(437f) Clustering of Ionic Liquids in Diluted Alcohol Solutions

Ionic liquids (IL) are design solvents that have a great potential in chemical engineering, e.g., for improving existing or designing new processes. Preliminary screening of a variety of possible cation-anion combinations requires reliable predictive tools based on molecular structure of system components. Elaboration and evaluation of those in turn challenge the deeper knowledge and understanding the real structure of a molecule. Particularly, on molecular level, IL can be considered as a single neutral species (molecular form), as an equimolar mixture of cations and anions (completely dissociated form) or as a combination of both forms. Furthermore, as shown by König et al. [1] and Wakisaka et al. [2], ILs can also form charged clusters in diluted solutions of water and methanol. The main objective of this work is to study the influence of different representations of IL on the predicted values of activity coefficients in the frame of the a-priori Conductor-like Screening Model for Real Solvation (COSMO-RS) [3].

A series of imidazolium-based ILs (1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium and 1-octyl-3-methylimidazolium chlorides) in n-alkanols (from methanol to butanol) have been studied systematically via experiments and calculations.

The measurements were carried out using a mass spectrometer QTrap (AB Sciex) with electrospray ionization source. It has been found that the considered ILs tend to cluster in alcohols. Depending on the concentration and type of IL and alcohol, 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 were observed.

The activity coefficients of IL were estimated from the pseudo-chemical potentials calculated using COSMOthermX with a special focus on the appropriate choice of the reference state.

The experimental and modelling results are analyzed here as a function of IL concentration, length of the side chain in IL cation, and polarity of the alcohol. Experimental trends are compared to those obtained theoretically to explore the effect of different structural representations and to get a first idea how to model IL molecules in solution: as completely dissociated to single ions, as neutral molecules, as charged clusters or as a mixture of those.

References

[1] König et al., Chem. Eng. Technol. 2012, 35, p.1-9

[2] Wakisaka et al., Anal. Sci. 2008, 24, p.1311-1314

[3] Klamt, J. Phys. Chem. 1995, 99, p.2224-2235