Phase Behaviour of Ionic Liquid Systems

Applications include the use of ionic liquids as electrolytes in electrochemical devices, as solvents in chemical synthesis and catalysis, separation technology, as lubricants and heat-transfer fluids. For the design of separation processes 

Edited by A. R. H. Goodwin, J. V. Sengers and C. J. Peters International Union of Pure and Applied Chemistry 2010 Published by the Royal Society of Chemistry, www.rsc.org 

Halides Ch, Br, l Sulfates CH30S03 Sulfonates CF3S03 Acetates CF3C02  

2 Phase Behaviour of Binary Ionic Liquid Systems 

The phase behaviour of binary ionic liquid systems can roughly be divided into three classes (i) ionic liquids with dissolved gases, (ii) ionic liquids with water and (hi) ionic liquids with organic solvents. The first class involves vapour-liquid equilibrium (VLE) data, whereas the last two classes mainly involve liquid-liquid equilibrium (LLE) data. 

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Phase Behaviour of Ionic Liquid Systems with Azeotropic Organic Mixtures... 

Because measurement of the phase behaviour of ionic liquid systems is time consuming, it is desirable to develop predictive methods to estimate this. Different approaches were proposed for modeling the phase behaviour of ionic liquid systems (i) molecular simulations, (ii) excess Gibbs-energy methods, (iii) equation of state modeling and (iv) quantum chemical methods. 

Different types of equations of state have been used to model the phase behaviour of ionic liquid systems. Cubic equations of state such as the Peng-Robinson equation and the Redlich-Kwong equation have been used to describe the solubility of carbon dioxide, trifluoromethane and organics in ionic liquids. Because cubic equations of state require the critical parameters of ionic liquids, which are unknown, these have to be estimated by using group-contribution methods. Thus estimates obtained from cubic equations of state for ionic liquid systems are unreliable. Moreover, cubic equations of state can only describe the carbon dioxide solubility in ionic liquids at low concentrations, but cannot predict the dramatic increase in bubble point pressure at higher carbon dioxide concentrations. ... 

The phase behaviours of binary and ternary ionic liquid mixtures with carbon dioxide,organics " and water " have been determined using COSMO-RS. In the COSMO-RS framework, ionic liquids are considered to be completely dissociated into cations and anions. Ionic liquids are thus taken as an equimolar mixture of two distinct ions, which contribute as two different compounds. Because ionic liquids only dissociate in the presence of strongly polar substances, the COSMO-RS prediction of the phase behaviour of ionic liquid systems with polar compounds (water and alcohols) is more accurate than that of ionic liquid systems with nonpolar compounds (carbon dioxide and organics). Especially the COSMO-RS prediciton of the solubility of (relatively nonpolar) carbon dioxide in ionic liquids shows considerable deviations ( 15 %) from experimental values. lUPAC Technical Reports document the measurements of the thermodynamic and thermophysical properties of l-hexyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl]amide and the recommended values. ... 

Blanchard LA, Gu Z, Brennecke JF (2001) High-pressure phase behaviour of ionic liquids/ COj systems. J Phys Chem B 105 2437-2444... 

Most (ionic liquid + organic) show immiscibility in the liquid phase with an upper critical solution temperature (UCST) found at low mole fractions of the ionic liquid. Therefore, the phase behaviour of (ionic liquid + organic) is similar to that of (ionic liquid + water) (Figure 11.4). However, unlike the system with water, the UCST of (ionic liquid + organic) decreases with increasing alkyl chain length of the cation. 58,60,84,89 92... 

Corresponding-states correlations, which were used for describing the phase behaviour of (ionic liquid- -organics) systems,also suffer from the same problem that the critical parameters of ionic liquids are required yet unknown. 

In contrast to the small effects which temperature change has on the phase behaviour of ionic surfactants [38] there is a very pronounced change in the appearance of phase diagrams of oil-water-non-ionic surfactant systems with increase in temperature. Changes induced by temperature in the relative positions and extent of isotropic and liquid crystal phases present in the ascorbic acid-water-polysorbate 80 system have been recorded by Nixon and Chawla [39] (Fig. 2.21). Temperature increase decreases the width of the liquid crystal band the most pronounced effect occurring between temperatures of 25 and 30° C where the polysorbate concentration at which liquid crystals first appear (Li + LC) is increased from about 35 to 36% to 44% polysorbate in the presence of ascorbic acid. 

Figure 11.2 (p,jc) section of the phase behaviour of binary ionic liquid + CO2 systems. 

The phase behaviour of many (ionic liquid + CO2) systems was subsequently studied, including CO2 solubility in imidazolium-based ionic liquids with tetra-fluoroborate anions, hexafluorophosphate anions, bis(trifluoro-methylsulfonyl)amide anions,and other ionic liquids. All these systems show similar phase behaviour as depicted in Figure 11.2. 

Phase Behaviour of Ternary Ionic Liquid Systems... 

The phase behaviour of four elasses of ternary ionic liquid systems has been determined (i) ionic liquid + carbon dioxide + organic, (ii) ionic... 

More reliable phase behaviour predictions for binary ionic liquid systems with carbon dioxide or organics come from group-contribution equations of state, such as the universal functional activity coefficient (UNIFAC) method, the group-contribution nonrandom lattice ffuid equation of... 

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