Shi Lixue, LaCour R Allen, Qian Naixin, Heindel Joseph P, Lang Xiaoqi, Zhao Ruoqi, Head-Gordon Teresa, Min Wei
Department of Chemistry, Columbia University, New York, NY, USA.
Kenneth S. Pitzer Theory Center and Department of Chemistry, University of California, Berkeley, CA, USA.
Nature. 2025 Apr;640(8057):87-93. doi: 10.1038/s41586-025-08702-y. Epub 2025 Mar 19.
Interfacial water exhibits rich and complex behaviour, playing an important part in chemistry, biology, geology and engineering. However, there is still much debate on the fundamental properties of water at hydrophobic interfaces, such as orientational ordering, the concentration of hydronium and hydroxide, improper hydrogen bonds and the presence of large electric fields. This controversy arises from the challenges in measuring interfacial systems, even with the most advanced experimental techniques and theoretical approaches available. Here we report on an in-solution, interface-selective Raman spectroscopy method using multivariate curve resolution to probe hexadecane-in-water emulsions, aided by a monomer-field theoretical model for Raman spectroscopy. Our results indicate that oil-water emulsion interfaces can exhibit reduced tetrahedral order and weaker hydrogen bonding, along with a substantial population of free hydroxyl groups that experience about 95 cm redshift in their stretching mode compared with planar oil-water interfaces. Given the known electrostatic zeta potential characteristic of oil droplets, we propose the existence of a strong electric field (about 50-90 MV cm) emanating from the oil phase. This field is inferred indirectly but supported by control experiments and theoretical estimates. These observations are either absent or opposite in the molecular hydrophobic interface formed by small solutes or at planar oil-water interfaces. Instead, water structural disorder and enhanced electric fields emerge as unique features of the mesoscale interface in oil-water emulsions, potentially contributing to the accelerated chemical reactivity observed at hydrophobic-water interfaces.
界面水表现出丰富而复杂的行为，在化学、生物学、地质学和工程学中发挥着重要作用。然而，关于疏水界面处水的基本性质，如取向有序性、水合氢离子和氢氧根离子的浓度、不规则氢键以及强电场的存在，仍存在诸多争议。这种争议源于测量界面系统所面临的挑战，即便使用现有的最先进实验技术和理论方法亦是如此。在此，我们报告一种溶液内界面选择性拉曼光谱方法，该方法借助拉曼光谱的单体场理论模型，采用多元曲线分辨技术来探测水包十六烷乳液。我们的结果表明，油水乳液界面可能呈现出降低的四面体有序性和较弱的氢键，同时存在大量游离羟基，与平面油水界面相比，其伸缩模式下会经历约95厘米的红移。鉴于油滴已知的静电zeta电位特性，我们提出存在一个从油相发出的强电场（约50 - 90兆伏/厘米）。该电场是间接推断出来的，但得到了对照实验和理论估计的支持。这些现象在由小溶质形成的分子疏水界面或平面油水界面中要么不存在，要么相反。相反，水结构的无序和增强的电场成为油水乳液中尺度界面的独特特征，这可能有助于解释在疏水 - 水界面观察到的化学反应加速现象。
