Differential capacity–potential curves, C(E), were obtained from electrochemical impedance spectra (12 kHz–2 Hz) for the interfaces between Hg and a series of alkyl imidazolium-based room temperature ionic liquids having the same anion, bis(trifluoromethanesulfonyl) imide: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [EMIM][Tf₂N], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [BMIM][Tf₂N], 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [HMIM][Tf₂N]. The electrocapillary curves were obtained from drop time measurements and the values of the pzc were calculated. The pzc apparently becomes more negative as the imidazolium alkyl chain length increases. A small effect of the cation is seen on the C(E) curves at negative potentials. The effect of the aromatic nature of the cation is assessed by comparing 1-butyl-1-methylimidazolium bis(trifluoromethanesulfonyl) imide, with 1-butyl-3-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide [BMPyr][Tf₂N]. The effects of temperature on the capacitance, drop time electrocapillary curve and on the pzc were also obtained. The capacity was found to increase with increasing temperature in the whole range of accessible potentials.

Mixing of ionic liquids provides new opportunities for their tuning, enabling the applications of ionic liquid mixtures to expand. At the same time, the genesis of the fundamental properties of ionic liquid mixtures is still poorly understood. In this study we carried out a molecular dynamics simulation of binary mixtures of 1-buthyl-3-methylimidazolium hexafluorophosphate, 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-buthyl-3-methylimidazolium tris(perfluoroethyl)trifluorophosphate ([C₄mim][PF₆] + [C₄mim][NTf₂], [C₄mim][PF₆] + [C₄mim][FAP], [C₄mim][FAP] + [C₄mim][NTf₂]) in a wide concentration range at 303.15 K and complemented it with quantum mechanical calculations. Three pure ionic liquids underwent the same kind of analysis for comparison purposes. We found that the addition of the [FAP]⁻–anion to a mixture enhances the segregation of non-polar domains and weakens the hydrogen-bond network. The H-bonds in the studied mixtures are rather weak, as follows from QTAIM analysis, with the rarest occurrence for the [FAP]⁻–anion. The competition of two anions in the mixtures for the most acidic hydrogen of the 1-butyl-3-methylimidazolium cation is reported. In most of the cases, the smaller anion ([PF₆]⁻ or [NTf₂]⁻) with stronger charge concentration displaces the bigger one ([NTf₂]⁻ or [FAP]⁻) from the preferred coordination site. The existing nano-segregation in some mixtures notably slows down ion diffusion. Our results show that the differences in anion size, shape and nature are the main reasons for nano-segregation and the non-ideal behavior of ionic liquid mixtures.