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Estimation of pressure-temperature critical loci of CO2 binary mixtures with methyl-tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane
THE JOURNAL OF
Supercri.tical fllilaS ELSEVIER
Journal of SupercriticalFluids 11 (1997) 15 20
Estimation of pressure-temperature critical loci of C O 2 binary mixtures with methyl-tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane T.L. Chester *, B.S. Haynes The Procter and Gamble Company, Miami Valley Laboratories, PO Box 538707, Cincinnati, OH 45253-8707, USA
Received 16 December 1996; accepted 26 July 1997
Abstract Pressure-temperature coordinates of critical points for binary mixtures of C O 2 and five co-solvents are estimated using a peak-shape-sensitive flow-injection procedure. CO2 is pumped continuously under pressure control into a thermostated capillary tube which is restricted at its outlet and interfaced to a flame ionization detector. The co-solvent is injected into the capillary tube inlet at ambient temperature, where the CO2 is a liquid at the test pressures and transported to the thermostated section of the capillary tube. The shape of the co-solvent peak as it exits the capillary tube and enters the detector is dependent on mass transport rates and phase behavior occurring earlier in the system. © 1997 Elsevier Science B.V. Keywords: Carbon dioxide; Critical loci; Binary mixtures; Flow-injection; Supercritical
1. Introduction Critical loci of binary mixtures are most often investigated using high-pressure view cells. The instrumentation required to generate (and sometimes monitor) the mixtures and to make the observations is often complicated and its use very time consuming. Researchers investigating critical loci also often limit their investigations to a small section of the curve applicable to their particular end use. Although there is much literature available involving binary mixtures including carbon dioxide, only a small fraction of it spans the entire critical loci of the mixtures investigated. * Corresponding author. 0896-8446/97/$17.00© 1997 ElsevierScienceB.V. All rights reserved. PH S0896-8446 (97) 00034-X
The selection of suitable initial temperatures and pressures during direct injection onto a retention gap in supercritical fluid chromatography (SFC) requires knowledge of the pressure-temperature projection of the appropriate critical locus for the mixture of the mobile phase and the sample solvent [ 1 ]. Knowledge of P - T projections of critical loci is also very beneficial in selecting pressure and temperature limits when modifiers are used in both SFC and supercritical fluid extraction (SFE). For example, it is trivial to ensure that the fluid will never separate into liquid and vapor phases, regardless of composition of the mixture, by keeping pressure and temperature in the one-phase region of the P - T projection. We developed a rapid, flow-injection method for determining the P - T projections of critical loci for
16
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15 20
binary mixtures containing CO 2 [2]. So far we have used the method to estimate P - T projections of critical loci of 17 binary CO2-solvent mixtures [2-5]. The previously investigated solvents include acetone, acetonitrile, 1-butanol, carbon tetrachloride, chloroform, cyclohexane, ethanol, n-heptane, n-hexane, methanol, n-octane, 1-octanol, 1propanol, 2-propanol, pyridine, tetrahydrofuran, and toluene. We present here recently completed work for CO2 binary mixtures made with five additional solvents: methyl-tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane.
2. Theory We continuously pump C O 2 into a capillary tube. The pump is operated under pressure control. The capillary tube has a flow restrictor at its outlet and the flow rate is kept slow enough that the pressure drop over the capillary tube is of the same order as our ability to measure and control the pressure (about +0.05 MPa in our case). We inject a plug of the selected co-solvent into the stream of CO2 using a room temperature injector. A short segment, usually 25-50 cm, of the inlet end of the capillary tube remains at room temperature while the remaining length is thermostated in an oven. In this method, pressures are always chosen above the CO2 boiling line. For Type I, mixtures [6] there will be no phase separation in the room temperature injector or the room temperature segment of the capillary tube under these conditions. Only a single liquid phase will be present for all possible mixture compositions while the temperature is below the critical temperatures of both components and the pressure is well above the boiling line of the COg. Our-method is based on a phase-behaviordependent limit in the rate of transport of the co-solvent to the detector when liquid-vapor (l-v) phase separation occurs in the thermostated segment of capillary tube. Mass transport to the detector occurs only in the moving phase, that is, the vapor, whenever 1-v phase separation occurs. (Any liquid present simply wets the capillary walls, and is spread by the moving vapor into a flooded zone whose length is determined by the diameter
