Anal. Chem. 1995,67,456-461
Estimation of Liquid-Vapor Critical Loci for Con-Solvent Mixtures Using a Peak-Shape Method James W. Ziegler and John 0. Dorsey*lt Department of Chemistty, University of Cincinnati, Cincinnati, Ohio 45221-01 72 T. L. Chester* and D. P. Innis
The Procter and Gamble Company, Miami Valley Laboratories, P.0. Box 398707, Cincinnati, Ohio 45239-8707
Critical-mixture curves for 13 COz-solvent binary mixtures were estimated using the peak-shape method.’ Mixture critical points were determined within 1 “C and 1 atm. The results for COz-toluene and Cop-methanol were compared to previously reported data from highpressure view cell studies. No more than a 3%difference was observed in the data generated by the two different techniques. A few abnormalitiesencounteredwhile using the peak-shape method are also discussed. Despite the growing interest in supercritical fluid techniques, the fundamental physical behavior of COz-solvent binary mixtures is not understood well enough to allow supercritical fluid chromatography (SFC) and extraction (SFE) to be conducted without the possibility of encountering serious phase behavior problems. With the exception of a few cases, phase equilibrium data that have been reported are far from adequate for SFC and SFE purposes. In many cases no data exist at all. To avoid tedious equilibrium studies, various equations of state have been developed in an effort to predict the phase behavior of any COz-solvent binary mixture. Prediction can result in considerable error for the critical-mixture curve estimation. This may lead to inadvertent operation in a two-phase (liquid-vapor) region of a phase diagram. Phase separation between COZ and a modifier or the injection solvent can occur anywhere in an SFC or SFE system. In a binary mixture, the phase state depends on the composition of the mixture and on the local temperature and pressure. Phase separation may be either desirable or undesirable, depending on the specific experimental circumstances. For example, phase separation on an SFC column can lead to broadened, split, and irregular peaks, excessive noise on the baseline, and irreproducible retention. However, controlled phase separation can be used advantageously for solute focusing during direct injection in opentubular SFC.ls2 Except for some analyte solubility studies, the effects of phase behavior in SFE have not been inve~tigated.~ It becomes apparent that knowledge of the phase behavior of COz combined with typical laboratory solvents would be very useful. However, as recently as 1992, Page, Sumpter, and Lee reported that “phase equilibria have been thoroughly studied for
’ Present
address: Department of Chemistry, Florida State University, Tallahassee, FL 323063006. (1) Chester, T. L.;Innis, D. P. J Microcolumn Sep. 1993,5, 261-273. (2) Chester, T. L.;Innis, D. P. n2e 5th International Symposium on Supercritical Fluid Chromatography and Extraction; Baltimore, MD, 1994. (3) Page, S. H.; Momson, J. F. n2e 5th International Symposium on Supercritical Fluid Chromatography and Extraction; Baltimore, MD, 1994.
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only 5 of the 33 binary compositions used in SFC”.4 This was updated in January 1994 by Page and Morrison to eight of the now 38 COz-solvent binary mixture^.^ These are reported to be COZcombined with acetone, benzene, ethanol, hexane, methanol, toluene, tributyl phosphate, and water. Of these mixtures, sufficient investigation to permit critical-mixture curve estimation has been performed for acetone, ethanol, methanol, toluene, and water. The limited amount of available phase behavior data stems from the methodology and equipment required to make such measurements. Most studies have been performed using highpressure view cells and either visual observation or light scattering detection. Generally, the experiments are performed under isothermal conditions, with a known mixture composition and varying pressure. Depressurizing from single-phase to two-phase conditions causes gas bubbles or liquid droplets to form, scattering the light. The technique requires equipment not commonly available in an analytical laboratory. In 1993, a simple solvent peak-shape method was described for estimating critical-mixture curves of COz-solvent combinations.’ This work is capable of being performed with chromatographic equipment. A pressure-controlled source of COZ is connected to an uncoated fused silica capillary tube. The capillary tube is housed in a temperature-regulated oven and connected by a restrictor to a flame ionization detector (FID). A small volume of test solvent is injected into the COZstream under the controlled temperature and pressure. If the resulting binary mixture remains in one phase over all compositions, then the injected solvent plug mixes freely with the mobile phase, broadens like any chromatographic peak, and assumes a skewed Gaussian shape (if the injection volume is not too large). However, if a liquid phase forms, then a dynamic film formation and evaporation process occurs, resulting in a distinctly rectangular peak shape. The present work is an extension and application of the peakshape method for estimating critical-mixture curves. Thirteen COz-solvent critical-mixture curves have been estimated, nine of which have never been reported in the literature. PHASE DIAGRAMS
Scott and van Konynenburg5 described six distinct types of phase behavior. Current applications of SFC and SFE primarily involve modifiers that form type I or I1 binary mixtures with COZ. (4) Page, S. H.; Sumpter, S. R; Lee, M. L. J. Microcolumn Sep. 1992, 4, 91122. (5) van Konynenburg, P. H.; Scott, R L. Phil. Trans. R. SOC.1980,298,495.
