Ionic liquids (ILs), considered to be a relatively recent magical chemical due their unique properties, have a large variety of applications in all areas of the chemical industries. The areas of application include electrolyte in batteries, lubricants, plasticizers, solvents and catalysis in synthesis, matrices for mass spectroscopy, solvents to manufacture nano-materials, extraction, gas absorption agents, etc. Non-volatility and non-flammability are their common characteristics giving them an advantageous edge in various applications. This common advantage, when considered with the possibility of tuning the chemical and physical properties of ILs by changing anion-cation combination is a great opportunity to obtain task-specific ILs for a multitude of specific applications. There are numerous studies in the related literature concerning the unique properties, preparation methods, and different applications of ILs in the literature. In this review, a general description of ILs and historical background are given; basic properties of ILs such as solvent properties, polarity, toxicology, air and moisture stability are discussed; structure of ILs, cation, anion types and synthesis methods in the related literature are briefly summarized. However, the main focus of this paper is how ILs may be used in the chemicals processing industries. Thus, the main application areas are searched and the basic applications such as solvent replacement, purification of gases, homogenous and heterogeneous catalysis, biological reactions media and removal of metal ions are discussed in detail. Not only the advantages of ILs but also the essential challenges and potentials for using ILs in the chemical industries are also addressed. ILs have become the partner of scCO2 in many applications and most of the reported studies in the literature focus on the interaction of these two green solvents, i.e. ILs and scCO2. The chemistry of the ILs has been reviewed in numerous papers earlier. Therefore, the major purpose of this review paper is to provide an overview for the specific chemical and physical properties of ILs and to investigate IL-scCO2 systems in some detail. Recovery of solutes from ILs with CO2, separation of ILs from organic solvents by CO2, high-pressure phase behavior of IL-scCO2 systems, solubility of ILs in CO2 phase, and the interaction of the IL-scCO2 system at molecular level are also included.

Infinite dilution partition coefficients of benzene, toluene, chlorobenzene and naphthalene in the ionic liquid, 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6])-CO2 system were measured with a chromatographic technique. Partition coefficients for each solute increased with increasing pressure and had the trend of at a given temperature. At pressures above 12 MPa, became one order in magnitude smaller than that of the other components, which is attributed to the [pi]-[pi] interactions between naphthalene and the imidazolium group of the ionic liquid. The Sanchez-Lacombe equation of state was found to provide adequate correlation of the data and their temperature and pressure trends.

Carbon dioxide solubility (vapor + liquid) equilibria: VLE in ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim][Ac]), has been measured with a gravimetric microbalance at four isotherms about (283, 298, 323, and 348) K up to about 2 MPa. (Vapor + liquid + liquid) equilibria (VLLE: or liquid-liquid separations) have also been investigated with a volumetric method used in our previous works, since the present analysis of the VLE data using our equation-of-state model has predicted the VLLE at CO2-rich side solutions. The prediction for the VLLE has been confirmed experimentally. CO2 solubilities at the ionic liquid-rich side show extremely unusual behaviors; CO2 dissolves in the ionic liquid to a great degree, but there is hardly any vapor pressure above these mixtures up to about 20 mol% of CO2. It indicates that CO2 may have formed a non-volatile or very low vapor pressure molecular complex with the ionic liquid. The thermodynamic excess properties (enthalpy, entropy, and Gibbs free energy) of the present system do support such a complex formation. We have conducted several other experiments to investigate the complex formation (or chemical reactions), and conclude that a minor chemical reaction occurs but the complex formation is reversible without much degradation of the ionic liquid.

Solubilities of CO2 in eight hydroxyl ammonium ionic liquids 2-hydroxy ethylammonium formate (HEF), 2-hydroxy ethylammonium acetate (HEA), 2-hydroxy ethylammonium lactate (HEL), tri-(2-hydroxy ethyl)-ammonium acetate (THEAA), tri-(2-hydroxy ethyl)-ammonium lactate (THEAL), 2-(2-hydroxy ethoxy)-ammonium formate (HEAF), 2-(2-hydroxy ethoxy)-ammonium acetate (HEAA) and 2-(2-hydroxy ethoxy)-ammonium lactate (HEAL) at the temperatures ranging from 303 to 323 K and the pressures ranging from 0 to 11 MPa were determined. The solubility data were correlated using Krichevisky-Kasarnovsky equation, from which Henry?s constants and the partial volumes of CO2 at different temperature were obtained. Results showed that Krichevsky-Kasarnovsky equation can correlate the solubilities of CO2 in these hydroxyl ammonium ionic liquids (ILs) well. Comparison showed that the solubility of CO2 in these eight hydroxyl ammonium ionic liquids was in sequence: THEAL > HEAA > HEA > HEF > HEAL > THEAA [approximate] HEL > HEAF.

