Phase Equilibria of Glycerol Tristearate and Glycerol Trioleate in

Article pubs.acs.org/jced

Phase Equilibria of Glycerol Tristearate and Glycerol Trioleate in Carbon Dioxide and Sulfur Hexafluoride Tina Perko, Ž eljko Knez, and Mojca Škerget* Faculty of Chemistry and Chemical Engineering, Laboratory for Separation Processes and Product Design, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia ABSTRACT: Phase equilibria of 1,3-di(octadecanoyloxy)propan-2-yl octadecanoate (glycerol tristearate) and 2,3-bis[[(Z)-octadec-9-enoyl]oxy]propyl (Z)-octadec-9-enoate (glycerol trioleate) in carbon dioxide and sulfur hexafluoride have been investigated. Compositions of the equilibrated phases have been determined experimentally at pressures up to 51 MPa in a variablevolume view cell. The solubility of glycerol tristearate in CO2 and SF6 was determined at temperatures (333, 343, and 363) K and over a pressure range from (1.6 to 45.1) MPa for CO2 and (1.6 to 31.0) MPa for SF6. The solubility of glycerol trioleate in CO2 and SF6 was measured at (333, 343 and 363) K and pressures up to 51 MPa. For the systems of triglyceride in CO2, the experimental data are in a good agreement with literature data. For glycerol tristearate-SF6 and glycerol trioleate-SF6 systems, the phase inversion was visually observed at all temperatures. Furthermore, the solid−liquid (S-L) phase transition of glycerol tristearate in CO2 and SF6 was investigated by using a high-pressure view cell. For both systems the three-phase solid−liquid−vapor (S-L-V) line with a temperature minimum in the p−T diagram was observed. High-pressure differential scanning calorimetry (HP DSC) was used to measure and compare the melting point of glycerol tristearate in CO2 with the results obtained by observation in a view cell.

1. INTRODUCTION In the past few years, supercritical fluids have been used as solvents or processing media in various processes for the applications in various fields, especially for pharmaceutical, nutraceutical, agriculture, food, cosmetic, chemical industries, medicine, coatings, textiles, electronics and semiconductors, and waste treatment. 1−3 For all of these processes, physicochemical data of pure compounds, phase equilibrium data, and mass- and heat-transport data are crucial.3 Using supercritical fluids is one possibility to carry out chemistry and chemical technologies in a sustainable manner (“green chemistry”).4−10 Technologies with supercritical CO2 and other dense fluids are an environmentally benign alternative to conventional industrial processes.10,11 The use of supercritical CO2 has increased significantly because of many advantages such as low cost, nontoxicity, nonflammability, and chemical inertness. Its supercritical conditions are easily attained (Tc/K = 304, pc/MPa = 7.38), and it can be removed from a system by simple depressurization.12 Another fluid with low critical conditions (Tc/K = 318.7, pc/MPa = 3.75) is sulfur hexafluoride (SF6), an inorganic, colorless, nontoxic, and nonflammable gas. It is poorly soluble in water but soluble in nonpolar organic solvents.10 Natural oils and fats are complex mixtures of lipid components.13 Even though the main components are triglycerides, fats and oils also contain mono- and diglycerides, free fatty acids, phospholipids, glycolipids, sterols, and other fatsoluble components.13 Previous investigations showed that CO2 has generally a low solvent power for fats, and consecutively the system vegetable oil + CO2 is heterogeneous over a wide range of conditions, both in the sub- and the supercritical range. 14 The © 2012 American Chemical Society

