Carbon dioxide solubility (vapor–liquid equilibria: VLE) in an ionic liquid, 1-ethyl-3-ethylimidazolium acetate ([eeim][Ac]) was measured using a gravimetric microbalance at four isotherms (about 283, 298, 323, and 348 K) up to about 2 MPa. An equation-of-state (EOS) model was used to analyze the VLE data and has predicted vapor–liquid–liquid equilibria (VLLE: or liquid–liquid separations) in CO2-rich solutions. The VLLE prediction was confirmed experimentally using a volumetric method and likely the liquid–liquid equilibria will intersect with the solid–liquid equilibria such that no lower critical solution temperature can exist and the binary system may be classified as Type III phase behavior. Carbon dioxide solubility in the ionic-liquid-rich solution show extremely unusual behavior. CO2 dissolves in the ionic liquid at large concentrations (up to about 20 mole % of CO2) with almost no vapor pressure above the mixtures. This result is similar to our previous findings with 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) and 1-ethyl-3-methylimidazolium acetate ([emim][Ac]). In all three cases the CO2 forms a molecular complex (or chemical reaction) with the ionic liquid. 13C NMR spectroscopy has identified the structure for CO2 absorbed in [eeim][Ac] to be [eeim]-2-carboxylate. Addition of water to the carboxylate leads to the dissolution of CO2. The thermodynamic excess properties (enthalpy, entropy, and Gibbs energy) for all three systems have been calculated using the EOS and support the complex formation of the type AB2 (where A is CO2 and B is ionic liquid). Isothermal differential scanning calorimetry has verified the heat of reaction calculations and found for CO2 absorbing in [emim][Ac], [eeim][Ac] and [bmim][Ac] to be about −38 kJ mol−1. Additional experiments have examined the effect of water on the density, viscosity and CO2 solubility in [eeim][Ac] and the CO2 solubility in mixtures of [eeim][Ac] with other acetate salts.
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