DOI: 10.1002/cphc.201100872 Determination of LFER Descriptors of 30 Cations of Ionic Liquids—Progress in Understanding Their Molecular Interaction Potentials Chul-Woong Cho, [a, b] Christian Jungnickel, [a, c] Stefan Stolte, [a] Ulrich Preiss, [b, d] Jürgen Arning, [a] Johannes Ranke, [a] Ingo Krossing,* [b, e] and Jorg Thçming* [a] 1. Introduction In the past decades the interest in ionic liquids (ILs) dramatical- ly increased both in industrial and academic fields. Due to their favorable properties, as for example a negligible vapor pressure or excellent solvation and electronic properties, they can be used as atmospheric environmentally friendly reaction media for synthesis of, among others, pharmaceutical drug molecules, biopolymer fibers and carbohydrates, [1–3] where ILs have the potential to partially replace organic solvents. Also, their physicochemical properties, such as density, moisture sta- bility, viscosity, or miscibility with other solvents, can be tail- ored by the appropriate selection of cation and anion. A very rough estimate indicates that these ion combinations can number at least a million as binary ion sets and 10 18 as ternary mixtures. [4] Despite the wide spectrum of applicability and dis- tinct advantages of ILs, a quantitative understanding of their molecular interaction potentials remains to be established. When molecular interactions of ILs come into play, two common situations can be differentiated: their roles as solvent and as solutes. For an understanding of the first situation, system parameters of ILs were determined by inverse gas chro- matography facilitated by their low vapor pressures. [5–7] How- ever, the second situation involves the possibility of separately investigating the two IL components, namely cations and anions. This presents both a challenge and a chance. It is a challenge because they will be isolated or more or less associ- ated in the form of ion pairs, clusters or aggregates depending on IL concentrations, their chemical environment, and their In order to understand molecular interaction potentials of 30 cations of ionic liquids (ILs), the well-known linear free energy relationship concept (LFER) was applied. The LFER descriptors for the excess molar refractivity and the molar volume were calculated in silico and for hydrogen-bonding acidity and ba- sicity, and the polarizability/dipolarity of IL cations were experi- mentally determined through high performance liquid chroma- tography (HPLC) measurements. For the study, three different columns (RP-select B, Cyan, and Diol) and buffered mobile phases, based on two organic solvents acetonitrile (ACN) and methanol (MeOH), were selectively combined to the HPLC sep- aration systems RP-select B-ACN, RP-select B-MeOH, Cyan- MeOH, Diol-ACN, and Diol-MeOH. By measuring the retention factors of 45 neutral calibration compounds and calculating LFER descriptors of three cations in the HPLC systems, the system parameters, including an ionic z coefficient, were deter- mined. Conversely, the LFER descriptors of 30 ionic liquid cat- ions were determined, based on the parameters of five sys- tems and their retention factors in the HPLC systems. The re- sults showed that the type of head group, alkyl chain length and further substituents of the cation have a significant influ- ence on the dipolarity/polarizability and the hydrogen-bonding acidity, and functionalized groups (hydroxyl, ether, and dime- thylamino) lead to hydrogen-bonding basicity of the cation. The characterization of cationic LFER descriptors opens up the chance for a more quantitative understanding of molecular in- teraction potentials and physicochemical properties of ILs. [a] C.-W. Cho, Dr. C. Jungnickel, Dr. S. Stolte, Dr. J. Arning, Dr. J. Ranke, Prof. J. Thçming Zentrum für Umweltforschung und nachhaltige Technologien (UFT)— Centre for Environmental Research and Sustainable Technology University of Bremen Leobener Straße, 28359 Bremen (Germany) Fax: (+ 49) (0)421 2187643 E-mail : thoeming@uni-bremen.de [b] C.-W. Cho, Dr. U. Preiss, Prof. I. Krossing Freiburger Materialforschungszentrum FMF University of Freiburg Stefan-Meier-Str. 21, 79104 Freiburg (Germany) Fax: (+ 49) (0)761 2036001 E-mail : krossing@uni-freiburg.de [c] Dr. C. Jungnickel Chemical Faculty Gdan ´sk University of Technology ul. Narutowicza 11/12, 80-233 Gdan ´sk (Poland) [d] Dr. U. Preiss Interdisciplinary Centre for Advanced Materials Simulation (ICAMS) Ruhr-Universität Bochum Stiepeler Str. 129, 44801 Bochum (Germany) [e] Prof. I. Krossing Institut für Anorganische und Analytische Chemie University of Freiburg Albertstraße 21, 79104 Freiburg (Germany) and FRIAS Fellow of the Section Soft Matter Science Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201100873. 780  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 780 – 787