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The excess Cibbs energy of liquid mixtures may be predicted using group contribution models (f.ex. UNIFAC and ASOG), or it may be correlated using molecular models such as UNIQUAC. Group contribution models and the UNIQUAC model contain a combinatorial contribution which accounts for size‐ and shape‐effects and a residual contribution which accounts for interactions among groups or molecules. A new expression for the combinatorial contribution is given which yields much improved predictions of the excess Gibbs energy of systems where the residual contribution may be expected to be zero (e.g., mixtures of aliphatic hydrocarbons). It is shown that it is advantageous to incorporate the new combinational expression into future modifications of the UNIFAC model.

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The phase behavior of a binary mixture consisting of a supercritical fluid and an imidazolium-based ionic liquid was studied experimentally. Carbon dioxide (CO 2 ) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF 4 ]) were the selected supercritical fluid and ionic liquid (IL), respectively. A synthetic method was used to measure the phase behavior of the binary system CO 2 + [hmim][BF 4 ]. Bubble-point pressures of this binary mixture were reported for CO 2 concentrations ranging from (10.3 up to 70.3) mol % and within a temperature range of (293.18 to 368.16) K. The experimental results obtained in this work were compared with the available phase behavior data of the binary system CO 2 + 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF 6 ]) to investigate the effect of the anion group on the phase behavior of this type of system. The three-phase liquid-liquid-vapor (LLV) equilibrium of this system was also measured. On the basis of the LLV measurements, the type of fluid phase behavior of this system most likely will be type III according to the classification of Scott and Van Konynenburg. The CO 2 + [hmim][BF 4 ] binary system has a two-phase liquid-vapor region extending up to very high pressures.