4476
Ind. Eng. Chem. Res. 2000, 39, 4476-4480
Complex Phase Equilibrium Phenomena in Fluid Mixtures up to 2 GPa-Cosolvency, Holes, Windows, Closed Loops, High-Pressure Immiscibility, Barotropy, and Related Effects Gerhard M. Schneider,* Arndt L. Scheidgen, and Dirk Klante Department of Chemistry, Physical Chemistry, NC 6/36, University of Bochum, D-44780 Bochum, Germany
First, the most important experimental techniques are briefly reviewed, including the diamond anvil cell (DAC) technique. As a first example, it is demonstrated that carefully selected type III carbon dioxide binaries exhibiting gas-gas-like fluid phase behavior can be combined to give a ternary system, which might show large cosolvency effects and, as a consequence, twophase holes in the three-phase liquid-liquid-gas surface and/or one-phase miscibility windows surrounded by heterogeneous states (e.g., carbon dioxide + 1-decanol + tetradecane). As a second example, type VI binary systems exhibiting closed immiscibility loops in isobaric temperature (composition) diagrams are considered. With increasing pressure, such closed loops can shrink or even disappear completely (e.g., tetrahydrofuran + water); have a tube-like shape, often with a narrowing at medium pressures (e.g., 3-aminopentane + water); or even appear at high pressures only (so-called “high-pressure immiscibility”, e.g., tetra-n-butylammonium bromide + water). Finally, isopycnic and barotropic effects are discussed in relation to the phenomena described above. 1. Introduction Over more than three decades, thermodynamic, phasetheoretical, and experimental investigations of fluid mixtures at high pressures have been a focus of the research activities in our laboratory. According to the temperatures and pressures involved and the systems under study, the experiments were made in several different high-pressure setups that were developed in our laboratory (for a review see ref 1). The measurements at very high pressures (here, up to 2 GPa) were performed in a diamond anvil cell (DAC) that has been described elsewhere.2-4 The results were presented in some forty doctoral dissertations, many original papers, and several review articles (see, e.g., refs 5-10) in which numerous references were also given. The present contribution is concerned with some selected phenomena of complex fluid phase behavior (see sections 3 and 4) and thus supplements the preceding papers and reviews. 2. Fluid Phase Equilibria of Binary Mixtures Fluid phase equilibria of binary mixtures at high pressures have already been extensively investigated, and their phase behavior is well understood from a phase-theoretical point of view, at least in principle. A classification scheme often used is that of Scott and van Konynenburg (Figure 1).11 It is so well-known that only some short explanatory remarks will be necessary in the following. For type I, the gas-liquid critical curve is not interrupted. No liquid-liquid immiscibility surface exists, or it is shifted to temperatures below the crystallization surface. * Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone: +49 234 32 24250. Fax: +49 234 32 14293
For type II, the gas-liquid critical curve is again not interrupted, but liquid-liquid phase separation with the exhibition of upper critical solution temperatures (UCSTs) exists above the crystallization surface. For type III systems, the liquid-liquid miscibility gaps are shifted to such high temperatures that the liquid-liquid critical curve merges into the gas-liquid critical line. Here, often pressure maxima or minima on the critical curves are no longer found, and the critical curves can even run through temperature minima, resulting in a so-called gas-gas equilibrium type (here, of the second kind). Types IV and V represent transitions between different types, e. g., II and III. Systems of type VI exhibit closed miscibility gaps (socalled closed loops) for liquid-liquid equilibria in isobaric T(x) sections (x ) mole fraction). For the example shown, the closed loop disappears with increasing pressure. In the recent literature, a type VII that is a combination of types IV and VI is also discussed. It has been shown elsewhere that continuous transitions exist between all types of fluid phase behavior shown in Figure 1.5,7,10 The details of the different types of phase behavior and the transitions between them can be complicated and involve many sophisticated effects such as azeotropy, critical azeotropy, hetero-azeotropy, double critical endpoints, tricritical phenomena, van Laar points, appearance of solid phases, etc., for which the reader is referred to the literature (see, e.g., ref 12). 3. Cosolvency, Windows, and Holes in Ternary Mixtures Ternary mixtures that consist of two low-volatile substances and one supercritical solvent can be considered as model systems for the study of supercritical fluid extraction (SFE), e.g., the extractive effect of the supercritical solvent, such as carbon dioxide, on the separation of two low-volatile substances or the effect of a
10.1021/ie0001669 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4477
Figure 1. p,T projections of fluid phase equilibria types I-VI: lg(A,B), vapor pressure curves of the pure components A,B; - - -, l1l2g three-phase llg line; O, l1 ) l2 + g critical endpoint, ll critical curve meets three-phase llg line; b, critical points of the pure components A,B; O, l1 ) g + l2 critical endpoint, lg critical curve meets three-phase llg line; s l1 ) l2, ll critical curve; s l ) g, lg critical curve.