0003-2700/95/0367-0456$9.00/0 0 1995 American Chemical Society
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Temperature Figure 1. Phase diagrams for type I (top) and II (bottom) binary mixtures. For our work, component a is COP,and component b is the injected solvent.
Figure 1 shows representative phase diagrams for type I and I1 binary mixtures. A type I mixture is formed when two solvents are miscible as liquids. Type I1 mixtures result when two solvents are mutually immiscible as liquids and become miscible near the critical region. This leads to the additional liquid-liquid-vapor (LLV) and the upper critical solution temperature (UCW lines. The UCST line indicates the temperature at which the two immiscible liquids merge to form a single liquid phase. What is important for the present work is that, between the critical point of the more volatile component (COZ)and the critical point of the less volatile component (the solvent), phase diagrams for type I1 mixtures exhibit the same features as type I mixtures. (See refs 3, 5, and 6 for thorough discussions of all types of binary mixtures.) Before proceeding, it is necessary to clarify some of the terminology that will be used. “Criticalpoint”, “criticalpressure”, and “critical temperature” will be used when a single component is being discussed. “Mixture critical point”, “mixture critical pressure”, and ‘‘mixture critical temperature” will be used to discuss phase behavior of a binary mixture. The critical-mixture curve is the locus of mixture critical points spanning the entire range of composition of the binary mixture.6 To fully map the phase behavior of a binary mixture, information is required in three dimensions: pressure, temperature, and composition (P-T-x). The L-V two-phase region is a volume in the P-T-x coordinate system. Figure 1 is a projection of the phase diagram into pressure and temperature dimensions maintaining the P-T values of the critical-mixture curve. The peakshape method estimates mixture critical pressures and temperatures but not the corresponding compositions. Thus, only twodimensional critical-mixture curves as in Figve 1can be estimated. However, P-Tcritical-mixture curves are very useful in SFC and (6) McHugh, M. A; Ktukonis, V.J. Superm’tical Fluid Extraction Principles and Practice; Butterworths: Stoneham, MA, 1986; pp 23-67.
EXPERIMENTAL SECTION Apparatus. A mod5ed7s8Varian Analytical Instruments (Palo Alto, CA) syringe pump, Model 8500, provided the pressurecontrolled COZstream for all studies. Solvents were injected using a Valco Instruments Co., Inc. (Houston, TX) ECI4W internal loop injector valve, with varying sample loop sizes of 60, 100,200, and 500 nL. A Hewlett-Packard Co. (Palo Alto, CA) 5830A gas chromatograph (GC) was used as both the oven and the FID. The temperature of the GC was calibrated using a data logger and a thermocouple. Temperatures to the nearest degree were obtained from the calibration curve. The FID was maintained at 350 “C. Polyimidecoated fused silica capillary tubing (50pm i.d.) from Polymicro Technologies (Phoenix, AZ) was used. For the solvents studied, it was found that deactivation of the fused silica was not necessary, and capillaries were used as received. All fused silica capillary connectionsmade outside the oven employed PEEK tubing sleeves with stainless steel fittings. All connections made inside the oven employed stainless steel fittings with graphite fused silica adapters. SFC-grade COZwas obtained from Matheson Gas Products, Inc. (Secaucus, NJ). Stainless steel tubing, 0.02-in. id., followed by approximately 25 cm of fused silica capillary (50pm i.d.) attached the pump to the injector. The injector was placed on top of the GC under room temperature conditions. Injection was performed by the waste port restrictor technique? using a short piece of fused silica capillary, 25pm i.d., as the waste port. A second section of 50pm i.d. fused silica capillary tube was wrapped around a support frame taken from a GC capillary column. Capillary tube lengths ranged from approximately 2 to 6 m. The support frame was mounted in the oven. The initial section of capillary tubing was pushed through a septum in the (unused) injection port of the GC and connected to the Valco injector. Depending on the injector loop size, between 25 and 75 cm of capillary tube (approximately 2-3 times the length of capillary filled by the injection volume) was left at room temperature for transport reasons.2 The other end of the capillary tube led to the FID via a frit restrictor, Dionex Corp. (Salt Lake City, UT), connected with a Valco ZU.5 union. Each solvent injection was made under isobaric and isothermal conditions. Detector attenuation was kept high so that the solvent peak could be observed. Solvents. All solvents were used without further purification. Ethanol, USP-Absolute 200 proof, was obtained from AAPER Alcohol & Chemical Co. (Shelbyville,KY). Tetrahydrofuran (high purity, W grade) was from Baxter (Arlington Heights, IL). All other solvents were obtained from Fisher Scientific (Pittsburgh, PA). Acetonitrile, hexane, and methanol were HPLC grade; acetone, 1-butanol, chloroform, octane, 1-octanol, 1-propanol, 2-propanol, and toluene were reagent grade. RESULTS AND DISCUSSION On the basis of the detector signals produced from different pressures at constant temperature, the corresponding mixture (7)Chester,T. L In Anal$ical Instrumentation Handbook Ewing, G., Ed.; Marcel Dekker: New York, 1990. (8) Van Lenten, F.J.; Rothman,L. D. Anal. Chem. 1976, 48, 1430. (9) Chester,T.L.; Innis, D. P.1.Microcolumn Sep. 1989, 1, 230. Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
457
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Figure 2. Series of acetone peaks at 150 "C. Detector attenuation was held constant across the pressure range studied.
