Solvents solute-ionic liquids interactions

Iwata, K., Kakita, M., Hamaguchi, H., Picosecond time-resolved fluorescence study on solute-solvent interaction of 2-aminoquinoline in room-temperature ionic liquids Aromaticity of imidazolium-based ionic liquids, /. Phys. Chem. B, 111, 4914-4919,2007. 

Claire Lisa Mullan is currently a PhD student in the School of Chemistry and Chemical Engineering at Queen s University Belfast. Her project focuses on the determination of structural properhes of ionic liquids and the nature of solute-solvent interactions, collaborating with both theorehcians and experimentalists. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

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. 

We review the results of experimental measurements of polarity in ILs and discuss how solute-solvent interactions should be viewed in ionic liquids. We focus primarily on the solvation of molecular species, though we include some discussion of ionic solvation at the end of the section. In the interest of brevity, we avoid discussion of mixtures and focus on dilute solutions. 

Typically, solute-solvent interactions are divided into two broad categories Specific and nonspecific interactions. Specific interactions include phenomena such as hydrogen bonding and ji-ji interactions, which depend on the presence of particular functional groups or steric structures. They are short ranged, and are specific in the sense that they involve individual solvent species within the first solvation shell of the liquid. In contrast, nonspecific interactions represent interactions that are not associated with the presence of individual functional groups. In molecular liquids, these include dispersion and electrostatic interactions, such as dipole-dipole forces. We will discuss the nature of each type of interaction in ionic liquids in the sections that follow. 

A potentially unique form of specific solute-solvent interaction has been proposed for ionic liquids. Blanchard and Brennecke [234] note that the solubilities of aromatic species are anomalously high in an imidazolium-based IL when compared to solutes of comparable molecular weight and dipole moment. This cannot be explained purely by ji-ji interactions, because while ji-ji interaction energies can be significant [235], the solubilization of a pure aromatic liquid must disrupt at least as many ji-ji contacts as it creates. However, work by Holbrey and co-workers [171] characterizes a cocrystalline clathrate form of an imidazolium-based IL with benzene, which shows distinctive ji-ji stacking. 

The theory is based on two observations. First, solute-solvent interactions are characterized by dipole-ion interactions, and so are much weaker than the ion-ion interactions between solvent species. Thus, the presence of the solute dipole should not greatly perturb the liquid from the electrostatic structure of the neat liquid. Second, because the ionic liquid is a conductor, the electric field of the solute must be screened by the solvent. This observation has been confirmed... 

One implication of this framework is that the electrostatic component of solute-solvent interactions should correlate strongly with the charge density of the liquid. This result is confirmed by study of the variation of the Kamlet-Taft n parameter with the number density of an ionic liquid. The result is shown in Fig. 7 (taken from [239]). This figure shows a clear relationship between it and the number density of the IL, where no such relationship exists in molecular liquids. It... 

This text discusses four aspects of ionic electrochemistry ion-solvent interactions, ion-ion interactions, ion transport in solution, and ionic liquids. 

Both examples highlight the importance of understanding ionic liquid - solute interactions to exploit fully the ionic liquids potential as solvents. 

The extent of mixing and the distribution of solutes in ionic liquids depend, therefore, on the relative solute-solute and solute-solvent interactions, which can have significant consequences on chemical reactivity and stabihty. In many ionic liquids, water-sensitive catalysts and chemical reactions are less sensitive to water compared with the situation in organic solvents because water dispersed throughout the ionic liquid cannot act like bulk water. 

We have attempted to show how electron spin resonance spectra may be used to reveal the presence and characteristics of solute-solvent interactions between aromatic radicals and non-aqueous solvents. Large areas of the application of electron spin resonance spectroscopy to problems involving solvation have been omitted, partly for reasons of space, partly because of recent reviews, and partly because their interpretation is largely centred on the solute. As a partial remedy we cite recent reviews of the study of solutes in liquid crystals and of the many experiments made on ionic associates. " ... 

SECTION 13l3 The solubility of one substance in another depends on the tendency of systems to become more random, by becoming more dispersed in space, and on the relative intermolecular solute solute and solvent solvent enei es compared with solute solvent interactions. Polar and ionic solutes tend to dissolve in polar solvents, and nonpolar solutes tend to dissolve in nonpolar solvents ( like dissolves like ). Liquids that mix in all proportions are miscible those... 

Two distinct groups of experimental techniques were presented in COlL-2 to study the behaviour of different solutes in ionic liquids, spectroscopic and thermodynamic, giving access to different scales of the properties studied - one microscopic and the other macroscopic. It was possible to explain microscopically the phase behaviour of ionic liquid solutions by balancing the effects of the solute-solvent interactions and the dynamics of the solutions. Bases were established to assess the microscopic mechanisms responsible by the properties observed and so to open the way to the rapid advancement of the field contributing to the development of novel applications in a growing variety of disciplines including catalysis, synthesis, nanomaterial synthesis or pharmaceutics. 

The solvation of neutral species in ionic liquids is distinct from those of charged ions because the solute-solvent interactions can be complex and the presence of neutral solutes can affect the structure of the solution. In COIL-2, several groups addressed solvation and its dynamics in ionic liquid media. Rebelo and collaborators [18] associated molecular simulation studies and... 

---