Supercritical Fluid Science and Technology - American Chemical

Chapter 6

Partition Coefficients of Poly(ethylene glycol)s in Supercritical Carbon Dioxide Downloaded by UNIV OF MISSOURI COLUMBIA on April 15, 2013 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch006

Manouchehr Daneshvar and Esin Gulari Chemical and Metallurgical Engineering Department, Wayne State University, Detroit, MI 48202

The coexisting phase compositions of poly(ethylene glycol) - carbon dioxide mixtures have been measured. Three polymer samples with average molecular weights of 400, 600, and 1000 were studied. The measurements were conducted at 313 and 323 Κ over a pressure range up to 28 MPa in a high pressure apparatus with countercurrent circulation. At equilibrium, both phases were sampled simultaneously and their compositions were determined. Poly(ethylene glycol) (PEG) amounts were measured by a colorimetric technique and molecular weight distribution of extracts were determined by Fast Atom Bombardment Mass Spectrometry (FAB-MS). It was observed that PEGs have appreciable solubilities in supercritical CO . At a given pressure and temperature, the lower molecular weight PEGs exhibited much higher solubility than the ones with higher molecular weights. The overall partition coefficients of the samples with different molecular weights indicated that the relative amounts of different fractions in a given mixture can be varied by changing the operating conditions. The individual n-mer partition coefficients for a sample of PEG(600) were calculated by combining the results from FAB-MS and colorimetric techniques. Two different regions of yield and selectivity with respect to the degree of polymerization were identified. 2

The polymers of ethylene oxide with the general formula of H-(OCH CH ) -OH are divided into two categories based on their molecular weights. The polymers with an average molecular weight in the range 200-20,000 are referred to as PEGs. The ethylene oxide polymers or polyethylene oxide) resins are the higher molecular weight members of this series. PEGs are soluble in water and they are used in cosmetics, lubricants and pharmaceuticals depending on their molecular weights. 2

2

0097-6156/89/Ό406-Ο072$Ο6.00/Ό © 1989 American Chemical Society

In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

n

Downloaded by UNIV OF MISSOURI COLUMBIA on April 15, 2013 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch006

6.

DANESHVAR A N D GULARI

Partition Coefficients ofPoly (ethylene glycol)s 73

In general, synthetic polymers exhibit a wide molecular weight distribution. Their separation into narrower molecular weight fractions is a fairly difficult task. Distillation is not an effective separation technique for these materials because of their low vapor pressures. The solubility of different fractions of a parent polymer in conventional liquid solvents are normally too high which makes the liquid extraction a nonselective technique for these materials. Fractionation of polymers by using supercritical fluids presents unique advantages and has attracted the attention of several investigators in the past few years (1-5). Supercritical fluid (SCF) extraction is a separation technique in which the extractant is a dense gas at temperatures and pressures above its critical point. The widespread interest in supercritical fluid processing is primarily due to the unique characteristics of the solvent. The solvation power of the solvent is controlled by its density; in the vicinity of the critical point, slight changes in pressure or temperature can produce large changes in density. In addition to the control of solvent power, there are other advantages in using a SCF as solvent. High diffusivity and low viscosity of a SCF cause it to penetrate into a polymer phase more effectively than a liquid solvent. The separation of the solute from the solvent can be achieved by pressure reduction or cooling or both. Therefore, it is possible to achieve a very selective and efficient fractionation. The feasibility of a supercritical polymer fractionation is determined by the variations of the solubility of a homologous series of compounds. Qualitatively, the solubility of a polymer decreases as the degree of polymerization increases. This concept has been illustrated by Krukonis (2) in supercritical fluid fractionation of some heat-labile, low molecular weight polymers. Another observation made by Kumar et. al. (3) is that, at a given pressure and temperature, the partition coefficient of an η-mer between the polymer phase and the SCF phase depends only on its chain length and doesn't depend on the compositions of polymers of different chain lengths in the two phases at equilibrium. This is an important observation because it implies the possibility of achieving optimum conditions for the separation of a given molecular weight fraction. In the literature, very little data on the solubility of polymers in supercritical fluids have been reported. The data are limited because experiments require precise detections of composition and molecular weight distribution. The study of the liquid solutes is even more complicated than solid solutes. For solid solutes, the solid phase can be assumed to remain pure and only the supercritical fluid phase is then sampled. In case of liquid solutes, the supercritical fluid is appreciably soluble in the solute and therefore both phases must be sampled. In this paper, the coexisting phase compositions of different average molecular weight PEG - carbon dioxide systems are presented. The equilibrium compositions have been measured at 313 and 323 Κ over a pressure range up to 28 MPa. The partition coefficients based on the average molecular weights of the polymer are reported. PEG with an average molecular weight of 600 is used to study the mass based partitioning of individual ti-mers between the SCF phase and the polymer phase. The experimental set up designed for sampling of both phases is described. The calibration data are reported for determining very small amounts of PEG in water by adapting a colorimetric technique. The effect of the molecular weight distribution of the parent polymer on the solubility data are discussed.


