Sensing the Potential of Zero Charge at Soft Microinterfaces via Collisional Ionosomes

Measuring interfacial phenomena on the microscopic level is a current frontier topic across science, technology, and engineering. The potential/point of zero charge (pzc) is a fundamental parameter used to describe the electrostatics of a polarizable electrochemical interface. The pzc is a reference quantity at which no excess net charge is present at either side of the electrical double layer (EDL) of an electrochemical interface. A polarizable molecular interface, known as the interface between two immiscible electrolyte solutions (ITIES), may be formed between two ionic conducting electrolytes. Electrochemistry at a microscopic ITIES is an emerging field with broad applications in chemical/biosensing, scanning electrochemical microscopy imaging, mechanistic studies, and single-entity electrochemistry (SEE). Herein, we demonstrate the direct measurement of the pzc and EDL structure at ITIES composed of molecular interfaces formed either between water and α,α,α-trifluorotoluene or between water and α,α,α-trifluorotoluene methylated derivatives by utilizing SEE in conjunction with the charged characteristics of ionosomes and a double potential step chronoamperometry technique. The harmony of qualitative and quantitative analyses proves the reliability of this method. For comparison, the pzc of each complementary macro-ITIES is also measured with the differential capacitance method in a four-electrode configuration. The pzc values obtained by both methods are basically consistent. The collisional ionosome method overcomes the difficulty in measuring the pzc at the ITIES at the microscopic scale. Besides, our work supports the validity of the discrete Helmholtz model for describing the EDL at a polarizable ITIES at least at micro- and/or nano-ITIES. Furthermore, this work provides new mechanistic insights into the universal emulsification route via ionosomes.