of the capillary tube, the amount of co-solvent injected, the pumping rate, the surface tension at the 1-v interface, and the resulting thickness of the liquid layer formed. Although some creeping of the liquid film probably occurs, it is of no consequence in this method as long as the capillary tube is long enough, compared with the length of the flooded zone, so that the liquid is completely evaporated before the liquid front reaches the detector.) When a liquid film is formed, if the liquid-flooded zone is sufficiently long compared with the pumping rate, then the vapor phase can approach saturation with co-solvent as the moving vapor phase passes through the zone containing the liquid film. The concentration of co-solvent in the vapor phase, and its rate of transport toward the detector, is therefore controlled by the temperature and pressure at the flooded zone. Thus, the observed detector signal is flat-topped when liquid-vapor separation occurs, being limited by the maximum possible co-solvent concentration in the vapor phase. Flat-topped peaks are easily tested to make sure they result from vapor-phase saturation by changing the volume of co-solvent injected and verifying that the maximum signal remains unchanged and that the duration of the signal is proportional to the volume injected. If the volume of co-solvent injected is too large compared with the length of the capillary tube, then the co-solvent liquid plug will not be completely consumed in the wall-wetting process; that is, liquid co-solvent will break through the capillary tube and will reach the detector. This may also produce a large, flat signal, but is easily distinguished from the signal produced by the saturation process described earlier. The large 'liquid' signal will subside as soon as the excess liquid is cleared from the detector, and the signal will fall to the value produced by the rate of co-solvent delivery in the vapor phase until the liquid film in the capillary tube is evaporated and removed. In addition, reducing the amount of co-solvent injected, or lengthening the capillary tube, will eventually eliminate the signal caused by transport of liquid co-solvent to the detector, but will not change the signal level resulting from the desired saturation condition. If the CO2 and co-solvent remain miscible under
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15-20
the test conditions, as will occur when the pressure exceeds the critical pressure at the oven temperature selected, then there is no limit on the signal level until the concentration of co-solvent in the center of the co-solvent plug reaches 100%. This allows the co-solvent peak to assume a rounded shape as caused by the normal chromatographic band broadening processes. Flat peaks resulting by phase separation and saturation are easily distinguished from the fiat peaks that may result from a delivery of 100% co-solvent in the heart of a large injected plug. A very large signal is produced in the latter case and requires a comparatively large injection volume to achieve this condition. Alternatively, critical pressures can be estimated using a peak-height method (as explained in Refs. [4,5]). However, in the present work, we relied entirely on peak-shape observations.
3. Experimental The mobile phase was delivered under pressure control using a modified [7] syringe pump (model 8500, Varian, Palo Alto, CA). Test solvents were introduced using a room temperature, internalloop injector (model ECI4W, Valco, Houston, TX). Injection loops of 60, 100, 200, and 500 nl were used. A gas chromatograph (GC) oven with a flame ionization detector (5890 Series II, Hewlett-Packard Co., Little Falls, DE) was used for temperature control and detection. A 10 m, 50 ~tm internal diameter fused-silica capillary tube (Polymicro Technologies, Phoenix, AZ) joined the injector to the detector. The first 30 cm of this tube remained outside the oven at room temperature, while the balance was wound on a wire GC column support in the oven. An integral restrictor [8] was prepared on the outlet of this tube and was used to interface the tube to the detector. The detector was separately thermostated at 400°C. This arrangement produced linear velocities in the range of about 1.5-15 cm/s depending on various conditions. (Fast flows are allowable for rangefinding measurements, but a restrictor producing a rather slow flow and minimum pressure drop over the capillary tube is necessary for the best estimates of critical pressures.)
17
Pump pressure was calibrated using a certified digital gauge (traceable to NIST), model DG-B/9512-07 (Transcat, Bridgeport, NJ). The oven temperature control was calibrated using a separate electronically compensated thermocouple model TAC-386JC (Omega Engineering, Stamford, CT) which, in turn, was checked against boiling water. The CO2 used was SFC/SFE grade (Air Products and Chemicals, Inc., Allentown, PA). The co-solvents used were all reagent grade or better and were purchased from a variety of suppliers.
4. Results and discussion Our estimates of pressure-temperature coordinates of critical points of CO2 mixed with methyltert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane are plotted in Figs. 1 and 2. The first and last points in each series are the critical points of CO 2 and the respective co-solvent. These are literature values [9] not measured points. For completeness, we have also plotted estimates of critical points for CO2 mixed with cyclohexane, pyridine, and n-heptane obtained using the same method and reported by Ziegler [5], but not yet published beyond his thesis. The coordinates are given in Table 1. We have purposely not drawn curves in Figs. 1 and 2, but only plotted data. We cannot easily 20.0
• • o o
tO0 oo O0 •
16.0 •
•m •
° • o0o 12.0 ,~
8.0
: •
decane cycl0hexane
n-heptane methyl-tertbutyl ether
E]
0..
4.0
D 0
0.0
I
100
I
200 Temperature, °C
I
I
300
400
Fig. 1. Estimates of critical loci for CO2-solvent mixtures as indicated. The first and last points in each series are the critical points for the pure materials. The cyclohexane and n-heptane data are from Ziegler [5].
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15-20
18 20.0
16.0
pyridine I dioxane I v methyl-ethyl ketone [
• [ ~ 3 D £3 Q D D (3°
• ethyl acetate I
~Dvvvvv v
12.0
! ~,
"
8.o
0.
