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Ind. Eng. Chem. Res. 2006, 45, 3373-3380

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Phase Behavior of Binary and Ternary Mixtures of Poly(decyl acrylate)-Supercritical Solvents-Decyl Acrylate and Poly(decyl methacrylate)-CO2-Decyl Methacrylate Systems Hun-Soo Byun* and Dong-Hyun Lee Department of Chemical Engineering, Yosu National UniVersity, Yosu, Chonnam 550-749, South Korea

Experimental cloud-point data up to 205 °C and 2300 bar are measured for binary and ternary mixtures of poly(decyl acrylate) [poly(DA)]-supercritical solvents-decyl acrylate (DA) and poly(decyl methacrylate) [poly(DMA)]-CO2-decyl methacrylate (DMA). Also, the cloud-point curves show the binary mixtures for poly(DA) in supercritical CO2, propane, propylene, butane, 1-butene, and dimethyl ether (DME). The phase behaviors for the poly(DMA)-CO2-DMA system are measured in changes of the pressure-temperature (P-T) slope and with DMA concentrations of 4.4, 6.7, 11.2, 22.8, and 32.9 wt %. Adding 41.6 wt % DMA to the poly(DMA)-CO2 solution significantly changes the phase behavior; the curves take on the appearance of a typical LCST boundary. The cloud-point curves for the poly(DA)-CO2 system with 0, 7.5, 14.9, 23.4, and 34.9 wt % DA changes the P-T curve from an upper critical solution temperature (UCST) region to a lower critical solution temperature (LCST) region as the decyl acrylate concentration increases. Adding 40.2 wt % DA to the poly(DA)-CO2 solution significantly changes the phase behavior. The cloud-point curve takes on the appearance of a typical lower LCST region. High-pressure phase behavior data are obtained for the CO2-DA and CO2-DMA systems at temperatures in the range 40-120 °C and pressures up to 189 bar. The CO2-DA and CO2-DMA systems exhibit type-I phase behavior with a continuous mixture-critical curve. The experimental results for the CO2-DA and CO2-DMA mixtures are modeled using the Peng-Robinson equation of state. A good fit of the data is obtained from the Peng-Robinson equation of state using two adjustable parameters for CO2-DA and CO2-DMA systems. Introduction Several studies have reported that a polar cosolvent can shift a polymer-supercritical fluid solvent cloud-point curve at extremely low temperatures and pressures.1-4 Recently, we have demonstrated that it is possible to dissolve polar (meth)acrylate polymer in supercritical CO2 over a large temperature range at modest pressure if (meth)acrylate monomer is used.5,6 A liquid monomer can greatly enhance polymer solubility in a given solvent due to several different reasons. If the solvent is highly expanded, the addition of a dense, liquid monomer reduces the free volume difference between the polymer and the solvent.7 Also, because the cosolvent has most of the same physicochemical properties as a repeat unit of the polymer, energetically favorable supercritical solvent-polymer interactions are expected to expand the single-phase region.8 Interpreting the effect of a cosolvent added to a supercritical fluid solvent is slightly more complicated, since increasing the system pressure reduces the free volume difference between the solvent and the polymer and increases the probability of interaction between the polymer, solvent, and cosolvent segments.1 The high-pressure, polymersupercritical fluid solvent-cosolvent studies reported in the literature show that cloud points monotonically decrease in pressure and temperature with the addition of a polar cosolvent as long as the cosolvent does not form a complex with the polar repeat units in the polymer.1-3 In these cases, the cosolvent effect is directly related to the polar forces of attraction attributed to the cosolvent and to the increase in solvent density resulting from the addition of a liquid cosolvent to a supercritical fluid solvent. * To whom correspondence should be addressed. Tel.: +82-61-6593296. Fax: +82-61-653-3659. E-mail: [email protected].

The location of the cloud-point curve is a reflection of the free volume difference between the dense polymer and the expanded CO2 rather than the balance of intermolecular interactions.5,9 The focus of this work is to present the determination of the impact of decyl acrylate (DA) and decyl methacrylate (DMA) cosolvent on the phase behavior of the poly(DA)-CO2 and poly(DMA)-CO2 systems. The phase behavior for these ternary poly(DA)-supercritical CO2-DA mixtures provides the information needed on the regions where homogeneous polymerization can occur in the presence of excess monomer. McHugh et al.9 have demonstrated that the poly(butyl acrylate)-CO2 cloud-point curves are almost vertical at ∼1100-2700 bar at high temperatures. Byun et al.10 have demonstrated that poly(hexyl methacrylate)-CO2 phase-behavior curves present the upper critical solution temperature (UCST) curve at ∼14202200 bar and below 190 °C. The intermolecular interaction between CO2 and a poly(DA) unit is expected to be similar in strength to that between CO2 and a poly(DMA) unit. Hence, the difference in phase behavior is attributed to the different degree of chain flexibility for these two polymers, which implies a more unfavorable conformational entropy of mixing for poly(octadecyl acrylate).6 The key issue is how to account for the intra- and intersegmental interactions of many segments of the polymer relative to the small number of segments in a solvent molecule. Experimental phase-behavior data on small molecules for binary CO2-DA and CO2-DMA systems is obtained to complement the poly(DA)-CO2-DA and poly(DMA)-CO2DMA studies presented here, since there are no literature phasebehavior data available on this mixture. The primary purpose for obtaining CO2-DA and CO2-DMA systems is to determine whether CO2 and DA or DMA form multiple phases in the

