J. Phys. Chem. B 2009, 113, 6579–6580
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COMMENTS Comment on “Characterization of the Ability of CO2 To Act as an Antisolvent for Ionic Liquid/ Organic Mixtures” Eliane Ku¨hne,† Lidia R. Alfonsı´n,† Maria Theresia Mota Martinez,† Geert-Jan Witkamp,† and Cor J. Peters*,†,‡ Delft UniVersity of Technology, Faculty of Mechanical, Maritime and Materials Engineering, Department of Process and Energy, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA, Delft, The Netherlands, and The Petroleum Institute, Chemical Engineering Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates ReceiVed: September 5, 2008; ReVised Manuscript ReceiVed: December 20, 2008 Introduction A few years ago, the capability of carbon dioxide to induce miscibility changes in ionic liquid/organic mixtures has been described by Scurto et al.1 In that communication, the split in phases was investigated at constant temperature and variable pressure for different initial compositions of ionic liquid in the mixtures. The point at which the homogeneous liquid phase splits into two liquid phases has been identified as the LCEP (lower critical end point, L1 ) L2 + V), and in addition, the pressure at which the upper liquid phase and the vapor phase become completely miscible has been identified as the K-point (L1 + L2 ) V). This nomenclature also has been used in related later publications by this group.2,3 However, an accurate analysis of their data shows that the phase transitions have been erroneously identified as critical end points. In our view, these phase transitions simply are of the type L + V f L1 + L2 + V and L1 + L2 + V f L1 + L2; i.e., they are normal phase transitions and do not meet the specific characteristics of criticality. This observation is supported by applying the Gibbs phase rule (F ) C - P + 2 - R, where F is the number of degrees of freedom, C is the number of components, P is the number of phases present at equilibrium, and R represent the extra conditions imposed on the system in the case of azeotropy or criticality). In the case of criticality, R ) 2 and, consequently, there are two additional conditions the system must satisfy. According to Heidemann4 and Heidemann and Khalil,5 the following two conditions for criticality apply for multicomponent-multiphase equilibria:
Q ) det(Q) ) 0
(1)
with the elements qij of the matrix Q defined as * To whom correspondence should be addressed. E-mail: c.j.peters@ tudelft.nl,
[email protected]. † Delft University of Technology. ‡ The Petroleum Institute.
qij )
( ) ∂2A ∂nj∂ni
and
C)
∑∑∑ i
j
k
(
)
∂3A ∆ni∆nj∆nk ) 0 ∂nk∂nj∂ni
(2)
with the normal vector ∆n fulfilling the condition
Q · ∆n ) 0 In these equations A is the molar Helmholtz energy and ni the number of moles of component i. If we apply the phase rule to the ternary systems in question (ionic liquid, organic, and CO2) with the coexisting phases L1 ) L2 + V or L1 + L2 ) V (as claimed to be present by Scurto et al.), the number of degrees of freedom is F ) 3 - 2 + 2 2 ) 1; i.e., we are dealing with a uniVariant system. In other words, only one intensive variable is left to be chosen arbitrarily. In the measurements presented by Scurto, Aki, and Mellein,1-3 the temperature was kept constant, in most cases at a value of 313 K, and various phase splits have been identified by changing two variables: the initial composition of ionic liquid and the pressure. As explained above, the phase rule dictates that for a ternary system at fixed temperature comprising one phase and a critical phase (e.g., L1 ) L2 + V and L1 + L2 ) V) F ) 0, there is only one set of conditions possible of all other intensive variables of the whole system. From the data presented by these groups, it becomes apparent that these intensive variables are having different values, which means that, if one of the phases is a critical phase, the data violate the phase rule. Consequently, the data presented by these groups correspond to normal phase transitions of the type L + V f L1 + L2 + V and L1 + L2 + V f L1 + L2 and, therefore, are erroneously indicated by the LCEP and K-point, respectively. For these phase transitions, the number of degrees of freedom of the system is F ) 2 (in case of an L1 + L2 + V equilibrium) and F ) 3 (for an L1 + L2 equilibrium). These values show that the system is able to present, at constant temperature, different phase transitions for different sets of pressure and composition, as it has been measured by these groups. To give a better insight into the phase behavior of the system ionic liquid + organic + CO2, we repeated one of the measurements performed by Aki and co-workers.2 The phase diagram of the ternary system bmimTf2N + acetonitrile + CO2 has been measured in our Cailletet facilities, applying a wellknown and established procedure.6 The results of these measurements are presented in Figure 1. In the measurements of Aki et al.,2 the initial concentration of bmimTf2N in acetonitrile was 9.63 mol %, with the phase split occurring at 7.37 MPa and 313.15 K. As can be seen in our measurements (9.821 mol % bmimTf2N in acetonitrile), the critical end point of the mixture can only be found at 336.73 K and 11.03 MPa. At 313.15 K, only a phase transition of the
10.1021/jp807912x CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
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Figure 1. P versus T diagram for the ternary system bmimTf2N + acetonitrile + CO2, with 9.821 mol % bmimTf2N in acetonitrile CO2free and 70.12 mol % CO2 (relative to the ternary mixture).
type L + V f L could be found. This may be due to the difference in water content between the ionic liquids: 450 ppm in the sample of Aki et al. and lower than 30 ppm in our sample. It should be noted that in particular a minor contamination with water of the sample very often may have a major influence on the location and nature of phase boundaries. Another influence may be the CO2 composition: our sample contained 70.12 mol % CO2 relative to the ternary mixture, while the sample of Aki et al. had 76.5 mol % CO2 just before the phase split. As already known, the CO2 composition may affect the location of the different phase transitions found in systems with ionic liquids, especially the L1 + L2 phase boundary.7 Therefore, another sample was measured with a higher CO2 composition (85.23 mol %), and the phase diagram was once more investigated. In this case, the phase split did occur at 313.17 K and 7.4 MPa,
Comments but a L + V f L1 + L2 + V phase transition occurred. The critical end point of the type L1 ) L2 - V or L1 - L2 ) V was not found for temperatures below 330 K. In summary, we would like to emphasize that, although the results presented by Scurto et al., Aki et al., and Mellein et al.1-3 might have value for the development of processes using ionic liquids, organics, and CO2, the interpretation of their experimental results is erroneous. The classification of their experimental data as being critical end points of the nature LCEP and K-point violates the Gibbs phase rule, and therefore, they should be presented as simple phase transitions rather than critical end points. Regrettably, no information is provided by Aki et al.2 as to what extent the high water content in their samples may have affected the location of the experimentally determined data or even may have led to observations that theoretically could not occur. The erroneous assignment of their experimental data easily may lead to misinterpretation of the related phase diagrams which, in turn, may have consequences in process design. References and Notes (1) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. J. Am. Chem. Soc. 2002, 124, 10276–10277. (2) Aki, S. N. V. K.; Scurto, A. M.; Brennecke, J. F. Ind. Eng. Chem. Res. 2006, 45, 5574–5585. (3) Mellein, B. R.; Brennecke, J. F. J. Phys. Chem. B 2007, 111, 4837– 4843. (4) Heidemann, R. A. The classical theory of critical points, in: Supercritical Fluids; Kluwer Academic Press: 1994. (5) Heidemann, R. A.; Khalil, A. M. AIChE J. 1980, 26, 769–779. (6) Peters, C. J.; de Roo, J. L.; de Swaan Arons, J. Fluid Phase Equilib. 1993, 85, 301–312. (7) Ku¨hne, E.; Witkamp, G. J.; Peters, C. J. Green Chem. 2008, 10, 897–1012.
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