0
4.0
•
I v
0.0
I
0
100
I
200 Temperature, °C
I
I
300
400
Fig. 2. Estimates of critical loci for CO2-solvent mixtures as indicated. The first and last points in each series are the critical points for the pure materials. The pyridine data are from Ziegler [5].
predict the results when attempting this flowinjection method on a mixture that is not Type I. For example, in an earlier work, we were unable to observe flat peaks for mixtures of CO2 and 1-octanol below 60°C. The behavior of this mixture is not consistent with Type I or Type II. In the present work, we were unable to observe fiat peaks for ethyl acetate, methyl-ethyl ketone, dioxane and decane co-solvents above 201°C, or for methylt-butyl ether above 176°C. The mixture CO2-decane is widely reported to be Type II. It is essential to realize that our method is actually a mass-transport-rate-sensitive procedure which, under ideal conditions, is influenced by the (pseudo)equilibrium established between the liquid and vapor phases in the flooded zone. The method is not a direct observation of 1-v phase separation. However, the flat-topped peaks are a consequence of the fact that removal of the liquid co-solvent from the flooded zone can never be faster than the product of the concentration of the co-solvent in the vapor phase and the total flow rate through the system. If the vapor-phase co-solvent concentration is limited by saturation, the peaks will be fiat. If the co-solvent mass-flow rate in the room temperature portion of the system or in the detector is ever slower than the co-solvent mass-flow rate in the oven, the method will not reflect the phase behavior occurring in the oven. Thus, care must be exercised in interpreting these data, particularly for mixtures more complicated than Type I.
However, we believe that the 1-v loci for mixtures with phase behavior more complicated than Type I can be estimated by this method, at least in some cases, over the segments of the 1-v critical loci where no other phase separation occurs. Better control of the temperature of the inlet system will be necessary to investigate mixtures with liquidliquid separations occurring at room temperature. For example, for a Type II mixture where injection is performed at a temperature above the upper critical solution temperature (but below the mixture critical temperature), the flow-injection method should behave as it does for a Type I mixture. When investigating mixture critical points near the critical point of pure C Q , it is possible that phase separation may~briefly occur when the mixture reaches the detector interface due to the temperature increase, even if there is no phase separation on the capillary tube at oven temperature. Artifacts can be prevented or minimized by operating the detector at a high temperature well above the critical temperature of the co-solvent, and with a well-designed interface to provide an abrupt temperature change, to minimize exposure of the mixture to the undesired intermediate conditions. Comparisons of our results with measurements reported by other methods have shown good agreement. Earlier, we compared our results with literature values derived from view-cell observations for CO2-toluene and CO2-methanol mixtures [3,4]. The average deviation in the mixture critical pressures (comparing our procedure with view-cell observations) was 1.1% and deviation never exceeded 2.3% when we ignored pressure drop in the capillary tube. The average deviation was 0.1% and never exceed 0.2% when we corrected for pressure drop [4]. Ziegler's experimental points for CO2cyclohexane mixtures using the flow-injection method [5] compare favorably with the observations of Shibata and Sandler [10]. The latter authors did not specifically report mixture critical points, but reported equilibrium phase compositions and densities over conditions apparently closely approaching mixture critical points. Although these authors only reported observations
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15 20
19
Table 1 Temperature and pressure coordinates for estimates of critical loci for the solvents shown in mixture with C O 2 Temperature
Pressure(MPa)
(of) Methyl-t-butyl ether 31.1 44 50 55 60 70 80 90 91 100 110 115 120 125 130 135 140 145 150 161 176 180 186 201 210 224.1 240 250.3 263.8 267.1 280.4 314 344.7 347
Ethyl acetate
Methyl-ethyl Dioxane ketone 7.38
Decane
7,38
7.38
7.38
7.38
8.8
9.3
9.2
9,6 10,1 10.7 11.0
10.1 11.0 11.7 12.2
10.2 10.9 11.8 12.5
11.3 11.3 11.2 11.3
12.6 12.8 12.9 13.0
13.1 13.6
14.1 14.9
16.5
13.8
15.3
11.1 10.9 10.5 10.3 9.9 9.0 7.5
13.0
13.9
15.7
17.9 14.4 18.2
13.0
13.8
16.1
18.3
12.9 12.5
13.7 13.3 12.8
16.4 16.4 16.3
18.5 18.5 18.4
11.0
16.2 15.8
18.2 17.7
10.9 10.5 12.5
Cyclohexanea 7.39 8.9 10.0 10.8
14.5
Pyridinea 7.39 8.5 9.1 9.7 10.3 11.7 13.0 14.2 14.3
7.39
9.8
12.2
13.3
17.2
