Xiong Hanqing, Lee Jae Kyoo, Zare Richard N, Min Wei
Department of Chemistry, Columbia University, New York, New York 10027, United States.
Department of Chemistry, Stanford University, Stanford, California 94305, United States.
J Phys Chem Lett. 2020 Sep 3;11(17):7423-7428. doi: 10.1021/acs.jpclett.0c02061. Epub 2020 Aug 25.
Chemical reactions in aqueous microdroplets often exhibit unusual kinetic and thermodynamic properties not observed in bulk solution. While an electric field has been implicated at the water interface, there has been no direct measurement in aqueous microdroplets, largely due to the lack of proper measurement tools. Herein, we employ newly developed stimulated Raman excited fluorescence microscopy to measure the electric field at the water-oil interface of microdroplets. As determined by the vibrational Stark effect of a nitrile-bearing fluorescent probe, the strength of the electric field is found to be on the order of 10 V/cm. This strong electric field aligns probe dipoles with respect to the interface. The formation of the electric field likely arises from charge separation caused by the adsorption of negative ions at the water-oil interface of microdroplets. We suggest that this strong electric field might account in part for the unique properties of chemical reactions reported in microdroplets.
水性微滴中的化学反应通常表现出在本体溶液中未观察到的异常动力学和热力学性质。虽然已经推测在水界面存在电场，但在水性微滴中尚未进行直接测量，这主要是由于缺乏合适的测量工具。在此，我们采用新开发的受激拉曼激发荧光显微镜来测量微滴水油界面处的电场。通过含腈荧光探针的振动斯塔克效应确定，电场强度约为10 V/cm。这种强电场使探针偶极子相对于界面排列。电场的形成可能源于微滴水油界面处负离子吸附引起的电荷分离。我们认为，这种强电场可能部分解释了微滴中报道的化学反应的独特性质。