investigations of phase behavior of vegetable oil−propane systems showed that propane has a high solvent power for vegetable oils and homogeneous systems are obtained at room temperature already at low pressures (e.g., for the sunflower oil and soybean oil in propane above 1.5 MPa14). Furthermore, for the vegetable oil-SF6 systems14,15 it was observed that the solubility of oils in SF6 is more than 100 times higher than in CO215 and that phase equilibria is influenced by the FA compositions (saturated/unsaturated) of vegetable oils.15 The SF6 presents therefore an alternative solvent for processing of oils and fats and fractionation of oil components. Triglycerides, the main constituents in most vegetable oils, are used as raw materials in a variety of industries ranging from petrochemical to pharmaceutical.16 The difference in molar mass and the number of unsaturated bonds of the fatty acid molecules composing the triglycerides characterize their physical and nutritive properties, these properties being important for specific uses in the food industry.4 2,3Bis[[(Z)-octadec-9-enoyl]oxy]propyl (Z)-octadec-9-enoate (glycerol trioleate or triolein) is an ester of glycerol and oleic (unsaturated) acid chains. It is one of the typical triglycerides, which is the most abundant component in many plant oils, such as canola, soybean, sunflower seed oils, and so forth.17 1,3Di(octadecanoyloxy)propan-2-yl octadecanoate (glycerol tristearate or tristearin) is an ester of glycerol and stearic (saturated) acid chains. It is used in the production of emulsified foodstuffs, in the cosmetic industry and medical industry.4 Received: July 17, 2012 Accepted: October 29, 2012 Published: November 9, 2012 3604

dx.doi.org/10.1021/je300801j | J. Chem. Eng. Data 2012, 57, 3604−3610

Journal of Chemical & Engineering Data

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cell. The desired pressure was set by introducing compressed N2 or CO2 into the measuring chamber through controlling pressure gauge. The HP DSC 1 instrument was calibrated with indium. Melting point and enthalpies of indium were used for temperature and heat capacity calibration. The materials were supplied by Mettler Toledo, and the purity of all materials was indicated to be higher than 0.99999 mass fraction. A sample of (5 to 10) mg of glycerol tristearate sample was weighed in aluminum pan (40 μL), and a pierced cover was sealed in place. An empty, hermetically sealed aluminum pan was used as a reference. Samples of glycerol tristearate were heated from (303 to 423) K with the heating rate of 5 K·min−1. High-Pressure View Cell. For the determination of melting points of substance under pressure of CO2 and SF6 by observation in a view cell, a method described in literature23,24 was used. Phase behavior data were measured by using a highpressure variable-volume view cell (NWA GMBh, Lorrach, Germany) of 120 mL volume. The cell has two sapphire windows and three openings: for introducing and emptying the gas, and for inserting a thermocouple. The apparatus is designed for maximum pressure 70 MPa and maximum temperature 473 K. The thermocouple was calibrated using pure substances with known melting points. Substance was filled in a clear glass vials (1.5 mL) and inserted in the high pressure cell next to the thermocouple.23 Pressurized gas was introduced with a high-pressure pump. Pressure was measured by electronic pressure gauge (WIKA to ± 0.1 %), and the cell was electrically thermostatted by a heating jacket to within ± 0.5 K. The temperature and pressure were recorded at the beginning and at the end of melting, and the mean value was calculated. For each melting point measurement, the cell was filled with the same amount of new substance. 2.2.2. Determination of Equilibrium Solubilities of Glycerol Tristearate and Glycerol Trioleate in CO2 and SF6. The same high-pressure variable-volume view cell as described above was used for determining the high-pressure equilibrium solubility data of glycerol tristearate-CO2, glycerol trioleate-CO2, glycerol tristearate-SF6, and glycerol trioleate-SF6. About 15 g of substance was placed in the cell. Afterward the CO2 or SF6 from a gas cylinder was cooled to a liquid state and compressed into the cell by a high-pressure pump. The content of the cell was mixed with a blade-turbine stirrer at 700 rpm under constant operating conditions (temperature and pressure) until equilibrium was reached. A minimum time for establishing the equilibrium as well as for the phase separation was determined to be 1 h each. Samples were taken from the lower and from the upper phase through sampling valves into the glass trap. The amount of CO2 or SF6 released was measured with a disposal of water in a 250 mL graduated cylinder (to within ± 1 mL). The mass of the substance was measured gravimetrically (accurate ± 0.0001 g). During the sampling procedure, the pressure drop was less than 0.3 MPa, while the temperature change was not detected.23