10.1021/ie0507070 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/29/2005

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viewed on a video monitor using a camera coupled to a borescope (Olympus Corp., model F100-038-000-50) placed against the outside of the sapphire window. Light is transmitted into the cell with a fiber optic cable connected at one end to a high-density illuminator (Olympus Optical Co., model ILK-5) and at the other end to a borescope. Mole fractions are accurate to within (0.0025. Cloud points are measured and reproduced at least twice to within (2.8 bar and (0.3 °C. Bubble-, dew-, and critical-point transitions for the CO2-DA and CO2-DMA mixtures are measured and reproduced at least twice to within (0.5 bar and (0.2 °C. Materials Figure 1. Schematic diagram of the high-pressure experimental apparatus used in this study.

pressure-temperature composition regions explored in the poly(DA)-CO2-DA and poly(DMA)-CO2-DMA studies. The experimental data of CO2-DA and CO2-DMA systems are fitted to the Peng-Robinson equation of state,11 and the phase behavior for these binary solvent mixtures is calculated at elevated operating temperatures and pressures. The design and operation of a separation process requires knowledge of phasebehavior experimental data. In particular, high-pressure phase equilibrium data for binary mixtures containing supercritical CO2 will be needed for the design and operation of processing plants, industrial application, and supercritical fluid extraction.12,13

Carbon dioxide (99.8% minimum purity) was obtained from Daesung Industrial Gases Co. (Korea), and poly(decyl acrylate) (GPC: Mw ) 130 000), poly(decyl methacrylate) (GPC: Mw ) 100 000), decyl acrylate (97% purity), and decyl methacrylate (97% purity) were obtained from Scientific Polymer Products, Inc. and used as received. To prevent decyl acrylate and decyl methacrylate polymerization, 2,6-di-tert-butyl-4-methyl phenol (Aldrich, 99% purity) was used as an inhibitor at a concentration of 0.005 times the amount of decyl acrylate and decyl methacrylate. Since the poly(decyl acrylate) was supplied in a toluene solution, the polymer solution was placed under vacuum for at least 10 h using a rotary evaporator (Tamato Scientific Co., model RE-47) for toluene removal.

Experimental Section The bubble-point, dew-point, and cloud-point curves are obtained with a high-pressure, variable-volume cell, described in detail elsewhere.14,16 Figure 1 shows a schematic diagram of the experimental apparatus used for obtaining pressurecomposition isotherms for the CO2-DA and CO2-DMA mixtures14,15 and cloud-point curves for poly(DA)-CO2-DA and poly(DMA)-CO2-DMA mixtures.16,17 Cloud points are measured for the polymer solutions at a fixed poly(DA) and poly(DMA) concentration of 5.0 ( 0.5 wt %, which is a typical value for the concentrations used for polymer-supercritical fluid solvent studies.21 Polymer is loaded into the cell to within (0.002 g, and then, the cell is purged with nitrogen followed by CO2 to ensure that all of the air is removed. Liquid monomer is injected into the cell to within (0.002 g using a syringe, and CO2 is transferred into the cell gravimetrically to within (0.004 g using a high-pressure bomb. The mixture is compressed to the desired pressure with an internal piston displaced with water in a high-pressure generator (HIP Inc., model 37-5.75-60). The pressure of the mixture is measured with a Heise gauge (Dresser Ind., model CM-108952, operating range 0-3450 bar, accurate to within (3.5 bar). The temperature in the cell is measured using a platinum-resistance thermometer (Thermometrics Corp., class A) connected to a digital multimeter (Yokogawa, model 7563, accurate to within (0.005%). The system temperature is typically maintained to within (0.2 °C below 200 °C. The mixture inside the cell is

Results and Discussion Table 1 lists the properties of the solvents used in this study.18-22 For the poly(DA)-solvent systems, CO2, propylene, propane, butane, 1-butene, and dimethyl ether (DME) were used as solvents. Propane and propylene also have similar critical properties and polarizabilities, but propylene possesses both dipole and quadrupole moments which favorably interact with the DA groups in the polymer. Hence, by using propane and propylene, the effect of solvent polarity can be studied independent of polarizability effects. On the other hand, the comparison of propane and butane, or propylene and 1-butene, yields information on the effect of solvent size and, hence, polarizability on the phase behavior independent of polarity effects. Like propane, n-butane does not possess an appreciable dipole moment and, therefore, it interacts with the polymer only through dispersion and induction forces. 1-Butene, like propylene, interacts with the DA segments through quadrupolar and dipolar forces. DME is a Bronsted base (proton acceptor) which can form strong hydrogen bonds with the DA units in the poly(DA) polymers. Phase Behavior of the Poly(DA)-CO2-DA System. Table 2 and Figure 2 show the cloud-point behavior of the poly(DA)CO2-DA mixture obtained in this study. The phase behavior of the poly(DA)-CO2-DA (0.0 wt % DA) mixture presented a negative slope that dissolved at a temperature of 206 °C and a pressure of 1936 bar. With 7.5 wt % DA added to the solution,

