(321am) Vapor-Liquid-Liquid Equilibria and Lower Critical Solution Temperatures of Hydrofluorocarbons in Room-Temperature Ionic Liquids

We have measured gas solubility (vapor-liquid equilibria) for methane (CH4-nFn where n = 1-3) and ethane (C2H6-nFn where n = 1 ? 5) series of hydrofluorocarbons (HFCs) in room-temperature ionic liquids (RTILs) using a gravimetric microbalance at temperatures from 283 to 348 K and pressures from 0.01 to 2 MPa [1-3]. Vapor-liquid equilibrium (VLE) data have been successfully correlated with a modified Redlich-Kwong equation of state (EOS), which was developed earlier for non-electrolyte solutions such as refrigerant + lubricant oil mixtures [4]. The present EOS predicts partial immiscibilities with lower critical solution temperatures (LCSTs) in the HFC-rich side solutions. This behavior is quite interesting and in contrast with ionic liquid solutions of various alcohols [5] where immiscibility gaps have been well studied experimentally and show upper critical solution temperatures (UCSTs). In order to verify our EOS predictions, we have conducted experiments to show vapor-liquid-liquid equilibria (VLLE) for some selected HFCs (R-23, R-41, R-125, R-134a, R-143a, R-152a, and R-161) + 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], mixtures. VLLE data have been obtained using a ?volume-mass-measurement? method at an isothermal condition [6]. According to the Gibbs phase rule, VLLE of a binary system is a univariant state. This means that at a given intensive variable, say temperature, there is no freedom for other intensive variables; all other variables such as compositions, pressure, densities of the system are uniquely determined regardless of any different extensive variables (volume of each phase and total mass of the system). LCSTs of HFC ionic liquid solutions have been confirmed by cloud-point measurements. For solutions of R-23 [7] and R-125 in [bmim][PF6], we show for the first time that these binary systems belong to Type V mixture behavior according to the classification of van Konynenburg and Scott [8,9] which supports our earlier predictions [10]. In addition to the phase behavior, we will discuss unusually large negative excess molar volumes discovered in the present VLLE experiments, and show that such large excess molar volumes are also explained by the present EOS model.

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