A literature review showed that phase equilibria of tristearate and trioleate were investigated in CO2, propane, and pressurized methanol.16,18−22 Straver et al.18 studied the phase behavior of the binary system propane and tristearate. Measurements were carried out in a temperature range from (300 up to 460) K and pressures up to 16 MPa. A year later, Bottini et al.19 experimentally determined and compared phase behavior of the binary systems (tristearate−propane and trioleate−propane), in the temperature range between (340 and 400) K and pressures up to 16 MPa. Tang et al.17 determined the phase behavior of methanol−trioleate system at (6.0, 8.0, and 10.0) MPa in the temperature range from (353.2 to 463.2) K. The same system was investigated later also by Glišić and Skala.20 Furthermore, Cismondi et al.21 determined the solubility of CO2 in tristearate at (333 and 353) K, and the pressure range was between (10 to 50) MPa. In the same year, Gracia et al.22 measured the solubility of CO2 in trioleate−oleic acid system in the temperature range from (313.15 to 333.15) K and pressure range from (20 to 30) MPa. In the present research, phase equilibrium data of glycerol tristearate in a system with CO2 and SF6 were determined at temperatures (333, 343, and 363) K and at pressures up to 42.9 MPa in the case of CO2 and up to 27.0 MPa in the case of SF6. The phase equilibrium data of glycerol trioleate in CO2 and SF6 were measured at temperatures of (333, 343, and 363) K and at pressures up to 50.0 MPa. Furthermore, S-L phase transitions data of glycerol tristearate in CO2 and SF6 were determined up to pressure 50.9 MPa. Phase equilibrium data of glycerol tristearate and glycerol trioleate in CO2 were compared to literature data, while phase equilibrium data for glycerol tristearate-SF6 and glycerol trioleate-SF6 systems could not be found in the literature. High-pressure differential scanning calorimetry (HP DSC) was used to measure the melting point of glycerol tristearate in CO2 and N2. The results were compared to the data obtained by observation in high-pressure view cell.

2. EXPERIMENTAL SECTION 2.1. Materials. Glycerol tristearate as a powder (catalog no. 555-43-1) and glycerol trioleate as a liquid (catalog no. 122-327) were purchased from Sigma Aldrich. SF6 was obtained from Linde plin (Celje, Slovenia). CO2 and N2 were provided by Messer (Ruše, Slovenia) (Table 1). Table 1. Properties of the Materials

a

compound

source

mass fraction purity

glycerol tristearate glycerol trioleate carbon dioxide nitrogen sulfur hexafluoride

Sigma Aldrich Sigma Aldrich Messer Company Messer Company Linde plin Company

≥ 0.99 ≥ 0.99 0.9995a 0.9999a 0.9999a

Mole fraction.

3. RESULTS AND DISCUSSION Melting Points of Glycerol Tristearate under High Pressure of CO2, SF6, and N2. The melting point of glycerol tristearate at atmospheric conditions was determined with HP DSC 1 and was 334.9 K. The experimental data of S-L phase transition of glycerol tristearate under pressure of CO2 and SF6 determined by observation in high-pressure view cell are presented in Table 2. Each data point represents the average of at least two measurements, and the uncertainties of the

2.2. Apparatuses and Methods. 2.2.1. Determination of Melting Points of Glycerol Tristearate under the Pressure of CO2 and SF6. High-Pressure Differential Scanning Calorimetry. A Mettler Toledo high-pressure differential scanning calorimeter (HP DSC 1) was used to measure the melting point of glycerol tristearate in the presence of CO2 and nitrogen at pressures up to 10 MPa. The HP DSC operates at pressures from (0.1 to 10) MPa and from room temperature up to 973 K. A pressure gauge displays the actual pressure in the 3605



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334.9 K at 0.1 MPa to 326.1 K at 9.0 MPa. The melting point of glycerol tristearate under the pressure of N2 generally does not change significantly with pressure. A comparison of measured data in the pressure range from (1.0 to 9) MPa by both high pressure methods, that is, the DSC and view cell methods, showed a deviation which was less than 1.0 %. Equilibrium Solubilities of Glycerol Tristearate and Glycerol Trioleate in Systems with CO2 and SF6. Phase equilibrium data for glycerol tristearate and glycerol trioleate in the systems with CO2 and SF6 are presented in Tables 4 and 5. Each data point represents the average of at least two measurements, and the uncertainties for the systems with CO2 were up to ± 0.0055 and ± 0.0103 for the upper and lower phase, respectively, while for the systems with SF6 they