Table 1. Critical Temperatures, Critical Pressures, Critical Densities, Polarizabilities, Dipole Moments, and Quadrupole Moments of the Solvents Used in This Study18-22 solvent

Tc (°C)

Pc (bar)

Fc (g/cm3)

R (×1025 cm3)

m (D)

Q (esu cm2)

other interactions

CO2 propane propylene n-butane 1-butene dimethyl ether

31.0 96.7 91.9 152.1 146.4 126.8

73.8 42.5 46.2 38.0 39.7 53.0

0.469 0.217 0.236 0.228 0.234 0.258

26.5 62.9 62.6 81.4 82.4 52.2

0.0 ∼0.08 0.37 -0.0 0.34 1.3

-4.3 1.2 2.5

polar moments, complexes with DA

2.5

polar moments, complexes with DA hydrogen bonds with DA

Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3375 Table 2. Experimental Cloud-Point Data for the Poly(DA)-CO2-(DA) System Measured in This Studya T (°C)

P (bar)

178.3 186.8 195.8

5.1 wt % Poly(DA) + 0.0 wt % DA 1936.2 CP 195.9 1485.9 1770.7 CP 206.5 1449.3 1500.3 CP

transition

T (°C)

P (bar)

transition CP CP

144.4 145.3 147.3 150.8

5.2 wt % Poly(DA) + 7.5 wt % DA 2096.9 CP 152.5 1350.0 1848.6 CP 165.2 1113.8 1558.3 CP 181.0 1031.7 1443.1 CP

CP CP CP

90.0 101.0 120.0

5.1 wt % Poly(DA) + 14.9 wt % DA 1543.1 CP 135.5 804.1 1225.9 CP 158.2 767.2 856.2 CP 180.5 751.0

CP CP CP

48.4 59.5 81.6 101.9

5.8 wt % Poly(DA) + 23.4 wt % DA 1141.0 CP 119.1 525.2 655.5 CP 140.4 538.3 534.1 CP 161.8 552.1 520.4 CP 177.1 557.2

CP CP CP CP

48.1 56.7 77.9 98.9

4.7 wt % Poly(DA) + 34.9 wt % DA 159.7 CP 117.5 296.6 181.0 CP 137.9 341.7 225.9 CP 160.5 373.5 267.2 CP 178.8 385.5

CP CP CP CP

a

BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

Figure 3. Phase behavior of the poly(decyl acrylate)-CO2-DA (40.2 wt % decyl acrylate) system obtained in this study. Open circles represent fluid f liquid + liquid transitions, closed squares represent fluid f liquid + vapor transitions, and closed circles represent liquid + liquid + vapor (LLV) data. The concentration of polymers in solution is ∼5 wt %. Table 3. Experimental Cloud-Point, Bubble-Point, and Liquid-Liquid-Vapor Data for the Poly(DA)-CO2-DA System Measured in This Studya 6.0 wt % Poly(DA) + 40.2 wt % DA T (°C)

P (bar)

transition

T (°C)

P (bar)

transition

234.8 200.3

CP CP

138.3

BP

200.0

LLV

159.5 142.4

296.9 270.0

Cloud-Point CP 121.5 CP 100.8

80.0 73.7

157.9 151.4

Bubble-Point BP 63.0 BP

100.0

183.0

Liquid-Liquid-Vapor LLV 111.0

a

BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

Figure 2. Impact of decyl acrylate on the phase behavior of the poly(decyl acrylate)-CO2-DA (x wt % decyl acrylate) system. The concentration of the polymer in solution is ∼5 wt %.

the cloud-point curve exhibits upper critical solution temperature (UCST) region type phase behavior with a negative slope. With 14.9 wt % DA in solution, the cloud-point pressure shows the UCST region behavior at a pressure up to 1540 bar and within a temperature range from 90 to 180 °C. The phase behavior of a poly(DA)-CO2-DA (23.4 wt % DA) mixture exhibits the U-LCST type phase behavior from a positive slope at low pressures to a negative slope which rapidly increases at 60 °C. If 34.9 wt % DA is added to the solution, the cloud-point curve exhibits lower critical solution temperature (LCST) type phase behavior of a positive slope at 48-179 °C. The phase behavior of the poly(DA)-CO2-DA (34.9 wt % DA) system shows 1.8 bar/°C in positive slope. The effect of DA cosolvent on the phase behavior is similar to that observed for the poly(butyl acrylate)CO2-butyl acrylate system.9 When 40.2 wt % DA is added to the poly(DA)-CO2 solution, the cloud-point curve shown in Figure 3 and Table 3 takes on the typical appearance of a lower critical solution temperature (LCST) boundary. At 120 °C, the phase boundary has shifted from 300 to 230 bar as the concentration of DA is increased from 34.9 to 40.1 wt %. The poly(DA)-CO2-DA (40.2 wt %

