Ion Distributions at Electrified Liquid-Liquid Interfaces: Microscopic and Macroscopic Measurements

An important unsolved problem in the study of ion distributions is the relationship of macroscopic electrochemistry measurements to the ion distribution determined by microscopic synchrotron x-ray scattering measurements. In this work, we present recent x-ray reflectivity and interfacial tension measurements of electrified oil/water interfaces as a function of interfacial electric potential for a series of interfaces between solutions of 10 mM alkali chloride (XCl, where X is the alkali metal ion of Li+, Na+, Rb+ and Cs+) in water and 5 mM organic supporting electrolyte BTPPATPFB in 1,2-dichloroethane. The potential is established by the use of electrodes and supporting electrolytes in the bulk phases. The impedance spectroscopy as a function of interfacial electric potential was also measured for each interface. The analysis of interfacial tension and impedance spectroscopy measurements were discussed and compared with the results from x-ray reflectivity analysis as well. Cyclic voltammogram was measured for each interface to determine the working potential range. Interfacial tension measurements were performed using a Cahn microbalance that measures the weight of a Teflon Wilhelmy plate fully submerged in the top water phase to determine the potential of zero charge (PZC) and the capillary wave roughness of the interfaces. The interfacial excess charge as a function of electric potential was determined from the tension measurements. Impedance spectroscopy was measured for each interface to determine the capacitance as a function of applied electric potentials. X-ray scattering experiments were carried out at the ChemMatCARS beamline 15-ID at the Advanced Photon Source (Argonne National Laboratory, USA). Large changes in reflectivity occur from intermediate potentials for each sample we study. These results disagree with predictions of the Gouy-Chapman (GC) theory. A Poisson-Boltzmann (PB) calculation that incorporates a potential of mean force (PMF) to describe the role of liquid structure on the ion distributions agrees well with the x-ray reflectivity data. In addition, we calculated the excess interfacial charge from these ion distributions. These calculated excess interfacial charges from the PB-PMF method are in good agreement with the tension measurement results. We also calculated the capacitance as a function of applied electric potentials for each sample from the PB-PMF calculations, and these results only showed limited agreements with the capacitance yielded from the impedance spectroscopy measurements. These comparisons make a direct connection between the molecular level x-ray measurements and macroscopic electrochemistry measurements of interfacial tension and impedance spectroscopy.