74

SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y

Apparatus and Procedure The apparatus was designed to obtain vapor-liquid equilibrium data at temperatures up to 373 Κ and pressures up to 35 MPa. A schematic diagram of the equipment is shown in Figure 1. A high pressure metering pump introduces C 0 to the system from a supply cylinder equipped with a siphon tube. Equilibrium is achieved by circulating both the liquid and vapor phases at approximate flow rates of 8 ml/min. The SCF phase is drawn from the top and driven to the bottom of the vessel by a reciprocating plunger pump while the polymer phase is drawn from the bottom and driven to the top by a second pump. The apparatus includes several quick-connects around the equilibrium vessel. These are helpful for loading the polymer and cleaning the system. The 150 ml vessel has an electric heating mantle with a temperature controller. The temperature is measured with a thermocouple installed inside the vessel to within ±0.3 K. The circulation lines are maintained at the vessel temperature with heating tapes and controllers which are run off the thermocouples attached to different locations on the lines. The pressure is measured by a pressure transducer installed on the vessel to within ±0.003 MPa. Two high pressure, six-port switching valves with sample loop volumes of 0.5350 (for the upper phase) and 0.1086 ml (for the lower phase) are installed on the circulation lines and are used for sampling.


2

After loading the polymer, the entire system is purged of air and then charged with C 0 . Once the desired C 0 loading is attained, the C 0 feed line is shut off and 2

2

2

the circulation pumps are started. At equilibrium, a sample from each phase is taken by switching the sample loop out of the system while the circulation is routed through a bypass. A typical equilibration time is approximately 30 min and the sampling is performed after allowing atleast an hour. The C 0 content of each phase is determined in a different manner. The details around the two sampling valves are indicated on the insets of Figure 1. When the sample loops are switched out of the system, the samples expand between two valves and the polymer precipitates in the line. The amount of C 0 in the supercritical phase is determined by throttling the expanded sample through valve 3 which is connected to a precision pressure gauge (Texas Instruments, Model 145). The gauge has a quartz Burdon tube element and measures pressure to a precision of 0.01%. The volume is known and the temperature of the sample is also measured after equilibration. In the polymer phase, the amount of C 0 is determined by carefully opening valve 4 and measuring the volume of C 0 2

2

2

2

corresponding to the displacement of water level in an inverted graduated tube which is immersed In C 0 saturated water. Over the range of data the volume of C 0 in the polymer phase measured at ambient conditions varies from 2 to 30 ml. The C 0 amounts in both phases are then determined from PVT data. 2

2

2

The polymer content of each phase is analyzed by flushing the sample lines with 50 ml of water. The concentration of the polymer in water is then determined by a colorimetric technique. Colorimetric Detection of P E G

A quantitative colorimetric detection of aqueous PEG mono-oleate solution was



6.



THERMOSTAT

Figure 1. Schematic diagram of the equipment and details of the sampling valves.