14.5
9.9
n-Heptanea
13.3
13.3
14.4
19.1
13.0
13.3
19.6
10.9
11.1
18.8
3.43 17.2 3.88 4.20 2.7 4.1 5.21 2.11 5.7
aAs reported by Ziegler [5]. at two temperatures, 93.5 a n d 137.9°C, a n d neither of these t e m p e r a t u r e s were used in Ziegler's experiments, the a p p a r e n t m i x t u r e critical pressures of Shibata a n d Sandler deviate by only 0.5% from i n t e r p o l a t i o n s o f Ziegler's flow-injection results. O u r results for C O z - d e c a n e do n o t c o m p a r e well with the d a t a for this mixture reported by Swaid et al. [11]. These a u t h o r s were also measuring e q u i l i b r i u m phase c o m p o s i t i o n s at several temperatures a n d were n o t directly observing mixture
critical points. They left m u c h larger gaps n e a r mixture critical points t h a n did Shibata a n d Sandler with CO2-cyclohexane. Thus, the actual m i x t u r e critical pressures are s o m e w h a t higher t h a n the highest pressures reported by Swaid et al., a n d close agreement with the c u r r e n t work should n o t necessarily be expected. (Our observations suggest that the critical pressures are a b o u t 3 - 1 3 % higher t h a n the highest pressures used by Swaid et al.) Regardless, more work will be needed to
20
T.L. Chester, B.S. Haynes/Journal of Supercritical Fluids 11 (1997) 15-20
validate the flow-injection procedure for estimating the l - v loci o f mixtures other than Type I. A l t h o u g h used here for CO2-co-solvent mixtures, the flow-injection peak-shape m e t h o d should be quite general. The only m a j o r requirement is the use o f a detector that can easily distinguish the injected co-solvent f r o m the p u m p e d primary fluid. Therefore, it should be possible to use this m e t h o d to examine mixtures such as toluene injected into butane using an ultraviolet absorption detector. In such an experiment, it would be necessary to ensure that conditions in the detector and tubing leading from the oven to the detector are always in the one-phase region. We think the easiest way to accomplish this would be to heat any ovendetector transfer tubing and the detector to a temperature well above the critical temperature corresponding with the pressure and the mixture under test. Setting the detector temperature well above the co-solvent critical temperature, when possible, and providing an abrupt temperature change f r o m the oven temperature, would be best to avoid potential confusion. With some fluids, the temperatures required will be too high for ordinary H P L C - t y p e U V detectors. Suitable safety precautions would also be required when investigating any flammable primary fluid since leaks in the oven or detector could lead to fire or explosion.
References [1] T.L. Chester, D.P. Innis, Quantitative open-tubular supercritical fluid chromatography using direct injection onto a retention gap, Analyt. Chem. 67 (1995) 3057.
[2] T.L. Chester, D.P. Innis, Dynamic film formation and the use of retention gaps with direct injection in open-tubular supercritical fluid chromatography, J. Microcolloid Separation 5 (1993)261. [3] J.W. Ziegler, J.G. Dorsey, T.L. Chester, D.P. Innis, Estimation of liquid-vapor critical loci for CO2-solvent mixtures using a peak-shape method, Analyt. Chem. 67 (1995) 456. [4] J.W. Ziegler, T.L. Chester, D.P. Innis, S.H. Page, J.G. Dorsey, in: K.W. Hutchenson, N.R. Foster (Eds.), Supercritical Fluid Flow Injection Method for Mapping Liquid-Vapor Critical Loci of Binary Mixtures Containing CO2, Innovations in Supercritical Fluids: Science and Technology, American Chemical Society, Washington, DC, 1995, p. 93. [5] J.W. Ziegler, 'Ain't Misbehavin'- fundamental investigations into supercritical fluid binary mixtures containing CO2 and supercritical fluid chromatography, Ph.D. dissertation, University of Cincinnati, 1996. [6] R.L. Scott, P.H. van Konynenburg, Van der Waals and related models for hydrocarbon mixtures, Discuss. Faraday Soc. 49 (1970) 87. [7] T.L. Chester, Supercritical fluid chromatography instrumentation, in: G. Ewing (Ed.), Analytical Instrumentation Handbook, Dekker, New York, 1990, p. 843. [8] E.J. Guthrie, H.E. Schwartz, Integral pressure restrictor for capillary SFC (supercritical fluid chromatography), J. Chromatogr. Sci. 24 (1986)236. [9] D. Ambrose, Critical constants, boiling points, and melting points of selected compounds, in: D.R. Lide (Ed.), Handbook of Chemistry and Physics, 73rd ed., CRC Press, Boca Raton, FL, 1992, pp. 6-49. [10] S.K. Shibata, S.1. Sandier, High-pressure vapor-liquid equilibria of mixtures of nitrogen, carbon dioxide, and cyclohexane, J. Chem. Engng Data 34 (1989) 419. [11] I. Swaid, D. Nickel, G.M. Schneider, NIR-spectroscopic investigations on phase behavior of low-volatile organic substances in supercritical carbon dioxide, Fluid Phase Equilibria 21 (1985) 95.