supercritical solvent plus a moderator on the supercritical fluid extraction of one low-volatile compound. For a more detailed discussion see refs 7 and 10. Here, sometimes cosolvency can play an important role. A cosolvency effect is characterized by the fact that a mixture of two low-volatile components B and C is more soluble in a supercritical solvent A than each of the pure components B or C alone. As a consequence, even closed homogeneous regions (so-called “miscibility windows”) surrounded by heterogeneous states in an red isobaric T(wred C ) diagram and/or an isothermal p(wC ) red diagram might appear. Here, wC is the solvent-free or reduced mass fraction of component C defined as wred C ) wC/(wB + wC). For the appearance of isobaric T(wred C ) windows, the situation is schematically represented in the threedimensional p,T,wred C diagram of the critical surface of the ternary system A + B + C in Figure 2. The sections red for wred C ) 0 and wC ) 1 correspond to the critical p,T curves of the binary systems A + B and A + C, respectively, which are of type III (see Figure 1c). For medium wred C values, however, the three-dimensional critical surface exhibits a distinct deepening. If isobaric sections are made between the ternary pressure minimum and the lower of the two binary pressure minima, isobaric T(wred C ) miscibility windows result. As an example, results recently obtained by Scheidgen13,14 are shown in Figures 3 and 4 for the system carbon dioxide (A) + 1-decanol (B) + tetradecane (C). In Figure 3, characteristic p,T sections for wred C ) 0.00, 0.84, and 1.00 are shown, and in Figure 4a, two isobaric T(wred C ) windows are given. For this system, the deepening of the ternary critical surface extends to such low pressures that it even penetrates the gas-liquid region (see Figure 3) with the additional exhibition of twophase “holes” in the three-phase liquid-liquid-gas
surface. In the p,T projection shown in Figure 3, for this hole, just a noncritical boundary between the two-phase region at lower pressures and the one-phase region at higher pressures exists. For details, see refs 13-16, and for the correlation of such miscibility windows, see ref 17. Another remarkable effect is also found in this ternary system. Because of cosolvency and because the binary critical p(T) curves run through temperature minima (see Figure 3), isothermal p(wred C ) windows are found in addition; that for T ) const ) 282 K is shown in Figure 4b. The arguments for the formation of these isothermal windows are analogous to those for the isobaric windows.13,15 4. Closed Loops and High-Pressure Immiscibility in Binary Mixtures Another accent of our research work was on liquidliquid phase equilibria at high pressures up to several hundred MPa, also including a systematic study of the effect of pressure on systems exhibiting closed isobaric temperature(composition) miscibility gaps (so-called closed loop systems).5,18,19 In Figure 5, the most important types of pressure dependence found for binary closed loop systems are represented schematically. For type 1, the closed loop in the isobaric temperature(composition) diagram shrinks with increasing pressure and finally disappears completely at the point HP1 (e.g., tetrahydrofuran + water20). It has been shown that an immiscibility surface of this type can be shifted to negative pressures, resulting in complete mutual miscibility of the components at normal pressure.5,21 It is, however, also possible that the miscibility gap reappears at high pressures starting from the point HP1′ and increases with further rising pressure. This type 1′ phase behavior was called “high-pressure immiscibility” and was found, e.g., for
4478
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Figure 2. Critical surface in a p,T,wred C cube for the ternary system carbon dioxide (A) + 1-octanol (B) + hexadecane (C). The surface has red the shape of a seat sagging at medium wred C (here, wC ) solvent-free or reduced mass fraction).
