Basics & Visualization

Ionic liquids, i.e. salts with melting points below 100° C, have attracted considerable interest in the past years as potentially very promising green solvents, i.e. environmentally benign solvents for reactions, extraction and separation [1]. The special advantage of these solvents is that they provide a wide range of solvent polarity and have almost no vapor pressure. By combining various large organic cations with a set of suitable anions a large number of different ionic liquids can be generated and thus the selection of ionic liquids that are suitable for solving a certain problem is an important issue.

One of the obstacles towards this goal is the limited access to experimental thermodynamic data for activity coefficients in ionic liquids, which could be a basis for such solvent selection. Although in the past years several groups have started to measure such data, the lack of data is even becoming larger because the number of suitable anions and cations and even more the number of ionic liquids are increasing faster than measurements can be performed. Thus the use of an appropriate prediction method like COSMO-RS is unavoidable for the selection of tailor-mate ionic liquids.

σ-surfaces and profiles of ionic liquids

The screening (polarization) charge density s, which is the key value of the COSMO-RS theory, of some cations and anions is shown in Figure 1 (mapped on the molecular surface) and Figure 2 (as a histogram). s is the response of an electric conductor to the charge density of the molecule and therefor a measure for the polarity of the different molecular regions. Please note that by definition, the sign is opposite to the sign of the molecular charge distribution.

hexafluorophosphate	ethylsulfate	bis((trifluoromethyl)sulfonyl)imide

pyridinium	1-buthyl-3-methylimidazolium

Figure 1: σ-surfaces of typical ionic liquid cations

Figure 2: σ-profiles of typical Ionic liquid anions and cations.

As can be seen from the upper σ-profiles, the typical ions used in ionic liquids show only moderate polarization charge densities. One can clearly see that the anions and cations show much less polarity hotspots than water. This is due to the delocalization of the molecular charge, which can be achieved in two ways. In the ions like the imidazolium, pydidiniuim, or nitrate and many others the charge is delocalized via conjugation and hyperconjugation. Other, mostly high symmetrical ions, like ammonium and phosphonium cations or anions like [PF6]-, [BF4]- delocalize the charge over their ligand surface.

Beside sterical aspects of the crystal packaging this is one of the reasons why the ionic liquids don't crystallize. It also explains why the COSMOtherm predictions for ionic liquid solutions can be done with the standard parametrization, which has not been adjusted for ions.
The following links provide a list of s-surfaces of the most important cations and anions of Ils:

References:

[1]
T. Welton. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev.
J. F. Brennecke, E. J. Maginn. Ionic Liquids: Innovative Fluids for Chemical Processing.
AIChEJournal 2001, 47, 2384-2389.