Electrochemical Processes Breaking Strict Phase Electroneutrality in Microemulsions

• Published in: Journal of the Electrochemical Society Vol. 172; no. 5; pp. 56501 - 56507
• Main Authors: Barth, Brian A., Imel, Adam E., Zawodzinski, Thomas
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.05.2025
• Subjects: electroneutrality; emulsions; ion coupled electron transfer; microemulsions
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1945-7111/add185

Phase electroneutrality is a near-omnipresent assumption in electrochemical reactions. Even media such as biphasic oil-water interfaces and kinetically stable emulsions follow this “rule.” This implies all electron transfer reactions must be accompanied by ion transfer to maintain charge balance within a phase. However, due to the small domain size, electroneutrality is known to break down in the electric double layer adjacent to the electrode. Therefore, the appropriateness of the electroneutrality approximation for media with nanoscale domains, such as microemulsions, is questionable. Here, we show definitively, for the first time, that phase electroneutrality can be violated during an electron transfer reaction in microemulsions. Cyclic voltammograms show that electrochemical oxidation of rubrene is possible in microemulsions when sodium hydroxide is the only supporting electrolyte. Rubrene is a hydrophobic redox compound that is insoluble in water in neutral and radical cation forms, and hydroxide anions are too hydrophilic to transfer to the oil (toluene) phase. Contrary to observations made in emulsion systems, rubrene oxidation in microemulsions seemingly occurs by a mechanism where coupled ion transfer required for rigorous phase electroneutrality is negated. Our results demonstrate that electron transfer reactions within nanometer-sized domains are not subject to restrictions that govern systems with larger domains.