Figure 3. p,T projections of binary and quasi-binary critical curves of carbon dioxide (A) + 1-decanol (B) + tetradecane (C). The quasi-binary isopleth at wred C ≈ 0.84 represents the minimum curve of the critical surface, interrupted by the lg hole. In the hatched area, closed isobaric miscibility windows occur.
several methylpyridine + water systems.5,18,19,21 Another interesting example found recently is tetra-n-butylammonium bromide + water.22 For type 2 in Figure 5, the mutual miscibility is lower; here, the low- and highpressure immiscibility surfaces are no longer separated, but instead, there exists a tube-like surface with a distinct narrowing at medium pressures.5,18,19,21 Binary mixtures that show a phase behavior according to Figure 5 exhibit exceptional thermodynamic properties, e.g., a change of sign from minus to plus of the excess molar enthalpy with increasing temperature and of the excess molar volume with increasing pressure. For details, the assumptions made, and the models for the correlation, see refs 5, 18, and 19 and, more recently, ref 23.
The binary system 3-aminopentane + water also exhibits a lower critical solution temperature (LCST) at normal pressure.4,24 In this paper, experimental highpressure data up to 2 GPa (corresponding to 20 kbar) are presented that have recently been obtained with a diamond anvil cell (DAC) in our laboratory by Klante.4 The apparatus and the measuring technique have been described previously.2-4 In Figure 6, the experimental T(p) isopleth for the mole fraction x(water) ) const ) 0.929 [corresponding to a mass fraction w(water) ) const′ ) 0.729] is plotted. The lower solution temperatures go through a temperature maximum at about T ) 408 K and p ) 600 MPa, whereas the temperature minimum of the upper solution temperatures is situated near T ) 400 K and p ) 400-450 MPa. Because the critical mole fraction does not change much with pressure for systems of this type, the shape of the T(p) isopleth shown in Figure 6 is also characteristic for that of the critical T(p) curve. Thus, 3-aminopentane + water has to be attributed to a type 2 system in Figure 5 and to a type VI system in Figure 1, where, however, the shape of the closed loop is changed. Thus, the Scott/van Konynenburg classification normally used can be extended by including a greater variety of the pressure dependencies of closed loop systems. 5. Isopycnic and Isooptic Behavior, Barotropy When variables such as temperature and/or pressure are varied, normally not only the compositions but also other properties (such as densities, refractive indices, etc.) of the coexisting phases change. Now, it can happen that, during such a variation, the density difference between the coexisting phases decreases and even changes the sign. This behavior is called “barotropy”, and the point where barotropy occurs is an “isopycnic point”.
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4479
Figure 6. Temperature T(pressure p) isopleth of the binary system 3-aminopentane + water for x(water) ) const ) 0.729 (x ) mole fraction; see text).
Figure 4. (a) Isobaric T,wred C sections through the critical surface of carbon dioxide (A) + 1-decanol (B) + tetradecane (C) at 7.4 and 7.8 MPasisobaric miscibility windows. (b) Isothermal p,wred C section through the critical surface of carbon dioxide (A) + 1-decanol (B) + tetradecane (C) at 282 Ksisothermal miscibility window.
Figure 7. (a) p[w(CO2)] diagram for (CO2 + 1-octanol) at 393 K. In the hatched region, (near-) isopycnic conditions exist. The full line represents a conode, and the dashed line is the three-phase llg line. (b) T[w(CO2)] diagram for (CO2 + 1-octanol) at 15 MPa. In the hatched region, (near-) isopycnic conditions exist (see text).
