Ionic liquids microscopic environment

The goal of this chapter is to compile existing knowledge on the behavior of ionic liquids and their influence on solvation and chemical reactivity. The intent is not to list reactions and their outcomes, but rather to review the results of studies that offer physical insight into the microscopic environment of ILs and their interaction with solute species. While many excellent reviews of ILs have been written [1, 4, 23, 30, 38 -0], this chapter is distinct in its attempt to identify the basic physical principles relevant to solvation in ILs. 

The goal of this chapter is to understand the behavior of ionic liquids as solvents and their influence on reaction based on their chemical structure and microscopic environment. We will therefore provide only a basic overview of their macroscopic physical properties. An online database, compiled by a research team operating under the auspices of the International Union of Pure and Applied Chemists (IUPAC), is now available detailing the physical properties of many known IL species [52],... 

As discussed below, ionic liquids often behave comparably to conventional polar organic solvents [6, 8, 10]. But the physics underlying solvation are entirely different. As noted above, ILs are characterized by considerable structural and dynamic inhomogeneity, and even simple concepts, such as the dipole moment, cannot be productively applied. We are therefore in the unusual position of needing to explain how an exotic microscopic environment produces conventional macroscopic behavior. To this end, we will review empirical characterizations of the ionic liquid environment, and then turn our attention to the underlying physics of solute-solvent interactions. 

By the time COlL-2 took place in 2007, the nanostructured nature of the ionic liquids had been postulated using molecular simulation [50] and evidenced by indirect experimental data [54, 85] or by direct X-ray or neutron diffraction studies [56]. This microscopic vision of these fluids changed the way their physico-chemical properties could be explained. The concept of ionicity was supported by this microscopic vision, and indirect experimental evidence came from viscosity and conductivity measurements, as presented by Watanabe et al. [54, 86]. This molecular approach pointed towards alternative ways to probe the structure of ionic liquids, not by considering only the structure of the conponent ions but also by using external probes (e.g. neutral molecular species). Solubility experiments with selected solute molecules proved to be the most obvious experimental route different molecular solutes, according to their polarity or tendency to form associative interactions, would not only interact selectively with certain parts of the individual ions but might also be solvated in distinct local environments in the ionic liquid. 

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