Ionic liquid-organic solute interactions

The current pool of y 3 data for ionic liquid-organic solute interactions exceeds those obtained by alternative techniques such as headspace chromatography, dilutor and the static technique. As mentioned previously, experimental investigations on ionic liquids from an industrial perspective have largely been guided by separation problems of interest to the chemical and petrochemical industries such as alkane-aromatic, cyclo-alkane-aromatic and alkane-alkene mixtures. In this regard, selectivities at infinite dilution (5j ) are presented in Table 3 for (i) n-hexane-benzene, (ii) cyclohexane-benzene and (iii) n-hexane-l-hexene separations in various ionic liquid and commercially significant solvents. 

It can be observed from Table 3 that the yj values decrease in the following hierarchy -hexane > cyclohexane > 1-hexene > benzene. This is of course in accordance with the relative strengths of the ionic liquid-organic solute interactions. Saturated non-polar aliphatic molecules in the form of... 

When it finally comes to continuous processing of transition metal catalysis in ionic liquid-organic biphasic reaction mode, some additional aspects have to be taken into account. First is the ease of phase separation that will determine the size of the separator unit and thus indirectly the ionic hquid hold-up required. Another very important aspect is the build-up of side-products or feedstock impurities in the ionic catalyst phase. Side-products and impurities that are likely to build up in the ionic liquid are relatively polar in nature and this brings along a significant risk of unfavorable interactions with the transition metal catalyst complex. Apart from this, all build-up of undesired components in the ionic hquid vnU also affect the ionic liquid s physicochemical properties. Therefore, a continuous build-up of components in the ionic catalyst phase that is not restricted by thermodynamic limits (e.g. solubility limits) will always require an extensive purge of the ionic catalyst solution. 

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

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. 

These detailed studies of the interactions and structure of mixtures of ionic liquids with aromatic organic compounds have not been yet extended to other families of molecular solutes. In the case of mixtures of ionic liquids with water or acetonitrile, although different experimental data were published with the aim of studying the limit of the low concentration of ionic liquid [48, 49], or the effect on the solubility of a third molecular species [50], no complete picture of the structure of the ionic liquid as a function of concentration has been established (Fig. 12). 

In liquids of low dielectric constant, dispersants tend not to form ionic species in solution, but can form ions in adsorbed films on particle surfaces where acid-base interactions and proton transfer occurs between the particle surface and the dispersant. Particle potentials develop when adsorbed dispersant ions desorb into the organic medium where they become the counter-ions. Zeta-potentials well over a hundred millivolts result from the stronger acid-base interactions. Debye lengths in concentrated dispersions are typically 5 to 20 nm, and the DLVO energy barriers, of ten exceed 25 kT with stability ratios of 10° or more. 

The master retention equation of the solvation parameter model relating the above processes to experimentally quantifiable contributions from all possible intermolecular interactions was presented in section 1.4.3. The system constants in the model (see Eq. 1.7 or 1.7a) convey all information of the ability of the stationary phase to participate in solute-solvent intermolecular interactions. The r constant refers to the ability of the stationary phase to interact with solute n- or jr-electron pairs. The s constant establishes the ability of the stationary phase to take part in dipole-type interactions. The a constant is a measure of stationary phase hydrogen-bond basicity and the b constant stationary phase hydrogen-bond acidity. The / constant incorporates contributions from stationary phase cavity formation and solute-solvent dispersion interactions. The system constants for some common packed column stationary phases are summarized in Table 2.6 [68,81,103,104,113]. Further values for non-ionic stationary phases [114,115], liquid organic salts [68,116], cyclodextrins [117], and lanthanide chelates dissolved in a poly(dimethylsiloxane) [118] are summarized elsewhere. 

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. 

An alternative description of a molecular solvent in contact with a solute of arbitrary shape is provided by the 3D generalization of the RfSM theory (3D-RISM) which yields the 3D correlation functions of interaction sites of solvent molecules near the solute. It was first proposed in a general form by Chandler, McCoy, and Singer [22] and recently developed by several authors for various systems by Cortis, Rossky, and Friesner [23] for a one-component dipolar molecular liquid, by Beglov and Roux [24, 25] for water and a number of organic molecules in water, and by Hirata and co-workers for water [26, 27], metal-water [26, 28] and metal oxide-water [31] interfaces, orientationally dependent potentials of mean force between molecular ions in a polar molecular solvent [29], ion pairs in aqueous electrolyte [30], and hydration of hydrophobic and hydrophilic solutes alkanes [32], polar molecule of carbon monoxide [33], simple ions [34], protein [35], amino acids and polypeptides [36, 37]. It should be noted that accurate calculation of the solvation thermodynamics for ionic and polar solutes in a polar molecular liquid requires special corrections to the 3D-RISM equations to eliminate the electrostatic artifacts of the supercell treatment employed in the 3D-RISM approach [30, 34]. 

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