Table 2. Melting Points of Glycerol Tristearate under the Pressures of CO2 and SF6 Determined by View Cell Methoda p/MPa

T/K

p/MPa

334.9 333.1 328.6 325.1 322.1 323.6 323.6 323.8 325.1 327.1 329.1 328.1

0.6 3.1 5.9 11.1 15.2 18.8 24.1 31.0

T/K SF6

CO2 0.1b 3.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

339.1 338.1 336.1 337.1 338.1 339.1 345.1 346.1

Table 4. Equilibrium Mole Fractions of CO2 in the Upper (y) and Lower Phase (x) for the CO2 (1) + Triglyceride (2)a Systems

a

u(p) = 0.05 MPa and u(T) = 0.8 K. bThis value was measured with DSC.

glycerol tristearate

temperature and pressure measurements were ± 0.8 K and ± 0.05 MPa, respectively. For both binary systems the three phase S-L-V line with a temperature minimum was observed. This type of phase behavior is influenced by competing effects of increasing solubility of the fluid in the component and increase in hydrostatic pressure.25 The melting point of glycerol tristearate decreased from 334.9 K at atmospheric pressure to a minimum of 322.1 K at 15 MPa. At pressures above 15 MPa, the melting point of glycerol tristearate increased with increasing pressure. A similar course of the S-L-V line with temperature minimum at 15 MPa for glycerol tristearate-CO2 was observed by Mandžuka and Knez24 and Spilimbergo et al.26 The melting point of glycerol tristearate under pressure of SF6 decreased from 339.1 K at 0.6 MPa to minimum of 336.1 K at 5.9 MPa. At pressures above 5.9 MPa, the melting point of glycerol tristearate increased with increasing pressure. The experimental data for the S-L transition of glycerol tristearate under pressure of CO2 and N2 in the pressure range from (0.1 to 9) MPa measured by HP DSC are presented in Table 3. Each data point represents the average of at least two measurements, and the uncertainties of the temperature and pressure measurements were ± 0.4 K and ± 0.05 MPa, respectively. It can be seen that the melting point of the glycerol tristearate under the pressure of CO2 decreases from

p/MPa

Table 3. Melting Points of Glycerol Tristearate under the Pressures of CO2 and N2 Determined by HP DSCa HP DSC/CO2

a

HP DSC/N2

p/MPa

T/K

p/MPa

kT/K

0.1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

334.9 332.6 331.5 329.9 328.3 327.1 326.0 326.4 326.7 326.1

0.1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

334.9 333.4 333.6 333.7 333.8 333.9 334.1 334.1 334.3 334.5

y1

glycerol trioleate x1

5.9 14.0 18.1 20.0 23.7 26.4 37.4 40.9

0.9965 0.9895 0.9909 0.9889 0.9970 0.9910 0.9574 0.9960

1.9 2.4 4.7 5.2 9.6 13.0 15.3 23.1 25.7 34.5 37.4 41.2 41.3

0.9958 0.9959 0.9956 0.9969 0.9964 0.9970 0.9981 0.9953 0.9970 0.9951 0.9965 0.9946 0.9966

3.1 9.9 14.2 18.7 23.3 25.9 30.7 35.5 36.3 42.9

0.9960 0.9961 0.9952 0.9942 0.9943 0.9970 0.9916 0.9972 0.9962 0.9899

p/MPa

T = 333 K 0.8455 1.9 0.8648 4.3 0.8937 6.0 0.8999 7.8 0.9069 13.2 0.9259 19.8 0.9238 26.9 0.9430 32.5 35.6 41.1 45.7 T = 343 K 0.4000 2.2 0.4548 5.6 0.6363 16.5 0.7138 19.1 0.9234 25.8 0.8392 31.7 0.8546 36.5 0.9208 41.5 0.9265 45.6 0.9491 0.9466 0.9354 0.9363 T = 363 K 0.5045 6.1 0.7993 10.4 0.8888 16.0 0.9256 21.7 0.9221 31.9 0.9522 36.0 0.9243 42.0 0.9297 46.0 0.9198 50.7 0.9657

