(441d) Phase Equilibria in Ionic Liquid- Alkanol Solutions

Ionic liquids (IL) can offer an enhanced potential in improvement of existing processes due to the fact that their properties can be specifically customized for a particular application by the corresponding combination of cations and anions. To provide preliminary screening of the great number of possible combinations, a-priori predictive tools such as Conductor-like Screening Model for Realistic Solvation (COSMO-RS) are of indisputable value.

To apply any molecular model, the right representation of a molecule is necessary. Till now, it is still not clear in what form IL molecules are present in solution and thus how to treat them in the molecular calculations: as a single neutral compound (molecular form/ion pair), as an equimolar mixture of cations and anions (dissociated form/single ions) or as a combination of both forms. Moreover, it has recently been shown that in diluted aqueous and methanol solutions ILs can also build ionic clusters, consisting of several cations and anions. So, IL may be present in solutions in any of the above-mentioned forms. The main objective of this work is to study the influence of different representations of IL on liquid-liquid (LLE) and solid-liquid (SLE) phase equilibria in the frame of COSMO-RS model and focuses on the IL-alcohol systems.

In the present study, the aggregation behavior of ionic liquids (1-alkyl-3-methylimidazolium chlorides) are experimentally studied in diluted alcohol (from methanol to butanol) solutions via mass spectrometry (QTrap, AB Sciex). Formation of positively charged clusters consisting of up to eight cations and seven anions as well as negatively charged clusters containing up to nine anions and eight cations have been observed. Cluster solvation energies calculated using COSMO-RS reproduce the experimentally obtained trends.

The COSMO-RS predictions of SLE and LLE are carried out in binary mixtures of 1-alkyl-3-methylimidazolium chlorides as well as with imidazolium-based cations and other anions (e.g., hexafluorophosphate, bis(trifluoromethylsulfonyl)amide, tetrafluoroborate) with alkanols (from methanol to octanol). The calculations are based on different representation of IL in the liquid phase (ion pairs, single ions, clusters) and compared to experimental data available in literature.

The experimental and modeling results are analyzed as a function of the length of the side chain in IL cation and polarity of the alcohol. The obtained trends are evaluated to explore the effect of different representations of IL and to gain preliminary understanding of how to model IL molecules in solution: as completely dissociated to single ions, as neutral molecules, as charged clusters or as a mixture of all those.