The Properties of Ions Constituting Ionic Liquids

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

The properties of ionic liquids, because they are dissociated to a major extent into ions, depend on those of the constituting cations and anions. Therefore, Chap. 2 of the book is devoted in part to the properties of isolated ions that do not interact with their environment. These properties comprise their mass and charge, their standard thermodynamics of formation, and their entropy and heat capacity under standard conditions. The sizes of ions, once they are in a condensed phase, depends on their being constrained by their neighboring particles due to the repulsion of their outer electron shells. These properties of the individual constituting ions of ionic liquids are assumed to be portable when they form the ionic liquid, some of them being subject to changes induced by the mutual interactions of the ions. 

Ionic liquids are however more just than a bulk medium and the dielectric constant may be not the best parameter to define ILs polarity. They are constituted by positive and negative ions which can exert various effects. Recently, the microscopic properties of ILs, i.e. the ability of these media to interact with specific dissolved species (reagents, transition states, intermediates and products), have been measured and several polarity scales, previously developed for common molecular solvents, have been extended to ILs. At variance with molecular solvents, ILs are characterized by complex interaction forces between anion and cation and these interactions are competitive with the ability of both anion and cation to interact with dissolved species thus, multiparameters solvatochromic correlations, better than single point measurements, resulted useful to understand the solvent polarity. ... 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

Liquid membranes are ideally suited for this application since they have the potential for removing toxic suhiLances from wastewater down to vety low levels. Both molecular and ionic species (anions, cations, and complex ions) have bean extracted successfully by property constituted liquid membranes. A perlial list of processes reported prior to 1978 is given in Table 19.4-1. These heve all been reviewed.2 3 Recent improvements in thase separations as well as new methods reportnd recently are summerlzed in this section. 

---