Figure 5. Three-dimensional p,T,x diagram of a binary closed loop system (x ) mole fraction; schematically; see text).
In Figure 7a, this behavior is demonstrated for the binary system carbon dioxide + 1-octanol for a change of pressure at a constant temperature according to measurements of Scheidgen.13,14 The borderline of the hatched range corresponds to the isopycnic conditions under which the densities of both coexisting phases are
the same. Within the hatched range, this density difference remains small, the phase that is rich in carbon dioxide having the slightly higher density. The reason is that, with further increasing pressures, the phases resemble each other more and more and become identical at the critical solution pressure where the densities are again exactly the same. Analogous effects can also be found for a variation of temperature at a constant pressure (see Figure 7b). Barotropy is often found for systems consisting of an organic compound with a relatively low liquid density
4480
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
(such as hydrocarbons, alkanols, etc.) and a supercritical solvent with a high critical density (such as carbon dioxide, sulfurhexafluoride, some fluorocarbons, xenon, etc.). For such systems, barotropy is more the rule than the exception. Barotropy might be important for separation methods where stratification of the coexisting phases is essential, such as supercritical fluid extraction (SFE). In addition, sometimes microgravity such as in space can be simulated under isopycnic conditions. Analogous effects also exist for other properties, e.g., a vanishing refractive index difference between two coexisting phases. Under such “isooptic” conditions, the interfaces between the coexisting phases are no longer visible, phase separation can no longer be detected by visual observation, and light scattering is minimized. Here, it might be of interest that the isooptic conditions depend on the wavelength of the light used and are, e.g., different for blue and red light. Barotropy as well as isopycnic and isooptic conditions have also been found for mixtures with liquid-liquid phase separation.25 In extremely rare cases, the demixing phases of a binary system can even be simultaneously (near-) isopycnic and (near-) isooptic; this is of interest for scattering experiments near critical states. Acknowledgment Financial support by the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie e. V. is gratefully acknowledged. The authors thank Dr. Dirk Tuma and Dipl.-Chem. Holger Nadolny for their assistance in the preparation of the manuscript. Literature Cited (1) Deiters, U. K.; Schneider, G. M. High-pressure phase equilibria: Experimental methods. Fluid Phase Equilib. 1986, 29, 145-160. (2) Grzanna, R. Investigations of fluid phase equilibria in a diamond anvil cell. Doctoral Dissertation, University of Bochum, Bochum, Germany, 1996. (3) Grzanna, R.; Schneider, G. M. High-pressure investigations on fluid mixtures with a diamond anvil cell. Z. Phys. Chem. (Frankfurt) 1996, 193, 41-47. (4) Klante, D. Investigations of fluid phase equilibria of binary and ternary aqueous systems in a diamond anvil cell up to 25 kbar. Doctoral Dissertation, University of Bochum, Bochum, Germany, 1999. (5) Schneider, G. M. Phase equilibria of liquid systems at high pressures. Ber. Bunsen-Ges. Phys. Chem. 1966, 70, 497-520. (6) Schneider, G. M. High-pressure investigations on fluid systemssA challenge to experiment, theory and applications. Pure Appl. Chem. 1991, 63, 1313-1326; J. Chem. Thermodyn. 1991, 23, 301-326. (7) Schneider, G. M. Physicochemical properties and phase equilibria of pure fluids and fluid mixtures at high pressures. In Supercritical FluidssFundamentals for Application; Kiran, E., Levelt Sengers, J. M. H., Eds.; NATO ASI Series E, Applied Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; Vol. 273, pp 91-115. (8) Schneider, G. M. Applications of fluid mixtures and supercritical solvents: A survey. In Supercritical FluidssFundamentals for Application; Kiran, E., Levelt Sengers, J. M. H., Eds.; NATO ASI Series E, Applied Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; Vol. 273, pp 739-757.