Figure 4. Impact of the phase behavior of poly(decyl acrylate) dissolved in supercritical propane, propylene, butane, 1-butene, and dimethyl ether (DME). The concentration of polymers in solution is ∼5 wt %.

DA) phase behavior curve intersects a liquid f liquid + vapor (LV) curve at ∼85 °C and ∼160 bar. A liquid and a vapor phase coexist at pressures below this curve, and the LV curve switches to a liquid1 + liquid2 + vapor (LLV) curve at temperatures greater than about 85 °C. The initial slope of the poly(DA)CO2-DA LCST curve at the lowest pressures is ∼1.6 bar/°C. Figure 4 and Table 4 show the phase-behavior curve of poly(DA) dissolved in supercritical DME, propane, propylene, butane, and 1-butene. The cloud-point behavior for these systems exhibits LCST curves with a positive slope. The poly(DA)alkane system is presented at the temperature range of 60-180 °C and a pressure up to 285 bar. At 120 °C, the phase-behavior

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Table 4. Experimental Cloud-Point Data for the Poly(DA)-Solvent Systems Measured in This Studya T (°C)

P (bar)

58.9 80.6 102.1 119.4

4.6 wt % Poly(DA) + 95.4 wt % Propane 169.0 CP 141.5 258.6 181.0 CP 162.2 273.5 205.9 CP 180.4 285.9 228.6 CP

CP CP CP

60.9 83.0 99.6 120.0

5.4 wt % Poly(DA) + 94.6 wt % Propylene 96.6 CP 142.2 245.2 142.4 CP 158.6 256.9 174.1 CP 180.5 277.6 208.6 CP

CP CP CP

81.4 102.4 120.9

4.8 wt % Poly(DA) + 95.2 wt % n-Butane 19.0 CP 140.9 102.4 50.0 CP 162.1 126.6 68.3 CP 181.6 143.1

121.9 141.7

6.5 wt % Poly(DA) + 94.4 wt % 1-Butane 53.5 CP 162.3 119.0 87.9 CP 181.7 139.7

101.5 119.3 141.7

transition

T (°C)

P (bar)

4.4 wt % Poly(DA) + 95.6 wt % DME 44.5 CP 159.7 156.9 81.0 CP 181.8 177.6 129.3 CP

transition

Table 5. Experimental Cloud-Point Data for the Poly(DMA)-CO2-DMA System Measured in This Studya T (°C)

P (bar)

164.4 166.7

4.3 wt % Poly(DMA) + 4.4 wt % DMA 2332.8 CP 171.1 1825.9 2074.1 CP 185.1 1493.4

CP CP

144.3 144.9 145.4

5.1 wt % Poly(DMA) + 6.7 wt % DMA 2094.8 CP 152.7 1565.1 1936.2 CP 165.2 1309.7 1863.8 CP 180.1 1205.2

CP CP CP

104.0 117.0

4.9 wt % Poly(DMA) + 11.2 wt % DMA 1656.9 CP 131.5 1095.2 1253.8 CP 147.8 1007.9

CP CP

CP CP CP

59.5 79.8 101.0

4.9 wt % Poly(DMA) + 22.8 wt % DMA 1014.1 CP 119.8 594.5 663.5 CP 147.5 591.4 600.7 CP

CP CP

41.8 61.9 80.0

4.9 wt % Poly(DMA) + 32.9 wt % DMA 221.7 CP 99.3 315.2 249.3 CP 120.3 350.0 280.0 CP 143.3 377.9

CP CP

transition

T (°C)

P (bar)

transition

CP CP

CP CP CP

a BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

a BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

Figure 6. Impact of decyl methacrylate on the phase behavior of the poly(decyl methacrylate)-CO2-DMA (x wt % decyl methacrylate) system. The concentration of polymers in solution is ∼5 wt %. Figure 5. Impact of the phase behavior of poly(decyl acrylate) dissolved in supercritical CO2 and dimethyl ether (DME). The concentration of polymers in solution is ∼5 wt %.

