Ion-Size Heterogeneity Controls Interfacial Electrostatic Response in Mixed Ionic Liquids

Ion-size heterogeneity provides a route to modifying the electrostatic response of ionic-liquid interfaces beyond ion size alone. Here, we use multistep chronoamperometry to measure differential capacitance and charging dynamics of [CnC1Im]+[NTf2]- (n = 3, 4, 6, 8) and their binary mixtures. The neat ionic liquids exhibit chain-length-dependent capacitance profiles, while mixtures with increasing cation-size disparity develop localized capacitance maxima that are suppressed for closely sized ions. The magnitude of these features depends nonmonotonically on composition, indicating nonideal interfacial mixing. Crucially, capacitance maxima coincide with peaks in the slower charging time scale, demonstrating coupled electrostatic and dynamic changes. These results are consistent with a mechanism where ion-size heterogeneity promotes potential-driven reorganization between competing configurations, offering an extra degree of freedom to tune dense ionic-liquid interfaces. Direct molecular-level characterization remains a natural next step to confirm these microscopic transitions.