Ionic Liquid Mixture–Electrode Interface: Size-Dependent Competition between Ions Determines Capacitance

Ionic liquid mixtures are promising electrolytes for electrochemical technologies; however, the microscopic principles that govern their interfacial behavior remain insufficiently understood. Here, we report molecular dynamics (MD) simulations of BMImPF₆–BMImFAP mixtures at the Au(100) surface, with systematic variation of both mixture composition and electrode surface charge density. The MD simulations predict differential capacitance–potential dependence for pure and mixed ILs and reveal the molecular-level structure of their electric double layer. The abnormal enhancement of capacitance upon small additions of FAP^–, reported experimentally and motivated this research, was not observed. However, the effect of ion size on the interfacial structure was revealed: the smaller PF₆^– adsorbs preferentially at low charge densities, while the bulkier FAP^– contributes increasingly at higher charges due to conformational flexibility and cation-assisted packing. This insight into the competition between PF₆^– and FAP^– for placement at the Au(100) surface provides a microscopic understanding of how steric constraints, adsorption energetics, and ion orientation govern the structure and capacitance of ionic liquid mixtures. The work links mixture composition to interfacial structure, guiding the rational design of ionic liquid electrolytes for energy storage applications.