Ion Distributions at the Water/1,2-Dichloroethane Interface: Potential of Mean Force Approach to Analyzing X-ray Reflectivity and Interfacial Tension Measurements

We present X-ray reflectivity and interfacial tension measurements of the electrified liquid/liquid interface between two immiscible electrolyte solutions for the purpose of understanding the dependence of interfacial ion distributions on the applied electric potential difference across the interface. The aqueous phase contains alkali-metal chlorides, including LiCl, NaCl, RbCl, or CsCl, and the organic phase is a 1,2-dichloroethane solution of bis(triphenylphosphor anylidene) ammonium tetrakis(pentafluorophenyl)borate (BTPPATPFB). Selected data for a subset of electric potential differences are analyzed to determine the potentials of mean force for Li+, Rb+, Cs+, BTPPA+, and TPFB–. These potentials of mean force are then used to analyze both X-ray reflectivity and interfacial tension data measured over a wide range of electric potential differences. Comparison of X-ray reflectivity data for strongly hydrated alkali-metal ions (Li+ and Na+), for which ion pairing to TPFB– ions across the interface is not ...