The role of long-range interactions in the phase behavior of ionic liquids

The COSMO-type liquid activity coefficient models are among the most successful methods for the prediction of the phase behaviours of fluids containing ionic liquids. However, these models consider only interactions between species that are in close contact; in other words, the long-range Coulomb interactions, which are known to be important for systems containing charged species, are completely ignored. Here we show that the inclusion of long-range terms (with, e.g., the Pitzer-Debye-Hückel model) is indeed critical to obtain the correct osmotic coefficients and the asymptotic behaviours in the infinite dilution of IL solutions. However, the direct combination of PDH and COSMO-type models does not produce accurate activity coefficient over the whole concentration range because both models contain short range interactions. We show that a recent revision of the COSMO-SAC model, which includes the PDH contributions and a specific treatment of solvent-ion and ion-ion interactions, is capable of providing an accurate description of the properties (phase behaviour, osmotic coefficients) of aqueous IL solutions over the whole concentration range.