Solvation properties, ionic liquids solutes

An example is the question of the pH-value of acidic solutes in ionic liquids. From a practical point of view, this issue is extremely difficult to resolve, as pH-electrodes cannot be used in ionic liquids. Furthermore, the pH-value is defined as the negative logarithm of the H30+ activity, but in a dry ionic liquid, water is not present to form this species with dissociated protons. Does this then mean that protons are naked , tending to exhibit super-acidic character The answer to this question lies of course in the solvation properties of the ionic liquid under investigation. 

HS-GC has been developed to serve as a sensitive tool to determine even small differences in the solvation properties of ionic liquids using a choice of model solutes featuring specific interactions molecular ion-dipol interactions, hydrogen bond donor and acceptor interactions, and n- and n-electron dispersion forces can be probed by model solutes such as acetonitrile, 1,4-dioxane, n-propanol, n-heptane and toluene, respectively. Bearing in mind that no solute exhibits exclusively one specific interaction, the systematic investigation of the effect of the variation of the structural elements of ionic liquids, i.e. choice of cation, cation substitution and anion, lead to the following conclusions. 

In a series of papers published throughout the 1980s Colin Poole and his coworkers investigated the solvation properties of a wide range of alkylammonium and, to a lesser extent, phosphonium salts. These ionic liquids were used as stationary phases for gas chromatography and the retention of a variety of probe compounds was analyzed using Abraham s solvation parameter model [6]. Since a wide variety of probe solutes were used any problems associated with the use of a single probe, which will inevitably have its own specific chemistry, were removed. 

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. 

Supercritical fluid (SCF) solvents are unique in that their densities can be varied continuously from gas-like to liquid-like values simply by varying the thermodynamic conditions. Because many of a fluid s solvating properties are strongly dependent on the fluid density, such large changes in density can have dramatic effects on solute reactivity [1,2]. For example, at low pressures supercritical water supports homolytic, free radical reactions, whereas at higher pressures, heterolytic, ionic reactions dominate [3,4]. Thus, thermodynamic control of SCF solvent densities promises to enable us to control reaction outcome and selectively produce desired products. 

By the time COlL-2 took place in 2007, the nanostructured nature of the ionic liquids had been postulated using molecular simulation [50] and evidenced by indirect experimental data [54, 85] or by direct X-ray or neutron diffraction studies [56]. This microscopic vision of these fluids changed the way their physico-chemical properties could be explained. The concept of ionicity was supported by this microscopic vision, and indirect experimental evidence came from viscosity and conductivity measurements, as presented by Watanabe et al. [54, 86]. This molecular approach pointed towards alternative ways to probe the structure of ionic liquids, not by considering only the structure of the conponent ions but also by using external probes (e.g. neutral molecular species). Solubility experiments with selected solute molecules proved to be the most obvious experimental route different molecular solutes, according to their polarity or tendency to form associative interactions, would not only interact selectively with certain parts of the individual ions but might also be solvated in distinct local environments in the ionic liquid. 

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