76



described by Brown and Hayes (6). Crabb and Persinger (7) have also described a similar procedure for determining the concentration of polyoxyethylene nonionic surfactants in parts per million concentration range. We adapted a procedure, similar to that of ref. (3) with small modifications. This procedure is based on the formation of a blue complex between PEG and ammonium cobaltothiocyanate solution. The ammonium cobaltothiocyanate solution is prepared by dissolving 15 gr of cobalt nitrate hexahydrate and 100 gr of ammonium thiocyanate in 500 ml distilled water. In a 60 ml separating funnel, 10 ml of PEG solution is reacted with 20 ml of ammonium cobaltothiocyanate solution at 20 °C. The complex is then extracted into 25 ml of chloroform in several repeated steps and the optical density of the blue chloroform solution at 320 nm is measured with a scanning diode array spectrophotometer (HP 8452A). Figure 2 shows a typical absorption spectra of PEG(600) at two different concentrations. There are two absorption peaks at 320 nm and 622 nm. The optical density of the peak at 320 nm is about six times larger than the one at 622 nm. Therefore, the sensitivity of 320 nm peak is much greater than that at 622 nm and it is used in our analysis. It is reported that the higher the degree of polymerization, the less is the amount of polyglycol required to form the blue complex (7). In fact, at least six ethylene oxide units are required in a chain for the color development. This implies that the absorbance varies with the length of PEG chain. Therefore, different calibration curves for each compound with different nominal molecular weight must be developed. Calibration data for the three polymer samples, PEG(400), PEG(600) and PEG(1000) were obtained using the colorimetric technique. In Figure 3, absorbance versus concentration is plotted for the three samples. The calibration 2

3

data were fitted to polynomials of the form y» a + bx + c x + dx where y is concentration in (mg of PEG/50 ml H 0 ) and χ is absorbance in AU. The calibration curves are summarized in Table I. 2

Table I.

Compound

Calibration data for polyethylene glycol)s with different molecular weights

(mg/50 ml) PEG(400) PEG(400) PEG(600) PEG(1000)

Coefficients

Cone. Range

0-10 10-50 1 -14 0-1.2

a

b

c

d

0 0 -0.535 -0.080

55.27 103.23 24.01 10.834

900.9 287.6 0 0

5723 2624 0 0

FAB • MS Fast Atom Bombardment Mass Spectrometry was used to obtain the molecular weight distribution of PEGs. FAB-MS like any mass spectrometry technique



Partition Coefficients ofPoly(ethylene glycol)s


Instrument Data

300

400

500 600 Wavelength (nm)

700

800

Figure 2. Absorption spectra of PEG(600) - Cobaltothiocyanate complex in Chloroform for two different concentrations.

0.7

PEG (mg/50ml) Figure 3. Calibration curves for polyethylene glycol) - cobaltothiocyanate complex at 320 nm.


78


utilizes the effects of electromagnetic fields to control the trajectory of isolated ions and thereby measures their mass to charge (m/z) ratio. In this work the formation of ions are accomplished by bombardment of polymer sample with a xenon gun. The samples are prepared by blending 1 μΙ of 1 wt% water solution of PEG with 0.5 μΙ of glycerol on the tip of a stainless steel probe. The xenon atoms striking the sample surface generate quasimolecular ions [M+H]+ resulting in a series of peaks, 44 units apart.


Results and Discussion The equilibrium phase compositions for PEG - carbon dioxide systems were experimentally measured. The data for PEG(400) - CO2 and PEG(600) - CO2 systems were obtained at 313 K. The PEG(400) - CO2 and PEG(1000) - CO2 systems were also studied at 323 K. The pressure versus composition diagrams are presented in Figures 4 and 5. The equilibrium compositions are reported as weight percent of the PEG. Appreciable solubilities of PEGs in supercritical carbon dioxide were observed. It was found that the solubility is a strong function of molecular weight of the polymer. Clearly, there is a minimum value of pressure above which the solubility of a given molecular weight polymer is detected. The minimum pressure increases with molecular weight along isotherms; the minimum pressure corresponding to the solubility limits are about 10 MPA for PEG(400) and about 15 MPA for PEG(600). At pressures above these limits, the solubility of C 0 in the polymer phase remains relatively constant as seen on the right hand branches of Figures 4 and 5. This condition affects the distribution of C 0 and polymer between the two phases. When the composition of the polymer phase is almost constant, a preferential partitioning of CO2 into the SCF phase drives a certain amount of polymer from the polymer phase into the SCF phase based on the criterion of phase equilibria. This effect together with the solvent density increase due to pressure cause the enhancement of the solubility of polymer in the SCF phase as observed on the left hand branches of Figures 4 and 5. 2