y1

x1

0.9887 0.9906 0.9967 0.9920 0.9941 0.9860 0.9835 0.9738 0.9798 0.9882 0.9860

0.4000 0.6443 0.8281 0.6898 0.8745 0.8753 0.8899 0.8918 0.8953 0.9066 0.9075

0.9883 0.9918 0.9881 0.9866 0.9863 0.9918 0.9936 0.9927 0.9912

0.4201 0.4842 0.8365 0.8683 0.8463 0.8745 0.9232 0.8993 0.8824

0.9900 0.9322 0.9479 0.9891 0.9854 0.9788 0.9897 0.9475 0.9744

0.6164 0.8573 0.8706 0.8868 0.8704 0.8555 0.8636 0.8718 0.9296

a

Glycerol tristearate u(p) = 0.05 MPa and u(T) = 0.5 K, u(y1) = 0.0037, u(x1) = 0.0095; glycerol trioleate u(p) = 0.05 MPa and u(T) = 0.5 K, u(y1) = 0.0055, u(x1) = 0.0103.

u(p) = 0.05 MPa and u(T) = 0.4 K. 3606



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system becomes highly viscous and mixing of the phases and taking of the samples at these conditions were impossible. Therefore, the phase equilibrium data of glycerol tristearate-SF6 system were measured at (343 and 363) K in the pressure range from (1.8 to 27.0) MPa. The solubility of glycerol trioleate in system with SF6 was measured at (333, 343, and 363) K in the pressure range from (2.5 to 50.9) MPa. For the systems glycerol tristearate-SF6 and glycerol trioleateSF6, the inversion of two-phase system was observed. A twophase system with inversed phases was recorded visually, at all temperatures under investigation as shown in Figures 1 and 2.

Table 5. Equilibrium Mole Fractions of SF6 in the Upper (y) and Lower Phase (x) for the SF6 (1) + Triglyceride (2)a Systems glycerol tristearate p/MPa

y1

2.0 6.2 6.8 7.0 10.6 14.2 19.9 22.9 23.7 27.0

0.9688 0.7841 0.6899 0.8837 0.3484 0.3604 0.3630 0.3796 0.4063 0.4482

1.8 3.2 4.9 6.3 6.8 8.0 12.5 15.3

0.9588 0.9428 0.9628 0.9475 0.9511 0.9481 0.5677 0.5771

glycerol trioleate x1

p/MPa

T = 333 K 2.7 5.0 5.3 11.3 11.6 11.6 20.3 20.4 31.6 37.3 47.2 T = 343 K 0.1905 2.5 0.9690 6.4 0.9658 7.7 0.8705 16.9 0.9671 20.9 0.9428 27.4 0.9763 31.7 0.9771 41.2 0.9699 50.9 0.9702 T = 363 K 0.1717 3.9 0.2256 6.5 0.2752 11.9 0.3646 22.2 0.4510 31.2 0.6910 41.4 0.8929 50.5 0.9222

y1

x1

0.9216 0.9238 0.9321 0.6722 0.5214 0.5455 0.2987 0.3047 0.3210 0.3243 0.3333

0.2250 0.2641 0.9032 0.9548 0.9588 0.9636 0.9772 0.9664 0.9640 0.9750 0.9542

0.9179 0.9195 0.4666 0.3193 0.3183 0.2979 0.3214 0.3128 0.3049

0.3618 0.9120 0.9716 0.9476 0.9591 0.9596 0.9385 0.9337 0.9643

0.9565 0.9108 0.3070 0.3540 0.3787 0.4526 0.4727

0.2555 0.9047 0.9413 0.9444 0.9217 0.9200 0.9591

Figure 1. Phase equilibria for the system SF6 (1) + glycerol tristearate (2): (a) at 343 K and (5.4, 7.3, and 8.6) MPa and (b) 363 K and (6.6, 8.7, and 9.7) MPa.

a

Glycerol tristearate u(p) = 0.05 MPa and u(T) = 0.5 K, u(y1) = 0.0073, u(x1) = 0.0107; glycerol trioleate u(p) = 0.05 MPa and u(T) = 0.5 K, u(y1) = 0.0101, u(x1) = 0.0146.

