Differential capacitance is a key quantity in the understanding of electrical double-layer charging of electrolytes. However, experimental observations of ionic liquid systems are controversial, inconsistent, and often unable of confirming or refuting existing theories as well as highlighting discrepancies between the measurement techniques. We study the differential capacitance in both pure and dilute ionic liquids at room temperature. Using chronoamperometry to measure the differential capacitance of the liquids at a polycrystalline platinum electrode, we find good agreement between the measured capacitance curves and the extended mean-field model of Goodwin–Kornyshev [Goodwin, Z. A.; et al. Electrochim. Acta.2017, 225, 190–197]. A crossover is found from the pure to the dilute regime, as shown by a transition from a camel-shape capacitance curve to a U-like one, together with a nonmonotonic dependence of capacitance with electrolyte concentration.

The interfacial behavior of ionic liquids promises tunable lubrication as well as playing an integral role in ion diffusion for electron transfer. Diluting the ionic liquids optimizes bulk parameters, such as electric conductivity, and one would expect dilution to disrupt the near-wall molecular ordering. We study this ordering in the ionic liquids [Emim]+[NTf2]−, [Emim]+[DCA]−, and [C4mpyr]+[NTf2]−, diluted in the solvent dimethyl sulfoxide. We found a structural crossover from well-ordered ionic liquids to a well-ordered solvent with increasing dilution, but this occurs nonlinearly, with solvent molecules initially space-filling and solvating and later disrupting the ionic layers. This is of key importance for ionic liquids as optimized tunable nanolubricants.

We extract the conservative and dissipative interactions of the ionic liquid [Emim] + [NTf 2 ] − on mica and HOPG. Charge-mediated layering on mica locks the counterions strongly in place, resulting in several liquid layers forming as the wall effect is integrated out, as well as much greater damping between the layers as compared to the bulk. Template-mediated layering on HOPG is much weaker than its Coulombic counterpart. In terms of local flow properties, layers on HOPG can glide easier over each other as compared to the bulk. This clearly demonstrates the importance of substrate for controlling near-wall dynamic response.

Protic ionic liquids are made via intermolecular transfer between cation and anion. This research employs theoretical calculation to gain understanding of the intermolecular interactions at single-ion-pair level and their consequences to the macroscopic properties of the ionic liquids. The protic ionic liquids used in this study are ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and butylammonium nitrate (BAN). Molecular structures of ions and ion pair of each liquid are optimized using the density functional theory. The ion pair binding energies and infrared spectra of optimized structures are investigated. Infrared spectra of separated cation and anion are compared with cation-anion ion pair to specify the vibrational band shifts due to intermolecular interactions. The infrared-spectra show the hydrogen-bond stretching modes in the far-infrared region at the wavenumber between 108-252 cm−1 and the red shifts of four vibrational bands due to intermolecular interactions between the ions. Three vibrational band shifts are mainly due to Coulomb force, and the other shift is caused by combination of intermolecular interactions. Moreover, the energy calculations show that the binding energy decreases as the alkyl chain length increases.

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