Supercri.tical fllilaS ELSEVIER
Journal of SupercriticalFluids 11 (1997) 15 20
Estimation of pressure-temperature critical loci of C O 2 binary mixtures with methyl-tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane T.L. Chester *, B.S. Haynes The Procter and Gamble Company, Miami Valley Laboratories, PO Box 538707, Cincinnati, OH 45253-8707, USA
Received 16 December 1996; accepted 26 July 1997
Abstract Pressure-temperature coordinates of critical points for binary mixtures of C O 2 and five co-solvents are estimated using a peak-shape-sensitive flow-injection procedure. CO2 is pumped continuously under pressure control into a thermostated capillary tube which is restricted at its outlet and interfaced to a flame ionization detector. The co-solvent is injected into the capillary tube inlet at ambient temperature, where the CO2 is a liquid at the test pressures and transported to the thermostated section of the capillary tube. The shape of the co-solvent peak as it exits the capillary tube and enters the detector is dependent on mass transport rates and phase behavior occurring earlier in the system. © 1997 Elsevier Science B.V. Keywords: Carbon dioxide; Critical loci; Binary mixtures; Flow-injection; Supercritical
1. Introduction Critical loci of binary mixtures are most often investigated using high-pressure view cells. The instrumentation required to generate (and sometimes monitor) the mixtures and to make the observations is often complicated and its use very time consuming. Researchers investigating critical loci also often limit their investigations to a small section of the curve applicable to their particular end use. Although there is much literature available involving binary mixtures including carbon dioxide, only a small fraction of it spans the entire critical loci of the mixtures investigated. * Corresponding author. 0896-8446/97/$17.00© 1997 ElsevierScienceB.V. All rights reserved. PH S0896-8446 (97) 00034-X
The selection of suitable initial temperatures and pressures during direct injection onto a retention gap in supercritical fluid chromatography (SFC) requires knowledge of the pressure-temperature projection of the appropriate critical locus for the mixture of the mobile phase and the sample solvent [ 1 ]. Knowledge of P - T projections of critical loci is also very beneficial in selecting pressure and temperature limits when modifiers are used in both SFC and supercritical fluid extraction (SFE). For example, it is trivial to ensure that the fluid will never separate into liquid and vapor phases, regardless of composition of the mixture, by keeping pressure and temperature in the one-phase region of the P - T projection. We developed a rapid, flow-injection method for determining the P - T projections of critical loci for
16
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15 20
binary mixtures containing CO 2 [2]. So far we have used the method to estimate P - T projections of critical loci of 17 binary CO2-solvent mixtures [2-5]. The previously investigated solvents include acetone, acetonitrile, 1-butanol, carbon tetrachloride, chloroform, cyclohexane, ethanol, n-heptane, n-hexane, methanol, n-octane, 1-octanol, 1propanol, 2-propanol, pyridine, tetrahydrofuran, and toluene. We present here recently completed work for CO2 binary mixtures made with five additional solvents: methyl-tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane.
2. Theory We continuously pump C O 2 into a capillary tube. The pump is operated under pressure control. The capillary tube has a flow restrictor at its outlet and the flow rate is kept slow enough that the pressure drop over the capillary tube is of the same order as our ability to measure and control the pressure (about +0.05 MPa in our case). We inject a plug of the selected co-solvent into the stream of CO2 using a room temperature injector. A short segment, usually 25-50 cm, of the inlet end of the capillary tube remains at room temperature while the remaining length is thermostated in an oven. In this method, pressures are always chosen above the CO2 boiling line. For Type I, mixtures [6] there will be no phase separation in the room temperature injector or the room temperature segment of the capillary tube under these conditions. Only a single liquid phase will be present for all possible mixture compositions while the temperature is below the critical temperatures of both components and the pressure is well above the boiling line of the COg. Our-method is based on a phase-behaviordependent limit in the rate of transport of the co-solvent to the detector when liquid-vapor (l-v) phase separation occurs in the thermostated segment of capillary tube. Mass transport to the detector occurs only in the moving phase, that is, the vapor, whenever 1-v phase separation occurs. (Any liquid present simply wets the capillary walls, and is spread by the moving vapor into a flooded zone whose length is determined by the diameter
