SCAN: Exploring the Nanostructured Nature of Ionic Liquids

Abstract
    Ionic liquids form a novel class of solvents with potential industrial applications in environmentally acceptable processes. They can act as sophisticated solvation or reaction media as they can dissolve a wide range of polar or nonpolar, organic or inorganic molecules, providing new paths to carry out chemical reactions or industrial separations.
    Several molecular simulation studies have recently shown that ionic liquids exhibit medium range ordering with the existence of persistent nanoscale domains in the liquid phase. In ionic liquids based in methylimidazolium cations with alkyl side-chains of medium length, two domains are observed: one formed by aggregates of the non-polar side chains, and the other by a network composed by the charged head groups and dominated by electrostatic interactions. The segregation of polar and nonpolar domains changes the way in which solvation can be understood in these liquids. It was demonstrated by molecular simulation that, depending on the nature of the solute (polar, nonpolar or associative), it would be solvated preferentially in one of the two domains described.
    Precise low-pressure gas solubility data can be used to assess the solute-solvent interactions and provide valuable informations about the dissolution process. When expressed in terms of Henry’s law constants, gas solubility is directly related to the standard Gibbs energy of solvation and, from its variation with temperature, the enthalpy and entropy of solvation can be derived. These thermodynamic properties of solvation provide information about the microscopic features (both structural and energetic) controlling the dissolution process.
    Several examples will be used to illustrate how the nanostructured nature of ionic liquids explains the solubility of different families of compounds and how different solutes can act as probes to the microscopic structure of these novel solvents.