boundary has shifted from 230 bar (propane) to 70 bar (butane), and this is due to the polarizability difference of propane (62.9 × 10-25 cm3) and butane (81.4 × 10-25 cm3). The poly(DA)alkene system is shown at the temperature range of 60-180 °C and a pressure as low as 277 bar. At a temperature of 140 °C, the cloud-point curve is reduced from 240 bar (propylene) to 80 bar (1-butene). As shown in Table 1, this seems to be due to the polarizability and dipole moment differences for propylene and 1-butene. Figure 5 is a comparison of the phase behavior between poly(DA)-supercritical CO2 and poly(DA)-supercritical DME. As shown in Figure 5, the DME has a dipole moment of 1.3 D18 and does not possess a quadrupole moment. The lower cloud point of the poly(DA)-DME system seems to be due to a hydrogen bond and a large dipole moment. Phase Behavior of the Poly(DMA)-CO2-DMA System. Table 5 and Figure 6 show the phase-behavior data for the poly(DMA)-CO2-DMA (x wt % DMA) system obtained in this study. The poly(DMA) does not dissolve in pure CO2 up to a temperature of 230 °C and a pressure of 2700 bar. When 4.4 and 6.7 wt % DMA is added to the poly(DMA)-CO2-DMA solution, the cloud-point curve exhibits UCST type phase

behavior of a negative slope within the range of temperature from 144 to 185 °C. The cloud-point curves for the poly(DA)CO2-DA (7.5 wt % DA) and poly(DMA)-CO2-DMA (6.7 wt % DMA) systems are shown in Figures 2 and 6, respectively. The phase behavior for the poly(DA)-CO2-DA mixture is shown at a temperature below 180 °C and a pressure of 1000 bar, and on the other hand, the behavior for the poly(DMA)CO2-DMA mixture is presented at 180 °C and 1200 bar. With 11.2 wt % DMA added to the solution, the cloud-point curve exhibits a UCST type phase of a negative slope. The cloud-point behavior is shown in a range of pressure from 1007 to 1656 bar and in the temperature range 104-147 °C. The poly(DMA)-CO2-DMA (22.8 wt % DMA) mixture exhibits the UCST type phase behavior of a negative slope at low pressures, that rapidly increases at 59 °C. The phase behavior of the poly(DMA)-CO2-DMA (32.9 wt % DMA) system shows 1.6 bar/°C in positive slope. When 41.6 wt % DMA is added to the solution, the phasebehavior curve exhibits LCST type cloud-point behavior with a positive slope. As shown in Figure 7 and Table 6, the poly(DMA)-CO2-DMA cloud-point (LCST) curve intersects the LV curve at 75 °C and 140 bar with 41.6 wt % DMA. A liquid and a vapor phase coexist at pressures below this curve. Note that the LV behavior curve switches to a liquid + liquid + vapor (LLV) curve at temperatures greater than 75 °C. The slope of

Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3377 Table 7. Experimental Isotherm Data for the CO2-DA System Obtained in This Studya DA mole frac

Figure 7. Phase behavior of the poly(decyl methacrylate)-CO2-DMA (41.6 wt % decyl methacrylate) system obtained in this study. Open circles represent fluid f liquid + liquid (L + L) transitions, closed squares represent fluid f liquid + vapor transitions, and closed circles represent liquid + liquid + vapor (LLV) data. The concentration of polymers in solution is ∼5 wt % Table 6. Experimental Cloud-Point, Bubble-Point, and Liquid-Liquid-Vapor Data for the Poly(DMA)-CO2-DMA System Measured in This Studya 5.6 wt % Poly(DMA) + 41.6 wt % DMA T (°C)

P (bar)

156.1 142.9 127.5

transition

T (°C)

P (bar)

transition

295.2 283.5 257.6

Cloud-Point CP 112.4 CP 97.7 CP 83.3

236.2 199.0 174.8

CP CP CP

71.2 63.0

139.7 128.6

Bubble-Point BP 53.1 BP

97.0

168.0

Liquid-Liquid-Vapor LLV 84.0

117.2

BP

153.0

LLV

P (bar)

transition

DA mole frac

P (bar)

