Voltage-Dependent Molecular Assembly at Ionic Liquid-Gold Interfaces: Quantifying Ion Structuring and Interaction Forces

Voltage-controlled ion structuring at ionic liquid (IL)-electrode interfaces critically affects the performance of electrochemical devices. Yet, the existing understanding of how molecular scale interactions correlate quantitatively with interfacial nanostructures remains poor. We hypothesize that introducing a novel quantitative descriptor, explicitly linking nanoscale ionic layering and molecular interactions, will enable precision tuning of interfacial properties, significantly advancing colloidal and interfacial science and engineering for energy storage technologies. We systematically characterized molecular interactions and interfacial structuring of the thin-layered phosphonium-based ionic liquid, [P₆,₆,₆,₁₄] [MEEA] (P₆M) at a gold electrode under methodically varied voltages (-4 to +4 V). The combination of gold colloid-probe atomic force microscopy (CP-AFM) with quartz crystal microbalance (QCM) enables accurate, quantitative mapping of IL-Au interactions. CP-AFM provided spatially resolved force mapping and local mechanical characterization under applied voltages, which we integrate with QCM observables into a new parameter, the Electro-Mechanical Structuring Factor (EMSF). Application of external voltages induced significant, measurable structural reorganization of ionic layers, resulting in increased stiffness, reduced thickness, and enhanced nanoscale ordering driven by electrostatic interactions. Our novel EMSF parameter provides explicit numeric correlations between molecular interactions and nanoscale structuring, directly addressing a critical gap in past literature, which has largely relied on qualitative/semiquantitative approaches. The EMSF parameter represents an essential advance in interfacial characterization, offering a powerful predictive tool for rationally optimizing IL-based colloidal and interfacial systems for targeted electrochemical applications.