Effect of the Structures of Ionic Liquids on Their Physical Chemical Properties

ILs are referred to as "designer solvents" [1], and one of their most important advantages is that their properties can be tuned/controlled by tailoring their structures. To do this, however, it is crucial to assume that ILs are solvents of which the local structural (that is, electronic and steric) features may be correlated with their properties and then deal with the effect of their cation and anion structures in altering the related properties. This is exactly the subject of this chapter. The structural factors of the cations are focused on the status of alkylation of H atoms on the ring and tail groups (the polar/nonpolar character, the chain length and its flexibility, the cyclic and branched structures, and the functional tail group). The anion characters include the symmetry, the size, the charge delocalization either by large volume of the central atom or by the presence of the perfluoroalkyl chain, the chain length and its flexibility, and the functional group. The general patterns through which the examined properties vary on changing the cation and anion structures are explored and the reasons behind the trends are briefly discussed on the basis of the structural effect on the interactions between the counterpart ions.