transition

71.6 58.1 47.8 37.4 30.7

BP BP BP BP BP

0.009 0.011 0.033 0.059 0.081 0.136

94.0 93.3 93.8 91.6 84.8 81.6

T ) 40 °C BP 0.213 BP 0.289 BP 0.408 BP 0.520 BP 0.611 BP

0.009 0.011 0.033 0.059 0.081 0.136

120.3 125.5 128.6 130.5 128.3 122.4

T ) 60 °C DP 0.213 DP 0.289 CP 0.408 BP 0.520 BP 0.611 BP

100.0 78.8 63.5 48.6 37.8

BP BP BP BP BP

0.009 0.011 0.033 0.059 0.081 0.136

145.7 154.8 167.6 169.0 166.9 154.7

T ) 80 °C DP 0.213 DP 0.289 CP 0.408 BP 0.520 BP 0.611 BP

128.3 105.3 80.0 59.3 44.1

BP BP BP BP BP

0.009 0.011 0.033 0.059 0.081 0.136

162.8 175.9 197.2 200.3 198.1 186.4

T ) 100 °C DP 0.213 DP 0.289 CP 0.408 BP 0.520 BP 0.611 BP

154.8 125.0 94.8 69.7 53.3

BP BP BP BP BP

0.009 0.011 0.033 0.059 0.081 0.136

170.7 187.8 220.0 222.4 222.6 212.4

T ) 120 °C DP 0.213 DP 0.289 DP 0.408 CP 0.520 BP 0.611 BP

179.0 144.1 106.9 79.3 60.3

BP BP BP BP BP

a

BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

the poly(DMA)-CO2-DMA LCST curve, ∼1.7 bar/°C, is approximately 45% greater than that observed for binary poly(isobutylene)-alkane mixtures, as reported by Zeman and Patterson.23 Phase Behavior of CO2-DA and CO2-DMA Mixtures. Phase-behavior data for the CO2-DA and CO2-DMA systems are measured and reproduced at least twice to within (0.3 bar and (0.2 °C for a loading of the cell. The mole fractions are accurate to (0.002. CO2-DA and CO2-DMA mole fractions for the solubility isotherms at 40-120 °C have an estimated accumulated error of less than (1.0%. Table 7 and Figure 8 present the experimental data for the CO2-DA system obtained in this study. Table 8 and Figure 9 show the phase-behavior data and the pressure-composition (P-x) curve of the CO2-DMA mixtures. Figure 8 shows the pressure-composition (P-x) isotherms at 40, 60, 80, 100, and 120 °C and at pressures up to 222 bar. Figure 9 presents the experimental phase behavior at 40-120 °C and at pressure up to 230 bar. The P-x isotherms shown in Figures 8 and 9 are consistent with those expected for type-I systems,12,24 where a maximum occurs in the continuous mixture-critical curve. Liquid-liquid-vapor (LLV) equilibrium was not observed under these conditions. The behavior exhibited in the CO2-DA and CO2-DMA systems is consistent with that of the CO2-butyl acrylate,9 CO2-butyl methacrylate,5 and CO2-octadecyl acrylate6 systems. Note that the bubble-point portions of the P-x isotherms for the CO2-DA and CO2-DMA systems are convex toward lower pressures, which means that

a BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

Figure 8. Experimental isotherms for the CO2-decyl acrylate system obtained in this work at 40, 60, 80, 100, and 120 °C.

CO2 is very miscible in the DA- and DMA-rich liquid phase. The lower solubility of CO2 in the DA is likely due to the steric hindrance of the decyl chain that prevents facile complex formation between CO2 and the acrylate carbonyl oxygen. Moreover, the strength of a CO2-DA complex is expected to be lower than that of a CO2-DMA complex, since the electron donating character of DA is less than that of DMA due to its larger molar volume. The high solubility of CO2 in the DMArich liquid is probably due to the formation of a weak complex between DMA and CO2. Kazarian et al.25 show that the carbon atom of CO2 acts as an electron acceptor that complexes with

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Table 8. Phase Behavior of the CO2-DMA System Obtained in This Studya DMA mole frac

P (bar)

transition

DMA mole frac

P (bar)

transition

T ) 40 °C BP 0.309 BP 0.397 BP 0.493 BP 0.593 BP 0.698

68.0 58.2 48.1 40.6 32.7

BP BP BP BP BP

0.050 0.081 0.133 0.194 0.239

90.4 89.3 83.5 78.6 74.7

0.050 0.081 0.133 0.194 0.239

137.3 133.2 120.1 107.3 100.5

CP BP BP BP BP

T ) 60 °C 0.309 0.397 0.493 0.593 0.698

88.5 74.8 62.2 49.9 39.5

BP BP BP BP BP

0.050 0.081 0.133 0.194 0.239

176.4 172.0 154.6 137.2 125.2

T ) 80 °C CP 0.309 BP 0.397 BP 0.493 BP 0.593 BP 0.698

107.8 90.7 75.1 58.7 45.5

BP BP BP BP BP

0.050 0.081 0.133 0.194 0.239

205.7 205.1 185.0 163.4 147.1

T ) 100 °C CP 0.309 BP 0.397 BP 0.493 BP 0.593 BP 0.698

126.2 106.1 86.4 67.6 51.0

BP BP BP BP BP

0.050 0.081 0.133 0.194 0.239

229.6 230.8 212.3 189.7 167.1

T ) 120 °C CP 0.309 BP 0.397 BP 0.493 BP 0.593 BP 0.698

146.6 119.8 97.9 75.6 55.3

BP BP BP BP BP

Figure 10. Comparison of the best fit of the Peng-Robinson equation of state to the CO2-decyl acrylate system at 80 °C. Table 9. Pure Component Critical Properties with the Peng-Robinson Equation of State18,19 component

Tb (°C)

Tc ( °C)

Pc (bar)

acentric factor

carbon dioxide decyl acrylate decyl methacrylate

154.15 210.45

31.1 447.39 471.60

73.9 17.09 15.87

0.2250 0.7266 0.7363

These two binary interaction parameters were determined by the regression of experimental data with the Peng-Robinson equation of state. The objection function (OBF) and the root mean squared relative deviation (RMSD) percent of this calculation were defined as follows. N

OBF )

a

BP stands for a bubble-point, CP for a critical-point, DP for a dewpoint, and LLV for liquid-liquid-vapor.

