Ion Distributions at Electrified Liquid-Liquid Interfaces: Microscopic and Macroscopic Measurements
Year: 2012
Type: dissertation
Abstract: 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\nX 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.\nCyclic 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.\nThe 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.\nX-ray scattering experiments were carried out at the ChemMatCARS beamline 15-ID at the\nAdvanced Photon Source (Argonne National Laboratory, USA). Large changes in reflectivity\noccur from intermediate potentials for each sample we study. These results disagree with\npredictions of the Gouy-Chapman (GC) theory. A Poisson-Boltzmann (PB) calculation that\nincorporates a potential of mean force (PMF) to describe the role of liquid structure on the ion\ndistributions agrees well with the x-ray reflectivity data.\nIn addition, we calculated the excess interfacial charge from these ion distributions. These\ncalculated excess interfacial charges from the PB-PMF method are in good agreement with the\ntension measurement results. We also calculated the capacitance as a function of applied electric\npotentials for each sample from the PB-PMF calculations, and these results only showed limited\nagreements with the capacitance yielded from the impedance spectroscopy measurements.\nThese comparisons make a direct connection between the molecular level x-ray measurements\nand macroscopic electrochemistry measurements of interfacial tension and impedance spectroscopy.
Source: Figshare
Author: Binyang Hou (1751215)
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W790601919