存在被视为水-疏水性界面的固有特性。本报告探讨了裸水-疏水性纳米乳液界面的分子结构和键合特性。我们报告了近零表面电荷的裸纳米乳液的制备，这些乳液在几天内仍保持适度稳定。制备这些低电荷纳米乳液（LCNE）的过程需要严格的清洁程序和适当的溶剂储存条件。我们使用振动和频散射光谱测量界面水相和疏水性相的结构和键合。这些 LCNE 样品的表面具有可测量的游离 OH 振动，这在以前的研究中未发现，表明界面清洁。通过添加阴离子表面活性剂来调整纳米乳液的电荷，模拟潜在的表面活性污染物，我们观察到游离 OH 消失，并且在微摩尔表面活性剂浓度下，界面疏水分子发生重新取向。值得注意的是，与类似的平面水-烷烃界面相比，游离 OH 振动为水分子与 LCNE 表面之间的分散相互作用提供了更强的证据。我们提出，更强的键合相互作用以及有序的界面水层有助于延迟液滴聚结和随后的相分离。
