During the last decade there has been an explosion of interest in ionic liquids. If the keywords “ionic liquid” or “ionic liquids” are used for a literature research in the Web of Science, more than seven thousand entries can be found for 2011. That is a tenfold increase in the number of publications dealing with ionic liquids over the past decade, and the field shows no signs of slowing down. Ionic liquids have changed from a lab curiosity to materials of tremendous academic and industrial interest. This new and remarkable liquid substance with unique and fascinating properties offers a phenomenal opportunity for new science and technology. The interest in these new materials is due to their wide range of possible applications as solvents for reaction and material processing, as extraction media or as working fluids in mechanical applications. Originating in the field of electrochemistry and based upon their wide electrochemical windows and good conductivities, ionic liquids have found useful applications in sensors, solar cells, solid-state photocells, batteries, separation devices and as thermal fluids, lubricants, hydraulic fluids, ionogels and fuels.
The generally accepted definition of an ionic liquid is a salt composed solely of ions with melting points below 100 °C. New combinations of ions provide changing physical properties and thus novel potential applications for this class of liquid material. To a large extent, the structure and properties of ionic liquids are determined by the intermolecular interaction between anions and cations. In particular, the subtle balance between Coulomb forces, hydrogen bonds, π stacking and repulsion–dispersion forces is of great importance for understanding the properties of ionic liquids. All the important properties such as structure, diffusion, viscosity, conductivity or melting points depend on these interactions between cations and anions in ionic liquids. Thus, an important goal is to develop a fundamental understanding of their chemical and physical properties at the molecular level. Reliable structure–property relationships can help us to synthesize ionic liquid compositions with the desired set of properties.
Since 2005 a number of research programs have been set up worldwide to advance ionic liquids research. Among those, the Japanese Priority Area “Science of Ionic Liquids” running from 2005 to 2009 and the priority programme SPP 1191 “Ionic Liquids” implemented by the German Science foundation (DFG) in 2006 and finished at the end of this year may be the most prominent ones in the academic arena. The efforts and developments achieved in ionic liquids research from these and other programmes are presented in this special issue. On the occasion of the International Bunsen Meeting 2012 in Leipzig, we put together a collection of almost forty articles that show the growing diversity within the field. In this special issue of ChemPhysChem, developments in experiment, theory and simulations for investigating ionic liquids are presented. The articles highlight the interaction within ionic liquids between anions and cations studied by a variety of experimental and theoretical methods including spectroscopy, ab initio simulations, classical molecular dynamics simulations and quantum molecular dynamics simulations. A particular focus is put on hydrogen bonding and dispersion forces, which are often assumed to be unimportant in Coulomb systems but are absolutely crucial to understanding ionic liquids. The critical review of theoretical and simulation approaches for describing the properties of ionic liquids is an important issue in this respect. Another topic is the interaction of ILs with water and organic solvents leading to modified local structures and polarities (Figure 1). In general, IL mixtures with water, alcohols and amines allow interesting tuning of important IL physical properties, including structural and transport properties such as diffusion coefficients, viscosities and conductivities. Walden plots, which compare the molar conductivity as a function of fluidity (inverse viscosity), show deviations from ideal, linear behaviour derived from well-understood aqueous solutions.
Why not consider mixing ILs themselves to change properties? Thus another focus is put on mixtures of ILs including different anion–cation combinations. By varying the cation or the anion only, structure and interaction strength can be modified in the desired way. Several examples are given for structure–property relations in ionic liquids, which may allow predictions for physical properties for a set of specially synthesized ILs. Chemical reactions and coordination in ionic liquids are also attracting increasing interest. In several articles the effects on physical properties of other materials such as gels and micelles in IL environments are reported. ILs have also become increasing important in biology. The behaviour of biomolecules, such as the folding and unfolding of proteins, are developing into an important facet in ionic liquids research. Of course, after having an idea about the origin of the physical properties, new ionic liquids need to be synthesized and characterized. New and special ionic liquids and their properties are introduced in some of the contributions. An important aspect in and of itself is the liquid/gas or the liquid/liquid interface of ionic liquids (Figure 2). How do these interfaces look like with respect to the polar and hydrophobic parts of anions and cations? How do the IL properties change at the interfaces? Combinations of experimental and theoretical methods try to provide answers to these questions.
All these fundamental issues are addressed in this special issue of ChemPhysChem. We hope that this compilation improves our understanding of the fascinating physical chemistry of ionic liquids. An understanding at the molecular level opens new potential for fundamentals and applications.