were up to ± 0.0101 and ± 0.0146 for the upper and lower phases, respectively. For the systems trygliceride-CO2 the results show that generally the amount of CO2 in the vapor phase does not change much with increasing temperature and pressure, while the solubility of CO2 in the liquid phase increases with increasing pressure and does not change much with temperature. The solubility of CO2 in glycerol tristearate at 333 K increases from 0.85 mole fraction at 5.9 MPa to 0.94 mole fraction at 40.9 MPa. The solubility of CO2 in glycerol tristearate at 343 K is in the range from (0.40 to 0.94) mole fraction, and at 363 K it is between (0.50 to 0.97) mole fraction at pressures from (1.9 to 42.9) MPa. The solubility of CO2 in glycerol trioleate at 333 K is between (0.40 to 0.91) mole fraction, at 343 K it is between (0.42 to 0.92) mole fraction, and at 363 K it is between (0.62 to 0.93) mole fraction at pressures from (1.9 to 50.7) MPa. For the system glycerol tristearate-SF6 it was observed that at specific experimental conditions, that is, at 333 K in whole pressure range investigated and at (343 and 363) K at pressures higher than (27.0 and 15.0) MPa, respectively, the two-phase

Figure 2. Phase equilibria for the system SF6 (1) + glycerol trioleate(2): (a) at 333 K and (4.4, 5.7, and 8.2) MPa; (b) at 343 K and (5.2, 7.2, and 8.4) MPa; (c) at 363 K and (6.4, 9.9, and 12.0) MPa. 3607



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For the system glycerol tristearate-SF6 (Figure 1) at a pressure of 5.4 MPa and a temperature of 343 K, the glycerol tristearate rich phase is the lower phase. At a pressure of 7.3 MPa and a temperature of 343 K, phase inversion takes place. At a pressure of 8.3 MPa and a temperature of 343 K the glycerol tristearate rich phase is the upper phase. Similar phase inversion was observed at 363 K at slightly higher pressure of 8.7 MPa. For the system glycerol trioleate-SF6 (Figure 2) phase inversions at (333, 343, and 363) K were observed at (5.7, 7.2, and 9.9) MPa, respectively. The composition of upper (y) and lower phase (x) given in Table 5 dramatically changes after phase inversions; the sharp decrease of the mole fraction of SF6 in upper phase (y) is observed. At pressures above phase inversion the amount of SF6 in upper phase increases with temperature at constant pressure. The equilibrium solubility data for both triglycerides in system with CO2 and SF6 are presented in Figures 3 to 7. The

Figure 5. Phase equilibria for the system CO2 (1) + triglycerides (2) at 363 K: □, glycerol tristearate upper phase; ■, glycerol tristearate lower phase; △, glycerol tristearate upper phase;24 ▲, glycerol tristearate lower phase;24 ○, glycerol trioleate upper phase; ●, glycerol trioleate lower phase.

Figure 3. Phase equilibria for the system CO2 (1) + triglycerides (2) at 333 K: ◇, glycerol tristearate upper phase; ◆, glycerol tristearate lower phase; △, glycerol trioleate upper phase; ▲, glycerol trioleate lower phase; ○, glycerol trioleate upper phase;27 ●, glycerol trioleate lower phase.27

Figure 6. Phase equilibria for the system SF6 (1) + triglycerides (2) at 343 K: ◆, glycerol tristearate upper phase; ◇, glycerol tristearate lower phase; ▲, glycerol trioleate upper phase; △, glycerol trioleate lower phase.

Figure 4. Phase equilibria for the system CO2 (1) + triglycerides (2) at 343 K: □, glycerol tristearate upper phase; ■, glycerol tristearate lower phase; △, glycerol tristearate upper phase;24 ▲, glycerol tristearate lower phase;24 ◇, glycerol trioleate upper phase; ◆, glycerol trioleate lower phase.

Figure 7. Phase equilibria for the system SF6 (1) + triglycerides (2) at 363 K: ◆, glycerol tristearate upper phase; ◇, glycerol tristearate lower phase; ▲, glycerol trioleate upper phase; △, glycerol trioleate lower phase.