(9) Schneider, G. M. High-pressure investigations of fluid mixturessreview and recent results. J. Supercrit. Fluids 1998, 13, 43-47. (10) Schneider, G. M.; Kautz, C. B.; Tuma, D. Physicochemical principles of supercritical fluid science. In Supercritical Fluidss Fundamentals and Applications; Kiran, E., Debenedetti, P. G., Peters, C. J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; pp 31-68. (11) Van Konynenburg, P. H.; Scott, R. L. Critical lines and phase equilibria in binary van der Waals mixtures. Philos. Trans. R. Soc. (London) 1980, A298, 495-540. (12) International Union of Pure and Applied Chemistry (IUPAC), Commission on Thermodynamics. Nomenclature for phase diagrams with particular reference to vapour-liquid and liquid-liquid equilibria (Technical Report). Pure Appl. Chem. 1998, 70, 2233-2257. (13) Scheidgen, A. L. Fluid phase equilibria of binary and ternary carbon dioxide mixtures with low-volatile organic substances up to 100 MPa. Doctoral Dissertation, University of Bochum, Bochum, Germany, 1997. (14) Scheidgen, A. L.; Schneider, G. M. Fluid phase equilibria of (carbon dioxide + a 1-alkanol + an alkane) up to 100 MPa and T ) 393 Kscosolvency effect, miscibility windows and holes in the critical surface. J. Chem. Thermodyn. 2000, 32 (No. 9). (15) Gauter, K.; Peters, C. J.; Scheidgen, A. L.; Schneider, G. M. Cosolvency effects, miscibility windows and two-phase lg holes in three-phase llg surfaces in ternary systems: A status report. Fluid Phase Equilib. 2000, 171, 127-149. (16) Po¨hler, H.; Scheidgen, A. L.; Schneider, G. M. Fluid phase equilibria of binary and ternary mixtures of supercritical carbon dioxide with a 1-alkanol and an n-alkane up to 100 MPa and 393 Kscosolvency effect and miscibility windows (part II). Fluid Phase Equilib. 1996, 115, 165-177. (17) Bluma, M.; Deiters, U. K. A classification of phase diagrams of ternary fluid systems. Phys. Chem. Chem. Phys. 1999, 1, 4307-4313. (18) Schneider, G. M. Phase behaviour of aqueous solutions at high pressures. In WatersA Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1973; Vol. 2, pp 381-404. (19) Schneider, G. M. High-pressure phase diagrams and critical properties of fluid mixtures. In Chemical Thermodynamics, Specialist Periodical Report; McGlashan, M. L., Ed.; The Chemical Society: London, 1978; Vol. 2, pp 105-146. (20) Wallbruch, A.; Schneider, G. M. (Liquid + liquid) phase equilibria at high pressures. Pressure-limited closed-loop behaviour of (tetrahydrofuran + water) and the effect of dissolved ammonium sulfate. J. Chem. Thermodyn. 1995, 27, 377-382; 1997, 29, 929. (21) Ochel, H.; Becker, H.; Maag, K.; Schneider, G. M. Influence of a third component on (liquid + liquid) phase equilibria in (butan2-ol + water) and in (butan-1-ol + water) at pressures up to 160 MPa. J. Chem. Thermodyn. 1993, 25, 667-677. (22) Weinga¨rtner, H.; Klante, D.; Schneider, G. M. Highpressure liquid-liquid immiscibility in aqueous solutions of tetran-butylammonium bromide studied by a diamond anvil cell technique. J. Solution Chem. 1999, 28, 435-446. (23) Rebelo, L. P. N. A simple gE-model for generating all basic types of binary liquid-liquid equilibria and their pressure dependence. Thermodynamic constraints at critical loci. Phys. Chem. Chem. Phys. 1999, 1, 4277-4286. (24) Stephenson, R. M. Mutual solubility of water and aliphatic amines. J. Chem. Eng. Data 1993, 38, 625-629. (25) Francis, A. W. Liquid-liquid equilibriums; Interscience: New York, 1963.
Received for review February 3, 2000 Revised manuscript received June 14, 2000 Accepted June 15, 2000 IE0001669