2

It was observed that, the solubility drops significantly with molecular weight at a given pressure and temperature. The difference in the relative solubility of PEG(400) and PEG(600) at 313 Κ is about an order of magnitude and that for PEG(400) and PEG(1000) at 323 Κ is more than an order of magnitude. The enhancement of solubility of PEG samples with increasing pressure is a complicated function of the molecular weight distribution. A sample with a very narrow molecular weight distribution is expected to exhibit a sharper solubility enhancement with pressure. The polydispersity or M / M of the PEG samples are typically high; value of 1.5 is quoted by the supplier. The measured solubility is a composite solubility with contributions from different molecular weight fractions. The composite solubility builds up slowly because the lowest molecular weight fraction starts dissolving at a certain threshold pressure and as pressure is increased, the next higher molecular weight fraction starts dissolving while a new equilibrium is established for the first fraction. It is evident from Figures 4 and 5 that, the solubility of C 0 in the polymer w

n

2


Partition Coefficients ofPoly (ethylene glycot)s


30.0-

ζ 25.0 H

"cT 20.0·


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£ 3

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i

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PEG600 POLY. PH.

•

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1 ι I • I MM III)—'

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Figure 4. Pressure-Composition diagrams for polyethylene glycol) carbon dioxide systems at 313 K. 30.0-

25.0 H •Γ

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r

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1

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10'

10" 10 WT. % PEG

10'

Figure 5. Pressure-Composition diagrams for poly(ethylene glycol) carbon dioxide systems at 323 K.


80


phase displays two types of behavior along an isotherm at low and high pressures. At low pressures, there is a linear relation between C 0 amount dissolved in the polymer and the overall pressure. The slope of a linear fit (wt% C 0 / MPa) in the low pressure region was found to be 2.04 for PEG(400) at 313 K, 2.13 for PEG(600) at 313 K, 1.00 for PEG(400) at 323 Κ and 2.08 for PEG(1000) at 323 K. At high pressures, the solubility of C 0 in polymer phase remains almost constant. The effect of temperature on the solubility of PEG(400) in the SCF phase and C 0 in the polymer phase is shown in Figures 6 and 7 respectively. In the SCF phase (Figure 6), a temperature change of 10 °C dose not affect the solubility of PEG(400) in C 0 . This observation suggests that the effects of the vapor pressure of solute and the density of the solvent are to some extent compensating. In the polymer phase (Figure 7), the solubility of C 0 drops with temperature because C 0 is very volatile and evaporates out of the liquid phase very effectively when temperature is increased from 313 to 323 K. If the PEGs are assumed to be monodisperse, then a partition coefficient can be calculated by dividing the weight fraction of PEG in SCF phase by the weight fraction of PEG in polymer phase. Figure 8 shows plots of these partition coefficients as a function of pressure and temperature for PEGs with different molecular weights. The behavior of the partition coefficient as a function of pressure displays interesting features for a separation process. Depending on the choice of pressure, the concentration of PEGs with different molecular weights can be varied relative to each other. If the partition coefficient of an η-mer does not depend on the compositions of other chain lengths, as claimed in ref.(2), the yield as well as selectivity of a multistage operation can be controlled by pressure programming. Polymer samples are inherently polydisperse. Molecular weight distribution of parent PEG(600) as measured by FAB-MS is shown in Fig. 9. For this nominal molecular weight PEG, the degree of polymerization ranges from n«8 to n»18. This level of polydispersity is quite typical of PEGs. In order to optimize a fractionation process, it is imperative to detect the overall solubility as well as nmer distribution in the two coexisting phases. The molecular weight distribution of polymer samples isolated from the coexisting phases for PEG(600) at T-313K and P=27.03 MPa were measured by FAB-MS. When the overall PEG solubility was weighted by the measured distribution, the solubility of each n-mer was calculated at the specified conditions. The individual n-mer partition coefficients are then defined as the weight fraction of the n-mer in the SCF phase divided by the weight fraction of the same n-mer in the polymer phase. Figure 10 shows a plot of the logarithm of the individual n-mer partition coefficients with respect to molecular weight. Two distinct relations between the partition coefficient and the molecular weight are observed. At a given temperature and pressure, there is a critical molecular weight below which the partition coefficients of the neighboring η-mers do not depend on molecular weight. Above this critical molecular weight, the partition coefficients of neighboring η-mers decrease exponentially with molecular weight. This behavior is expected to change with pressure. In the low molecular weight region, the relative yields are high while the selectivity is low. In the high 2

2

2


2

2

2

2


6.