of the capillary tube, the amount of co-solvent injected, the pumping rate, the surface tension at the 1-v interface, and the resulting thickness of the liquid layer formed. Although some creeping of the liquid film probably occurs, it is of no consequence in this method as long as the capillary tube is long enough, compared with the length of the flooded zone, so that the liquid is completely evaporated before the liquid front reaches the detector.) When a liquid film is formed, if the liquid-flooded zone is sufficiently long compared with the pumping rate, then the vapor phase can approach saturation with co-solvent as the moving vapor phase passes through the zone containing the liquid film. The concentration of co-solvent in the vapor phase, and its rate of transport toward the detector, is therefore controlled by the temperature and pressure at the flooded zone. Thus, the observed detector signal is flat-topped when liquid-vapor separation occurs, being limited by the maximum possible co-solvent concentration in the vapor phase. Flat-topped peaks are easily tested to make sure they result from vapor-phase saturation by changing the volume of co-solvent injected and verifying that the maximum signal remains unchanged and that the duration of the signal is proportional to the volume injected. If the volume of co-solvent injected is too large compared with the length of the capillary tube, then the co-solvent liquid plug will not be completely consumed in the wall-wetting process; that is, liquid co-solvent will break through the capillary tube and will reach the detector. This may also produce a large, flat signal, but is easily distinguished from the signal produced by the saturation process described earlier. The large 'liquid' signal will subside as soon as the excess liquid is cleared from the detector, and the signal will fall to the value produced by the rate of co-solvent delivery in the vapor phase until the liquid film in the capillary tube is evaporated and removed. In addition, reducing the amount of co-solvent injected, or lengthening the capillary tube, will eventually eliminate the signal caused by transport of liquid co-solvent to the detector, but will not change the signal level resulting from the desired saturation condition. If the CO2 and co-solvent remain miscible under
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15-20
the test conditions, as will occur when the pressure exceeds the critical pressure at the oven temperature selected, then there is no limit on the signal level until the concentration of co-solvent in the center of the co-solvent plug reaches 100%. This allows the co-solvent peak to assume a rounded shape as caused by the normal chromatographic band broadening processes. Flat peaks resulting by phase separation and saturation are easily distinguished from the fiat peaks that may result from a delivery of 100% co-solvent in the heart of a large injected plug. A very large signal is produced in the latter case and requires a comparatively large injection volume to achieve this condition. Alternatively, critical pressures can be estimated using a peak-height method (as explained in Refs. [4,5]). However, in the present work, we relied entirely on peak-shape observations.
3. Experimental The mobile phase was delivered under pressure control using a modified [7] syringe pump (model 8500, Varian, Palo Alto, CA). Test solvents were introduced using a room temperature, internalloop injector (model ECI4W, Valco, Houston, TX). Injection loops of 60, 100, 200, and 500 nl were used. A gas chromatograph (GC) oven with a flame ionization detector (5890 Series II, Hewlett-Packard Co., Little Falls, DE) was used for temperature control and detection. A 10 m, 50 ~tm internal diameter fused-silica capillary tube (Polymicro Technologies, Phoenix, AZ) joined the injector to the detector. The first 30 cm of this tube remained outside the oven at room temperature, while the balance was wound on a wire GC column support in the oven. An integral restrictor [8] was prepared on the outlet of this tube and was used to interface the tube to the detector. The detector was separately thermostated at 400°C. This arrangement produced linear velocities in the range of about 1.5-15 cm/s depending on various conditions. (Fast flows are allowable for rangefinding measurements, but a restrictor producing a rather slow flow and minimum pressure drop over the capillary tube is necessary for the best estimates of critical pressures.)
17
Pump pressure was calibrated using a certified digital gauge (traceable to NIST), model DG-B/9512-07 (Transcat, Bridgeport, NJ). The oven temperature control was calibrated using a separate electronically compensated thermocouple model TAC-386JC (Omega Engineering, Stamford, CT) which, in turn, was checked against boiling water. The CO2 used was SFC/SFE grade (Air Products and Chemicals, Inc., Allentown, PA). The co-solvents used were all reagent grade or better and were purchased from a variety of suppliers.
4. Results and discussion Our estimates of pressure-temperature coordinates of critical points of CO2 mixed with methyltert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane are plotted in Figs. 1 and 2. The first and last points in each series are the critical points of CO 2 and the respective co-solvent. These are literature values [9] not measured points. For completeness, we have also plotted estimates of critical points for CO2 mixed with cyclohexane, pyridine, and n-heptane obtained using the same method and reported by Ziegler [5], but not yet published beyond his thesis. The coordinates are given in Table 1. We have purposely not drawn curves in Figs. 1 and 2, but only plotted data. We cannot easily 20.0
• • o o
tO0 oo O0 •
16.0 •
•m •
° • o0o 12.0 ,~
8.0
: •
decane cycl0hexane
n-heptane methyl-tertbutyl ether
E]
0..
4.0
D 0
0.0
I
100
I
200 Temperature, °C
I
I
300
400
Fig. 1. Estimates of critical loci for CO2-solvent mixtures as indicated. The first and last points in each series are the critical points for the pure materials. The cyclohexane and n-heptane data are from Ziegler [5].
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15-20
18 20.0
16.0
pyridine I dioxane I v methyl-ethyl ketone [
• [ ~ 3 D £3 Q D D (3°
• ethyl acetate I
~Dvvvvv v
12.0
! ~,
"
8.o
0.