Figure 3. Comparison of the Con-toluene critical-mixture curve estimated by our work with the data previously reported by a highpressure view cell technique (ref 10). The last point of the curve is the critical point of toluene, reported from ref 13.
Table 1. Toluene Comparison
Table 2. Methanol Comparison
TIME
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38.1 48 60 71 79.4 80 89 100 110 120 120.6 130 140 150 160 170
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(10) Ng, H.-J.; Robinson, D.B.]. Chem. Eng. Data 1978,23, 325-327. (11) Bmnner, E. J. Chem. " n o d y n . 1985, 17, 671-679.
458 Analytical Chemistry, Vol. 67, No. 2, January 75, 7995
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critical pressure can be estimated. At pressures below the criticalmixture curve, the peak shape is rectangular; above the criticalmixture curve, the shape is skewed Gaussian. This can be seen in Figure 2 for a series of acetone peaks at 150 "C. A transition from rectangular to skewed Gaussian can be observed over the narrow pressure range of 112-114 atm. The peak at 113 atm seems to be a cross between the two peaks on either side. (Due to the limits of our calibration procedure and equipment, conditions could not be determined more precisely than within 1 "C and 1atm.) S i a r behavior was observed for all solvents studied. Comparison of Toluene and Methanol. COz toluene and COZ methanol were chosen for comparison of the peak-shape method with existing literature data. Table 1 lists the available mixture critical points obtained in a high-pressure view cell study with toluenelo and the data obtained in this work. A pictorial representation of the comparison is provided in Figure 3. The four previously reported points, as determined with high-pressure view cells, are in excellent agreement with the critical-mixture curve estimated in this work. Table 2 lists previously reported data for methanol1lJZand the
this
refs 11 and 12
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Figure 4. Comparison of the CO2-methanol critical-mixture curve estimated by our work with the data previously reported in refs 11 and 12. The last point of the cuwe is the critical point of methanol, reported from ref 13.
