Simulations of Solutions and Mixtures

Another example of the same effect was found by Kim and coworkers [139] when they studied the effects of turning on a dipole in a probe diatomic solute dissolved in a range of imidazolium ionic liquids. In the absence of the dipole there was 

Ions comprise another class of solutes which have been studied extensively by Wipff and coworkers, who have been particularly interested in lanthanide and uranyl ions and their chloro-complexes [44,142]. Their studies show that the chloro-complexes are stabHized by solvation in the ionic liquids based on imidazolium cations and the [PFe] anion. The principal interaction of the naked ions is with the anions, but on chlorination the cations move closer to complexes such as [EuQe] and [U02Cl4] . In a further study [45] they showed that in an equimolar mixture of water and ionic liquid, water molecules tend to fill the first solvation shell of naked ions in preference to [PFs] ions, and tend to solvate the chloro-complexes in preference to the imidazolium cations. 

One important solute is water, which is completely miscible in imidazolium salts with short alkyl side chains and hydrophilic anions such as d, but is only partially miscible with ionic liquids with longer side chains and less hydrophilic anions. Extraction into an aqueous phase is important for product recovery from an ionic liquid medium. Hanke and Lynden-BeU [143] used simulation to investigate thermodynamic properties and local structure in mixtures of water with [MMIM]Q and [MMIM][PF6] liquids. They found that the excess energy of solvation was negative for the chloride and positive for the [PF ] liquid, as shown in Fig. 4.2-12. There is a similar difference in the molar volumes of mixing shown in Fig. 4.2-13. This is consistent with the perception of the [PFe] anion as being more hydrophobic than the chloride anion. 

The simulations showed that water molecules tend to be more strongly associated with anions than with other water molecules. At low concentrations each water molecule is surrounded by ions and isolated from other water molecules. It is only at quite high mole fractions (75%) that significant clusters of water are found. 

5 trifluorobenzene with [MMIM][PFe] showed negative, near-zero and strongly positive values of the energy of mixing, respectively [89]. This was correlated with the different degrees of interaction of the three substances with the ionic liquid. In particular the 1,3,5-trifluorobenzene has a very small electrostatic field around it and interacts weaHy with the ions. 

---