0
4.0
•
I v
0.0
I
0
100
I
200 Temperature, °C
I
I
300
400
Fig. 2. Estimates of critical loci for CO2-solvent mixtures as indicated. The first and last points in each series are the critical points for the pure materials. The pyridine data are from Ziegler [5].
predict the results when attempting this flowinjection method on a mixture that is not Type I. For example, in an earlier work, we were unable to observe flat peaks for mixtures of CO2 and 1-octanol below 60°C. The behavior of this mixture is not consistent with Type I or Type II. In the present work, we were unable to observe fiat peaks for ethyl acetate, methyl-ethyl ketone, dioxane and decane co-solvents above 201°C, or for methylt-butyl ether above 176°C. The mixture CO2-decane is widely reported to be Type II. It is essential to realize that our method is actually a mass-transport-rate-sensitive procedure which, under ideal conditions, is influenced by the (pseudo)equilibrium established between the liquid and vapor phases in the flooded zone. The method is not a direct observation of 1-v phase separation. However, the flat-topped peaks are a consequence of the fact that removal of the liquid co-solvent from the flooded zone can never be faster than the product of the concentration of the co-solvent in the vapor phase and the total flow rate through the system. If the vapor-phase co-solvent concentration is limited by saturation, the peaks will be fiat. If the co-solvent mass-flow rate in the room temperature portion of the system or in the detector is ever slower than the co-solvent mass-flow rate in the oven, the method will not reflect the phase behavior occurring in the oven. Thus, care must be exercised in interpreting these data, particularly for mixtures more complicated than Type I.
However, we believe that the 1-v loci for mixtures with phase behavior more complicated than Type I can be estimated by this method, at least in some cases, over the segments of the 1-v critical loci where no other phase separation occurs. Better control of the temperature of the inlet system will be necessary to investigate mixtures with liquidliquid separations occurring at room temperature. For example, for a Type II mixture where injection is performed at a temperature above the upper critical solution temperature (but below the mixture critical temperature), the flow-injection method should behave as it does for a Type I mixture. When investigating mixture critical points near the critical point of pure C Q , it is possible that phase separation may~briefly occur when the mixture reaches the detector interface due to the temperature increase, even if there is no phase separation on the capillary tube at oven temperature. Artifacts can be prevented or minimized by operating the detector at a high temperature well above the critical temperature of the co-solvent, and with a well-designed interface to provide an abrupt temperature change, to minimize exposure of the mixture to the undesired intermediate conditions. Comparisons of our results with measurements reported by other methods have shown good agreement. Earlier, we compared our results with literature values derived from view-cell observations for CO2-toluene and CO2-methanol mixtures [3,4]. The average deviation in the mixture critical pressures (comparing our procedure with view-cell observations) was 1.1% and deviation never exceeded 2.3% when we ignored pressure drop in the capillary tube. The average deviation was 0.1% and never exceed 0.2% when we corrected for pressure drop [4]. Ziegler's experimental points for CO2cyclohexane mixtures using the flow-injection method [5] compare favorably with the observations of Shibata and Sandler [10]. The latter authors did not specifically report mixture critical points, but reported equilibrium phase compositions and densities over conditions apparently closely approaching mixture critical points. Although these authors only reported observations
T.L. Chester, B.S. Haynes / Journal of Supercritical Fluids 11 (1997) 15 20
19
Table 1 Temperature and pressure coordinates for estimates of critical loci for the solvents shown in mixture with C O 2 Temperature
Pressure(MPa)
(of) Methyl-t-butyl ether 31.1 44 50 55 60 70 80 90 91 100 110 115 120 125 130 135 140 145 150 161 176 180 186 201 210 224.1 240 250.3 263.8 267.1 280.4 314 344.7 347
Ethyl acetate
Methyl-ethyl Dioxane ketone 7.38
Decane
7,38
7.38
7.38
7.38
8.8
9.3
9.2
9,6 10,1 10.7 11.0
10.1 11.0 11.7 12.2
10.2 10.9 11.8 12.5
11.3 11.3 11.2 11.3
12.6 12.8 12.9 13.0
13.1 13.6
14.1 14.9
16.5
13.8
15.3
11.1 10.9 10.5 10.3 9.9 9.0 7.5
13.0
13.9
15.7
17.9 14.4 18.2
13.0
13.8
16.1
18.3
12.9 12.5
13.7 13.3 12.8
16.4 16.4 16.3
18.5 18.5 18.4
11.0
16.2 15.8
18.2 17.7
10.9 10.5 12.5
Cyclohexanea 7.39 8.9 10.0 10.8
14.5
Pyridinea 7.39 8.5 9.1 9.7 10.3 11.7 13.0 14.2 14.3
7.39
9.8
12.2
