| Recently, much more attention has been focused on green chemistry along with people's awareness of the importance of environment. Ionic liquid can be used in electrochemical reactions as environmentally benign solvent. However, the high viscosity and surface tension restrict its applications in electrochemistry. Because CO2 is readily available, inexpensive, nontoxic, nonflammable, has low viscosity and has a mild critical temperature (304.2 K) and critical pressure (7.38MPa), supercritical CO2 (scCO2) has been recognized as an environmentally benign solvent and has attracted much attention in many chemical processes, such as extraction and fractionation, chemical reactions, and so on. Ionic liquids were introduced to supercritical carbon dioxide to endow the latter with conductivity in order to decrease the high viscosity and high surface tension of traditional electrochemical medium. The related phase behaviors have also been studied in this work. The contents of our work are as following:The knowledge of high pressure phase behavior about carbon dioxide mixtures, especially the vapor-liquid equilibria (VLE) are essential in the design, development, and operation of supercritical fluid separation processes. This work has studied the phase behaviors of CO2+ 1-bromobutane, CO2+1-chlorobutane, CO2+1-methylimidazole, which are related to synthesis of imidazolium based ILs. The vapor-liquid equilibrium datas of the three mixtures were measured with concentrations of solute mole fractions (0.0102 to 0.1495), temperatres from (308.2 to 337.4) K and pressures from (6.21 to 19.04) MPa. The VLE data of CO2+ 1-methylimidazole were also determined using analytic method under (293.15,309.75,323.2) K and (2.83 to 14.16) MPa. The results indicate that the phase behaviors of three binary systems have similar tendency in the temperature range we studied. And the density of mixtures increases with increasing pressure and decreasing temperature, and with increasing solute mole fraction. It is also demonstrated that the density is sensitive to the pressure as the pressure approaches the phase transition points of binary mixtures; that is, KT is large and increases significantly. When the pressure is much higher than the phase transition pressure or the composition is far from the critical composition, KT is rather small, and the effect of pressure on KT is fairly limited. The phase boundary data of the binary mixtures can be correlated well by the Peng-Robinson equation of state (PR EoS) with two binary parameters. Th results also indicate that the liquid-liquid-vapor equilibria (LLV) of the CO2+1-methylimidazole binary system exists under temperatures from 297.85K to 313.95K and pressures from 6.20MPa to 8.67MPa. The experimental data were also correlated well by the Peng-Robinson equation of state with two van der Waals mixing rules. According to the experimental results, the phase behavior of the binary system might be classified to Type-IV or Type-V according to the classification of six principal types of binary phase diagrams. The solubility of carbon dioxide decreases along with increasing temperatre and increases as pressure increasing.The solubilities of ionic liquid ([Bmim]BF4) in supercritical CO2 with cosolvent (acetonitrile) have been investigated using static method at temperatures from 313.2K to 332.2K. The mole fraction of acetonitrile was from 0.1495 to 0.1990. And it is demonstrated that the solubility of ionic liquid in supercritical carbon dioxide increases with increasing pressure and mole fraction of acetonitrile, while it decreases with increasing temperature.The conductivity of ionic liquid in supercritical carbon dioxide has been studied In high-pressure variable-volume view cell, the conductivity of supercritical carbon dioxide with and without ionic liquids were measured. And the mole fraction of cosolvent (acetonitrile) was from 0.1390 to 0.1971, temperatures from 313.2K to 332.2K. At presence of identical concentration of acetonitrle, the conductivity of supercritical carbon dioxide without ionic liquid is very small, while it increases greatly with ionic liquid. And the coductivity decreases with increasing temperature, while it increases with increasing pressure and mole fraction of acetonitrile.
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