Short-range correlation effects at molecular level in the electrical double layer of imidazolium-based electrolytes

The nanostructure of room temperature ionic liquids (RTILs) is known for its contribution to their unique bulk liquid properties, especially the alkyl chain length effect in the 1-alkyl-3-methylimdazolium cation ([C_nmim]⁺). In this study, a constant potential method for a charge development simulation of a supercapacitor model using [C₃mim]⁺, [C₄mim]⁺, [C₆mim]⁺ and bis(trifluoromethyl sulfonyl)imide anion ([NTf₂]^-) was carried out. The multi-perspective analysis revealed that each RTIL behaved differently and that alkyl chain length prominently contributed to the electrical double layer (EDL) structure. While all three cations were able to form a dense EDL, [C₃mim][NTf₂] formed disordered layers that were more inert to the voltage bias than those of [C₄mim][NTf₂] which rearranged and reoriented into a highly organized EDL structure. Meanwhile, the disordered EDL packing in [C₆mim][NTf₂] was due to the significantly higher alkyl chain aggregation and a larger non-polar domain, which mitigated molecular reorganization in the EDL region. All RTIL physical properties reflected the differential capacitance profile, where [C₃mim][NTf₂] and [C₄mim][NTf₂] were able to be electronically described by the Goodwin-Kornyshev EDL theory.