∑i

(

RMSD(%) )

Figure 9. Pressure-composition isotherms for the CO2-decyl methacrylate system obtained in this study at 40, 60, 80, 100, and 120 °C.

the carbonyl oxygen of poly(DMA); hence, the same behavior can be reasonably expected for CO2 with DMA monomer. The experimental data obtained in this work are modeled using the Peng-Robinson equation of state. The equation of state is briefly described here. The Peng-Robinson equation of state11 is used with the following mixing rules.

amix )

∑i ∑j xixjaij

aij ) (aiiajj)1/2(1 - kij) bmix )

∑i ∑j xixjbij

bij ) 0.5[(bii + bjj)](1 - ηij)

(1) (2) (3) (4)

)

Pexp - Pcal Pexp

2

× 100 xOBF ND

(5)

(6)

ND in eq 6 represents the number of data points. We used Marquardt’s26 method to optimize the objection function. All isotherms were included for calculation. Table 9 lists the pure component critical temperatures, the critical pressures, and the acentric factors for CO2, DA, and DMA used with the Peng-Robinson equation of state. The critical properties of DA and DMA are obtained by Lydersen’s method in group contribution.18 The vapor pressures were calculated by the Lee-Kesler method.18 Figure 10 presents the comparison of the CO2-DA experimental results with the calculated data obtained using the PengRobinson equation of state at a temperature of 80 °C. The values of the optimized parameters (bubble-point data ) 8, RMSD ) 1.80%) of the Peng-Robinson equation of state for the CO2DA system are kij ) 0.0195 and ηij ) -0.0005. A reasonable fit of the data is obtained over most of the composition range even if no binary interaction parameters are used. We compared the experimental results with the calculated P-x isotherms at temperatures of 40, 60, 100, and 120 °C for the CO2-DA system using the optimized values of kij and ηij determined at 80 °C. Therefore, Figure 11 shows the comparison of the experimental results with calculated P-x isotherms at temperatures of 40, 60, 100, and 120 °C for the CO2-DA system using the adjusted values of kij and ηij determined at 80 °C. A good fit of the data is obtained with the Peng-Robinson equation using two adjustable mixture parameters for the CO2-DA system. The RMSD at five temperatures for the CO2-DA system was 3.94% of the bubble-point number, 42. Figure 12 shows the comparison of the experimental data with the calculated results at 40, 60, 80, 100, and 120 °C for

Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3379

Figure 11. Comparison of the experimental data (symbols) for the carbon dioxide-decyl acrylate system with the calculations (solid lines) obtained with the Peng-Robinson equation of state with kij equal to 0.0195 and ηij equal to -0.0005.

Figure 12. Comparison of the experimental data (symbols) for the carbon dioxide-decyl methacrylate system with the calculations (solid lines) obtained with the Peng-Robinson equation of state with kij equal to 0.0240 and ηij equal to -0.0345.

the CO2-DMA system. These isotherms are calculated using the optimized values (bubble-point data ) 9, RMSD ) 0.94%) of kij equal to 0.0204 and ηij equal to -0.0345 determined at 80 °C in the same way as above (Figure 10). The RMSD at five temperatures for the carbon dioxide-DMA system was 3.05% of the bubble-point number, 47. The calculated mixturecritical curve is type-I, in agreement with experimental results. Figures 13 and 14 show the mixture-critical curve for the CO2-DA and CO2-DMA systems predicted by the PengRobinson equation of state. The calculated mixture-critical curve is type-I, which shows an agreement with experimental observations. As shown in Figures 13 and 14, the solid lines represent the vapor pressures for pure CO2,18,19 DA,18 and DMA.18 The solid circles represent the critical points for pure carbon dioxide, DA, and DMA. The upper part of the dashed line is single phase (fluid), and the lower part is two phase (vapor-liquid). The solid squares are the mixture-critical points determined from isotherms measured in this experiment. The dashed lines represent the calculated value obtained using the PengRobinson equation of state. Conclusions The phase-behavior data for the poly(DA)-CO2-DA system are measured with DA concentrations of 0.0, 7.5, 14.9, 23.4, and 34.9 wt %. This system changes the pressure-temperature slope of the phase-behavior curves from an UCST region to a LCST region as the DA concentration increases. When 40.1 wt % DA is added to the poly(DA)-CO2 solution, the poly(DA)-

Figure 13. Pressure-temperature diagram for the CO2-decyl acrylate system. The solid lines and the solid circles represent the vapor-liquid lines and the critical points, respectively, for pure CO2 and decyl acrylate. The open squares are critical points determined from isotherms measured in this work. The dashed lines represent calculations obtained using the Peng-Robinson equation of state with kij equal to 0.0195 and ηij equal to -0.0005

Figure 14. Pressure-temperature diagram for the CO2-decyl methacrylate system. The solid lines and the solid circles represent the vapor-liquid lines and the critical points, respectively, for pure CO2 and decyl methacrylate. The open squares are critical points determined from isotherms measured in this work. The dashed lines represent calculations obtained using the Peng-Robinson equation of state with kij equal to 0.0240 and ηij equal to -0.0345.