data obtained in this work. Figure 4 illustrates this comparison. Again, over the entire critical-mixture curve, the data from the two different methods are in excellent agreement. On a numerical comparison, slight deviations can be seen at low temperatures. However, these deviations are no more than about 3%. No signiiicant deviation is observed at higher temperatures at the precision attainable with this method. Figures 3 and 4 suggest
that the peak-shape method is an acceptable technique for studying phase behavior of COz-solvent binary mixtures in P-T space. Additional Critical-MixtureCurves. Critical-mixture curves for 11 additional COz-solvent binary mixtures were generated by the peak-shape method. These are COZ combined with acetone, acetonitrile, 1-butanol, chloroform, ethanol, hexane, octane, 1-octanol,1-propanol,2-propanol,and tetrahydrofuran.The critical-mixture curves are plotted in Figures 5 and 6. O l e raw data are available as supplementary material.) Some of the curves have gaps between the last reported point and the critical point of the modi6er. The reasons for these gaps will be discussed later. With the exception of 2-propanol, Figure 5 shows criticalmixture curves for a series of primary alcohols. The criticalmixture curves increase along both the temperature and pressure axes as the carbon chain length increases. Including that for
2-propanol, the behavior of the critical-mixture curves seems to follow the individual solvent critical points on the temperature scale. The critical-mixture curves do not cross each other. Although this is the case for alcohols, it cannot be concluded as a general rule for all solvents. This is noted by comparing Figures 5 and 6. The critical temperature of 2-propanol is less than that of both tetrahydrofuran and chloroform. The critical-mixture curve of 2-propanol displays a maximum above 150 atm, whereas the critical-mixture curves of tetrahydrofuran and chloroform do not exceed 140 atm. The critical-mixture curve of 2-propanol crosses the critical-mixture curves of tetrahydrofuran and chloroform. Applying the Peak-ShapeMethod. The peak-shape method is very simple and easy, yet some practical considerations must be made. All that is necessary to estimate a mixture critical pressure at a given temperature is to determine where the peak shape changes from rectangular to skewed Gaussian. This has important implications for choosing practical capillary tubing lengths and injection volumes. Too short a tube may not provide sufficient area for film formation and removal. Another consideration in choosing capillary length is the possible movement of the .tilmonce it forms. Two different modes of film creeping have been reported. The fist method of creeping is caused by unequal pressure across the film. Studies have been performed using long bubbles to investigate the creeping motion of a liquid film.15J6(To be applicable to this work, the flowing vapor phase is considered to be a bubble of infinite length moving along the film.) It was observed that when the film thickness was equal along the length of the bubble, film migration completely stopped. However, if a pressure drop existed along the bubble and film thickness was not constant, then a pressure gradient in the film along the bubble was established. This caused the liquid to flow in an effort to achieve constant curvature at the liquid-vapor interface along the entire length of the bubble.15J6 Since in SFC both a pressure drop along the length of the film can be expected and, as evaporation occurs at the trailing end, the film gets thinner and then disappears, similar movement of the liquid film should be expected. The second method of film creeping is a result of Rayleigh instability,17J8and has been investigated for liquid films in retention gaps employed in GC.19 More important is the choice of injection volume, which is dependent on both the solvent being studied and the conditions. Too small an injection volume will not provide enough liquid for film formation and saturation of the vapor phase. A peak shape that is indistinguishable as either rectangular or skewed Gaussian results. A larger injection volume will form a longer film, but the vapor phase composition at saturation will not change. Thus the length of the rectangular peak increases, while the height is not affected. As the temperature is increased, the vapor phase solvent concentration increases. At some point, the injection volume will become insufficient for the conditions. The temperature at which the injection volume had to be increased varied among solvents. Plotting Peak Height versus Pressure. Over a small pressure range, the COZmass flow rate is approximately constant, and thus the output signal is proportional to the vapor phase
(12) Brunner, E.; Hultenschmidt, W.; Schlichtharle, G. J. Chem. ntennodyn. 1987,19,273-291. (13) CRC Handbook of Chemistry and Physics; 71st ed.; Lide, D. R, Ed.; CRC Press, Inc.: Boca Raton, FL, 1990-1991. (14) CRC Handbook of Chemistry and Physics; 72nd ed.; Lide, D.R, Ed.; CRC Press, Inc.: Boca Raton, FL, 1991-1992.
(15) Goldsmith, H. L.; Mason, S. G.J Colloid Sci. 1963,18,237-261. (16) Everett, D. H.; Haynes, J. M. J Colloid Interface Sci. 1972,38, 125-137. (17) Bartle, K. D.; Woolley, C. L.; Markides, K. E.; Lee, M. L.; Hansen, R S. J. High Resolut. Chromatogr., Chromatogr. Commun. 1987,10,128-136. (18) Goren, S. L.J. Fluid Mech. 1962,12, 309-319. (19) Grob, K. J.J. Chromatogr. 1981,213,3-14.
Ethanol a 1-Propanol 0 2-propanol l-Butanol I 1-Octanol
I
Temperature (OC) Figure 5. Critical-mixture curves for COZ combined with different alcohols estimated using the peak-shape method. The last point of each curve is the critical point of the solvent, reported from refs 13 and 14. 0
Chloroform
Tetrahydrofuran A
Acetonitrile
+ Acetone 0
Hexane
9
n-Octane
- 02
Temperature (“C) Figure 6. Critical-mixture curves for CO2 combined with various solvents estimated using the peak-shape method. The last point of each curve is the critical point of the solvent, reported from refs 13 and 14.
Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
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Pressure (atm) Figure 7. Plot of peak height vs pressure for CO2-methanol at 100 “C, pressure range of 120-190 atm. Detector attenuation was chosen so that it could remain constant.
concentration of the solvent. The solvent concentration in the vapor phase remains relatively constant with increasing pressure, except when the pressure is very near the mixture critical pressure. There the solvent concentration in the vapor phase rapidly changes with pressure. A study was performed using methanol as the solvent, a temperature of 100 “C, and various pressures in the range of 120190 atm. The peak heights were measured by hand to the nearest 0.5 mm and plotted against pressure. The sigmoidal curve shown in Figure 7 resulted. We assume that the mixture critical pressure exists at the inflection. At pressures less than the mixture critical pressure, the plot remains relatively flat. Near the mixture critical pressure, the vapor phase concentration of solvent increases considerably,as indicated by the increas‘e in peak height. Above the mixture critical pressure, only a single phase exists. No
saturation occurs, and the peak heights are again relatively constant. Because the height of the peak is dependent on the rate of delivery of solvent to the detector, which will increase slightly with increasing flow rate caused by increasing pressure, a slight positive slope is observed in the “flat lines” before and after the inflection point. These plots add objectivity to the subjective peak-shape change for the determination of a mixture critical pressure. Unconventional Detector S i s . Noisy Peaks. Sometimes single, clean peaks were not observed. Figure 8 shows peaks obtained for 1-propanol at 70 “C and increasing pressure. Although spikes appear on the detector signals as pressure is increased toward the mixture critical pressure, the general trend of rectangular to skewed Gaussian peak-shape can still be observed. The phase transition can still be estimated, because a basically flat top peak with spikes occurs at 120 atm. A skewed Gaussian peak occurs at 122 atm, and 121 atm shows a cross between the two. The spiked signals are believed to indicate the presence of an aerosol. Indistinguishable Peaks at High Temperatures. Figure 9a shows a series of peak shapes obtained for hexane at 170 “C and a pressure range of 83-87 atm. By interpolation of the criticalmixture curve, it can be seen that the phase transition should have occurred somewhere in this pressure range. It is possible to conclude that the peak at 83 atm has a flat top, that at 84 atm has a cross between flat and skewed Gaussian top, and that at 85 atm has a skewed Gaussian top. However, these peaks are not the shapes expected, and the data are at best questionable. These peaks were obtained with the largest available injection loop (500 nL) . The presence of large tails and two distinctly different curves on the back side of the signal suggests that at least two different mass transfer phenomena are occurring. Other types of “indistinguishable peaks” are shown in Figure 9b. These peaks were obtained for 1-octanol at 250 “C. Again, 122
atm
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116
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1MINW
114
12 1
atm
atm
112
TIME Flgure 8. COP-1-propanol at 70 “C. During the experiment, detector attenuation had to be increased by a factor of 4. 460 Analytical Chemistry, Vol. 67, No. 2,January 15, 1995
us
^.
86
87
atm
atm
TIME 180 am
180 atm
182 am
183 atm
TIME
Figure 9. Unconventional peak shapes: (a, top) hexane, 170 "C and an injection volume of 500 nL; (b, bottom) 1-octanol, 250 "Cand an injection volume of 500 nL.
no conclusive phase behavior can be determined. The first two signals were produced under identical injection conditions, yet differentsignals resulted. This indicates the possibility of sporadic mass transfer. In all cases, if indistinguishable peaks occurred at higher temperatures, they were detected approaching the critical temperature of the pure modifier. This is why there are gaps in the higher temperature regions for some of the solvents in Figures 5 and 6. These signals are indicative of nonideal film formation and removal behavior and cannot be explained at this time.
CONCLUSION With the peak-shape method, estimation of a critical-mixture curve for a single mixture can be performed in approximately 1 day. The equipment required can be found in any analytical laboratory in which SFC or SFE is done. It was found that the peak-shape method was less labor- and equipment-intensivethan the previous methods for studying phase equilibria with highpressure view cells. Knowledge of the critical-mixturecurves for COZwith typical injection solvents allows one to successfully employ a retention gap for large volume injections in capillary SFC. Observations made during this work suggest that characteristics of the dynamically formed liquid film and associated mass transfer may require further investigation. Only as the understanding of physical phenomena that occur with supercriticalfluids increases can the related techniques be developed and expanded in an analytical fashion. ACKNOWLEDGMENT J.G.D. gratefully acknowledges support of this work by the Procter and Gamble Co. and by NIEHS ES04908. We thank Barbara Stout and Joe Caruso for helpful suggestions. SUPPLEMENTARY MATERIAL AVAILABLE The raw data used to generate the critical-mixture curves from this work (Figures 3-6) can be obtained as supplementary material (1 page). See any current masthead page for ordering information. Received for review June 27, 1994. Accepted October 24, 1994.a AC940637Z *Abstract published in Advance ACS Abstracts, December 1, 1994.
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