Deviations from ideality in mixtures of two ionic liquids containing a common ion

Excess molar volumes of six binary mixtures composed of two ionic liquids of the 1-methyl-3-alkylimidazolium bis(trifluoromethylsulfonyl)amide family-([C_mmim] + [C_nmim])[NTf₂] with n and m ranging from 2 to 10 - were measured for several compositions at 298 and 333 K. Similarly, three other binary systems containing [C₄mim]⁺ (1-methyl-3-butylimidazolium) as a common cation have been studied: [C ₄mim]([NTf₂] + [PF₆]), [C₄mim] ([NTf₂] + [BF₄]) and [C₄mim]([BF₄] + [PF₆]). Thus, the mixing process of two distinct ions of equal sign embedded in a constant field network of a given counterion was analyzed. All systems exhibit small, positive V^E values of the order of a few tenths of cm³ mol^-1, which are essentially temperature and pressure independent. The results show additive trends, for instance, the V^E values become larger as the difference between the alkyl chain lengths of the two cations increases. The Flory theory of mixtures provides a useful, semiquantitative correlation between the excess volumes and excess enthalpies in these systems, allowing for comparison with analogous results obtained within the linear primary alcohols.