CO2-DA (40.2 wt % DA) phase-behavior curve intersects a liquid f liquid + vapor (LV) curve at ∼85 °C and ∼160 bar. The phase-behavior curve is shown for poly(DA) dissolved in supercritical DME, propane, propylene, butane, and 1-butene. The cloud-point behavior for poly(DA)-alkane and poly(DA)alkene systems exhibits LCST curves with a positive slope. Cloud-point data are obtained for poly(DMA)-CO2-DMA mixtures at DMA concentrations of 4.4, 6.7, 11.2, 22.8, and 32.9 wt %. With 41.6 wt % DMA added to the solution, the phase-behavior curve exhibits LCST type cloud-point behavior with a positive slope. The CO2-DA and CO2-DMA systems exhibit type-I phase behavior characterized by an uninterrupted mixture-critical curve. The P-x bubble-point curves are convex, which indicates that CO2 exhibits high solubility in DA and DMA probably due to the formation of a weak complex between the carboxylic oxygen in DA or DMA and the carbon in CO2. The Peng-Robinson equation of state can be used with two adjustable parameters to calculate a reasonable representation of the phase behavior of the CO2-DA and CO2-DMA systems. Temperature-independent interaction-parameter quantitative agreement can be obtained between the experimental data and the calculated phase behavior.

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Acknowledgment The authors gratefully acknowledge the financial support from the Korea Ministry of Commerce, Industry & Energy and the Korea Energy Management Corporation. Note Added after ASAP Publication. The E-mail address of the corresponding author has been changed from that in the version published on the Web 10/29/2005. The corrected version was posted 4/5/2006. Literature Cited (1) LoStracco, M. A.; Lee, S.-H.; McHugh, M. A. Comparison of the Effect of Density and Hydrogen Bonding on the Cloud-Point Behavior of Poly(ethylene-co-methyl acrylate)-Propane-Cosolvent Mixtures. Polymer 1994, 35, 3272. (2) Hasch, B. M.; Meilchen, M. A.; Lee, S.-H.; McHugh, M. A. Cosolvency Effect on Copolymer Solutions at High Pressure. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 429. (3) Meilchen, M. A.; Hasch, B. M.; Lee, S.-H.; McHugh, M. A. Poly(ethylene-co-methyl acrylate)-Solvent-Cosolvent Phase Behavior at High Pressures. Polymer 1992, 33, 1922. (4) Rindfleisch, F.; DiNoia, T. P.; McHugh, M. A. Solubility of Polymers and Copolymers in Supercritical CO2. J. Phys. Chem. 1996, 100, 15581. (5) Byun, H. S.; McHugh, M. A. Impact of “Free” Monomer Concentration on the Phase Behavior of Supercritical Carbon Dioxide-Polymer Mixtures. Ind. Eng. Chem. Res. 2000, 39, 4658. (6) Byun, H. S.; Choi, T. H. Effect of the Octadecyl Acrylate Concentration on the Phase Beavior of Poly(octadecyl acrylate)/Supercritical CO2 and C2H4 at High Pressures. J. Appl. Polym. Sci. 2002, 86, 372. (7) Patterson, D. Polymer Compatibility With and Without a Solvent. Polym. Sci. Eng. 1982, 22, 64. (8) Wolf, B. A.; Blaum, G. J. Measured and Calculated Solubility of Polymers in Mixed Solvents: Monotony and Cosolvency. J. Polym. Sci., Part B: Polym. Phys. 1975, 13, 1115. (9) McHugh, M. A.; Rindfleisch, F. P.; Kuntz, T.; Schmaltz, C.; Buback, M. Cosolvent Effect of Alkyl Acrylates on the Phase Behavior of Poly(alkyl acrylates)-Supercritical CO2 Mixtures. Polymer 1998, 39, 6049. (10) Byun, H. S.; Kim, J. G.; Yang, J. S. Phase Behavior of the Poly[hexyl(meth)acrylate]-Supercritical Solvents-Monomer Mixtures at High Pressure. Ind. Eng. Chem. Res. 2004, 43, 1543. (11) Peng, D. Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Res. Fundam. 1976, 15, 59. (12) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth: Stoneham, MA, 1994. (13) Hsu, R. C.; Lin, B. H.; Chen, C. W. The Study of Supercritical Carbon Dioxide Extraction for Ganoderma Lucidum. Ind. Eng. Chem. Res. 2001, 40, 4478.

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ReceiVed for reView June 15, 2005 ReVised manuscript receiVed September 23, 2005 Accepted September 27, 2005 IE0507070