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WT


P,

0.80.4- ι — j —

0.0-

5.0

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10.0

15.0

1

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1

— Γ

20.0

25.0

30.0

Pressure (MPa) Figure 6.

Solubility of PEG(400) in supercritical C 0 as a function of 2

pressure at 313 and 323 K.

50.0· 40.0Ο Ο

30.020.0Legend

10.00.0

—ι

0.0

5.0

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Pressure (MPa) Figure 7.

Solubility of CO2 in PEG(400) as a function of pressure at 313 and

323 Κ.




40.0 Legend 30.0 H

20.0 H

•

M.W. 1000 AT 323 Κ

O

M.W. 400 AT 323 Κ

•

M.W. 400 AT 313 Κ

Δ

M.W. 600 AT 313K

/ Ο

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Pressure (ΜΡα) Figure 8. Partition coefficients for poly(ethylene glycol)s with different average molecular weights.



6. DANESHVAR A N D GULARI

Partition Coefficients ofPoly (ethylene glycol)s

100% τ

12

13 14

MWD of Parent

11

PEG(600)

H-(OCH CH ) -OH

15

2

2

n

10

,48*

60 +

16 7Z3

n=8 40 +

17

18

19 20

lilil 300

r q l l l l l l l l l p ι 1 l'V I f 1 f j I Ί f I I f I t ί |" I Ρ Γ I Ί l'Vll | f I ! I I Ί .1 I I I ι 400 500 600 700 800 900

Figure 9.

FAB mass spectrum of parent PEG(600).


83



200.0

400.0

600.0

800.0

1000.0

Molecular Weight Figure 10. Individual n-mer partition coefficients of PEG(600) at 313 Κ and 27.03 MPa.


6.


Parution Coefficients ofPoly (ethylene glycol)s

molecular weight region, the selectivity is high while the yield is low. The n-mer partition coefficients can provide means for optimizing a supercritical fractionation process in terms of yield and selectivity.


Conclusions The equilibrium solubility data have been presented for polyethylene glycol)s carbon dioxide systems at 313 and 323 Κ over a pressure range up to 28 MPa. The solubility of lower molecular weight PEGs in supercritical CO2 were found to be appreciable. At a fixed temperature, there is a minimum pressure above which a given molecular weight PEG is soluble in CO2. PEG solubility in supercritical CO2 drops significantly with molecular weight. The enhancement of PEG solubility occurs over a pressure range when the amount of CO2 soluble in the polymer phase becomes almost constant. The partitioning of individual n-mers between the S C F phase and the polymer phase exhibits two types of behavior in terms of yield and selectivity. The higher molecular weight neighboring η-mers exhibit high selectivity and relatively lower yield while the lower molecular weight neighboring n-mers show almost no selectivity and higher yield.

Acknowledgement The authors gratefully acknowledge the financial support from the National Science Foundation Grant No. RII-8503643 and the University Science Partners, Inc..

Literature Cited 1. Kumar, S. K.; Suter, U. W.; Reid, R. C. Fluid Phase Equilibria 1986, 29, 373. 2. Krukonis, V. Polym. News 1985, 11, 7. 3. Kumar, S. K.; Chhabria, S. P.; Reid, R. C.; Suter, U. W. Macromolecules 1987, 20, 2550. 4. Yilgor, I.; McGrath,J.M. Polymer Bulletin 1984, 12, 499. 5. Scholsky, Κ. M.; O'Connor, Κ. M.; Weiss, C. S. J. Applied Polym Sci. 1987, 33, 2925. 6. Brown, E. G.; Hayes, T. J. Analyst 1955, 80, 755. 7. Crabb, N. T.; Persinger, Η. E. J. Am. Oil Chem. Soc. 1964, 41, 752. RECEIVED June 9,

1989


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Supercritical Fluid Science and Technology - American Chemical

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