13.3
17.2
14.5
9.9
n-Heptanea
13.3
13.3
14.4
19.1
13.0
13.3
19.6
10.9
11.1
18.8
3.43 17.2 3.88 4.20 2.7 4.1 5.21 2.11 5.7
aAs reported by Ziegler [5]. at two temperatures, 93.5 a n d 137.9°C, a n d neither of these t e m p e r a t u r e s were used in Ziegler's experiments, the a p p a r e n t m i x t u r e critical pressures of Shibata a n d Sandler deviate by only 0.5% from i n t e r p o l a t i o n s o f Ziegler's flow-injection results. O u r results for C O z - d e c a n e do n o t c o m p a r e well with the d a t a for this mixture reported by Swaid et al. [11]. These a u t h o r s were also measuring e q u i l i b r i u m phase c o m p o s i t i o n s at several temperatures a n d were n o t directly observing mixture
critical points. They left m u c h larger gaps n e a r mixture critical points t h a n did Shibata a n d Sandler with CO2-cyclohexane. Thus, the actual m i x t u r e critical pressures are s o m e w h a t higher t h a n the highest pressures reported by Swaid et al., a n d close agreement with the c u r r e n t work should n o t necessarily be expected. (Our observations suggest that the critical pressures are a b o u t 3 - 1 3 % higher t h a n the highest pressures used by Swaid et al.) Regardless, more work will be needed to
20
T.L. Chester, B.S. Haynes/Journal of Supercritical Fluids 11 (1997) 15-20
validate the flow-injection procedure for estimating the l - v loci o f mixtures other than Type I. A l t h o u g h used here for CO2-co-solvent mixtures, the flow-injection peak-shape m e t h o d should be quite general. The only m a j o r requirement is the use o f a detector that can easily distinguish the injected co-solvent f r o m the p u m p e d primary fluid. Therefore, it should be possible to use this m e t h o d to examine mixtures such as toluene injected into butane using an ultraviolet absorption detector. In such an experiment, it would be necessary to ensure that conditions in the detector and tubing leading from the oven to the detector are always in the one-phase region. We think the easiest way to accomplish this would be to heat any ovendetector transfer tubing and the detector to a temperature well above the critical temperature corresponding with the pressure and the mixture under test. Setting the detector temperature well above the co-solvent critical temperature, when possible, and providing an abrupt temperature change f r o m the oven temperature, would be best to avoid potential confusion. With some fluids, the temperatures required will be too high for ordinary H P L C - t y p e U V detectors. Suitable safety precautions would also be required when investigating any flammable primary fluid since leaks in the oven or detector could lead to fire or explosion.
References [1] T.L. Chester, D.P. Innis, Quantitative open-tubular supercritical fluid chromatography using direct injection onto a retention gap, Analyt. Chem. 67 (1995) 3057.
[2] T.L. Chester, D.P. Innis, Dynamic film formation and the use of retention gaps with direct injection in open-tubular supercritical fluid chromatography, J. Microcolloid Separation 5 (1993)261. [3] J.W. Ziegler, J.G. Dorsey, T.L. Chester, D.P. Innis, Estimation of liquid-vapor critical loci for CO2-solvent mixtures using a peak-shape method, Analyt. Chem. 67 (1995) 456. [4] J.W. Ziegler, T.L. Chester, D.P. Innis, S.H. Page, J.G. Dorsey, in: K.W. Hutchenson, N.R. Foster (Eds.), Supercritical Fluid Flow Injection Method for Mapping Liquid-Vapor Critical Loci of Binary Mixtures Containing CO2, Innovations in Supercritical Fluids: Science and Technology, American Chemical Society, Washington, DC, 1995, p. 93. [5] J.W. Ziegler, 'Ain't Misbehavin'- fundamental investigations into supercritical fluid binary mixtures containing CO2 and supercritical fluid chromatography, Ph.D. dissertation, University of Cincinnati, 1996. [6] R.L. Scott, P.H. van Konynenburg, Van der Waals and related models for hydrocarbon mixtures, Discuss. Faraday Soc. 49 (1970) 87. [7] T.L. Chester, Supercritical fluid chromatography instrumentation, in: G. Ewing (Ed.), Analytical Instrumentation Handbook, Dekker, New York, 1990, p. 843. [8] E.J. Guthrie, H.E. Schwartz, Integral pressure restrictor for capillary SFC (supercritical fluid chromatography), J. Chromatogr. Sci. 24 (1986)236. [9] D. Ambrose, Critical constants, boiling points, and melting points of selected compounds, in: D.R. Lide (Ed.), Handbook of Chemistry and Physics, 73rd ed., CRC Press, Boca Raton, FL, 1992, pp. 6-49. [10] S.K. Shibata, S.1. Sandier, High-pressure vapor-liquid equilibria of mixtures of nitrogen, carbon dioxide, and cyclohexane, J. Chem. Engng Data 34 (1989) 419. [11] I. Swaid, D. Nickel, G.M. Schneider, NIR-spectroscopic investigations on phase behavior of low-volatile organic substances in supercritical carbon dioxide, Fluid Phase Equilibria 21 (1985) 95.