A general treatment of fugacities of liquid mixtures yields the proper thermodynamic functions for dealing with nonidealities of liquid phases in equilibrium with vapor mixtures for systems containing supercritical components. The activity coefficients of the supercritical components may be based on standard state fugacities for the hypothetical pure liquids or on Henry?s constants. A comparison of the rigorous thermodynamic equations which apply to the liquid phase for the two alternatives shows that they are equivalent.

The high-pressure phase diagram and other thermodynamic properties of the water + carbon dioxide binary mixture are examined using the SAFT-VR approach. The carbon dioxide molecule is modeled as two spherical segments tangentially bonded. The water molecule is modeled as a spherical segment with four associating sites to represent the hydrogen bonding. Dispersive interactions are modeled using the square-well intermolecular potential. The polar and quadrupolar interactions present in water and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Galindo and Bias (Fluid Phase Equilib. 2002, 194-197, 502; J. Phys. Chem. B 2002, 106, 4503) and Clark et al. (Mol. Phys. 2006, 22-24, 3561) for carbon dioxide and water, respectively. The phase diagram of the mixture exhibits a number of interesting features: type-III phase behavior according to the classification of Scott and Konynenburg, three-phase behavior at low temperatures with its corresponding upper critical end point, a gas-liquid critical line at high temperatures and pressures that continuously changes from gas-liquid to liquid-liquid as the pressure is increased and gas-gas immiscibility of second kind. Only one unlike interaction parameter is fitted to give the best possible representation of the temperature minimum of the gas-liquid critical line of the mixture. This unlike parameter is then used in a transferable manner to study the complete pressure-temperature-composition phase diagram. The phase diagram calculated with SAFT-VR is in excellent agreement with the experimental data taken from the literature in a wide range of thermodynamic conditions. The theory is also able to predict a good qualitative description of the excess molar volume and enthalpy of the mixture as well as the most important features of the Henry?s constants at different temperatures. \copyright 2007 American Chemical Society.

Abstract: We take advantage of the recent advances in statistical mechanics on mixtures to examine a century-old problem in solution thermodynamics, specifically for the popular activity coefficient model, with regard to the absence of a standard state for the non-condensable gases in the mixture. This defect is traced back to the excess Gibbs free energy formalism where insistence on a pure liquid-state reference fluid is incorporated. By examining the molecularly derived counterparts, we propose a new division of the component chemical potential along the line of the molecular theory. A new definition of a reference fugacity and that of a molecular-inspired activity coefficients are formulated to cure this defect. We employ the Ornstein-Zernike equations to actually evaluate the molecular activity coefficients for a mixture of methane and n-pentane. The system temperature is 444 K. Thus, methane is the suprecritical component and does not fit into the classical activity coefficient model. We demonstrate that the molecular activity coefficients of methane and n-pentane can be evaluated and do not suffer nonexistence. Furthermore, these values are used to determine the dew point and bubble point of the mixture. The results compare favorably with the experimental data of Sage and Lacey.

Using a variable volume high-pressure cell, bubble and dew points of the binary systems carbon dioxide + methylcyclopentane and carbon dioxide + isopropylcyclohexane were measured at temperatures ranging from 292.75 to 373.45 K. The saturation pressures, ranging from 10.6 to 169.8 bar were measured at carbon dioxide mole fractions between 0.1021 and 0.9700. For these two systems, no literature data were available. The experimental results were correlated with the PPR78 model. The average absolute deviations between experimental and predicted saturation pressures are respectively of 4.90 and 5.54 bar for the carbon dioxide + methylcyclopentane and the carbon dioxide + isopropylcyclohexane systems.