Theoretical Description of Ionic Liquids

Ionic liquids are universally considered to be materials of the future. Their peculiar properties appeal to the most diverse technological areas such as chemical industry, electrochemistry, optics, environmental chemistry, medicine and nanotechnology. Their extraordinary macroscopic properties originate from the peculiar ionic nanoscopic structure and the possibility of modulating these properties depends ultimately on small modifications of the material at the molecular level. Theoretical simulations of ionic liquids are becoming increasingly important in predicting the properties of these newmaterials and in helping to interpret the experimental determinations. From the theoretical point of view, ionic liquids are peculiarmaterials because they are entirely composed of ions. They can be defined as pure electrolytes. All the technologically interesting properties that are proper to this class of compounds are due to the strong electrostatic interactions that govern the fluid behavior. Though these interactions are quite strong, the size and nature of the cationic partners (more often) are such that the fluid remains liquid even at room temperature. Its behavior, from a microscopic point of view, is fairly similar to the more conventional high temperature molten salts. The theoretical framework in which it is possible to provide high quality studies of the microscopic structure of the ionic liquid is mainly represented by classical molecular mechanics and, only very recently, by ab-initio molecular dynamics. While the employed theoretical techniques are not very different from those used for conventional fluids, many difficulties arise because of the microscopic nature of ionic liquids. In particular these substances are extremely viscous and simulation times become quickly prohibitive if one wants to describe dynamical properties, even as simple as diffusion coefficients. Recent technological advances such as the introduction of GPU clusters might allow unprecedented possibilities in the simulation of these material opening the route to the simulation of rare events and long time scale phenomena. The choice and validation of the many force fields which have been established in the last twenty years, some of which are explicitly tailored for a specific class of ionic liquid, represent a crucial step in performing classical trajectory studies. This task has to be performed by taking into account many different sources of experimental data: on the one hand structure studies with X-ray or neutron diffraction provide indications on the short-range structure, on the other hand measurements of bulk properties such as evaporation enthalpies point to the overall energetic behavior of the fluid. In this chapter we would like to summarize and review the recent developments and possible future opportunities of theoretical simulations of ionic liquids. In particular we will review some of the issues connected to the theoretical simulations of ionic liquids, focusing on our 5