In Figure 3 vapor−liquid equilibria of the systems glycerol tristearate-CO2 and glycerol trioleate-CO2 at 333 K is presented in the pressure range from (8.45 to 45.65) MPa. The maximum solubility of CO2 in glycerol tristearate at 333 K was 0.94 mole

results obtained for both investigated triglycerides in CO2 and SF6 were compared to ascertain how the type of fat (saturated/ unsaturated) influences phase equilibria. The experimental data were also compared to data available in the literature. 3608



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consequence of rapid increase of the density of SF6 with the pressure at these conditions to around 1000 kg·m−3.

fraction at 41.1 MPa. The maximum solubility of CO2 in glycerol trioleate at the same temperature was 0.91 mole fraction at 45.7 MPa. The solubility of CO2 in glycerol tristearate is somewhat higher as compared to glycerol trioleate at pressures above 15 MPa. The composition of the upper phase does not change significantly with pressure. The vapor− liquid equilibrium data for glycerol trioleate-CO2 at 333 K are in good agreement with the data reported by Weber et al.27 The results presented in Figure 4 show that experimental data for glycerol tristearate-CO2 at 343 K are in a good agreement with the literature data.24 Small differences could be a consequence of different polymorphic forms of fats. The maximum solubility of CO2 in glycerol tristearate at 343 K was 0.95 mole fraction at 37.5 MPa. The experimental results (Figure 4) show that the solubility of CO2 in glycerol trioleate is somewhat lower as compared to glycerol tristearate at pressures above 2.5 MPa. The phase composition of the upper phase for both systems is similar. Similar results of vapor−liquid phase behavior were observed also at 363 K (Figure 5); the solubility of CO2 in glycerol tristearate at 363 K is somewhat higher than in glycerol trioleate. The maximum solubility of CO2 in glycerol trioleate at 363 K was 0.97 mole fraction at 39.6 MPa. Solubility data for glycerol tristearate-SF6 and glycerol trioleate-SF6 systems are presented in Figures 6 and 7. As can be seen in Figure 6 for both systems at 343 K, the phase inversion was observed at approximately the same pressure. The obtained results show that SF6 is better soluble in glycerol tristearate at the pressures above 10 MPa as compared to glycerol trioleate, while the solubility of glycerol trioleate in SF6 is somewhat higher than the solubility of glycerol tristearate. Similar conclusions can be made for the results obtained at 363 K and pressures from (10 to 15) MPa (Figure 7).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +386 2 22 94 463. Fax: +386 2 25 27 774. Funding

The authors are grateful to the Slovenian Ministry of High Education, Science and Technology for the financial support of this work. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Knez, Ž .; Škerget, M.; Perva Uzunalić, A. Phase equilibria of vanillins in compressed gases. J. Supercrit. Fluids 2007, 43, 237−248. (2) Colussi, S.; Elvassore, N.; Kikic, I. A comparison between semiempirical and molecular-based equations of state for describing the thermodynamic of supercritical micronization processes. J. Supercrit. Fluids 2006, 39, 118−126. (3) Škerget, M.; Knez, Ž .; Knez-Hrnčič, M. Solubility of solids in suband supercritical fluids. J. Chem. Eng. Data 2011, 56, 694−719. (4) Soares, B. M. C.; Gamarra, F. M. C.; Paviani, L. C.; Gonçalves, L. A. G.; Cabral, F. A. Solubility of triacylglycerols in supercritical carbon dioxide. J. Supercrit. Fluids 2007, 43, 25−31. (5) Ndiaye, P. M.; Franceschi, E.; Oliveira, D.; Dariva, C.; Tavares, F. W.; Vladimir Oliveira, J. Phase behavior of soybean oil, castor oil and their fatty acid esters in carbon dioxide at high pressures. J. Supercrit. Fluids 2006, 37, 29−37. (6) Mandžuka, Z.; Škerget, M.; Knez, Ž . High pressure micronization of tristearate. J. Am. Oil Chem. Soc. 2010, 87, 119−125. (7) Szydłowska-Czerniak, A.; Karlovits, G.; Lach, M.; Szłyk, E. X-ray diffraction and differential scanning calorimetry studies of β′−β transitions in fat mixtures. Food Chem. 2005, 92, 133−141. (8) Oh, J. H.; McCurdy, A. R.; Clark, S.; Swanson, B. G. Characterization and thermal stability of polymorphic forms of synthesized tristearin. J. Food Sci. 2002, 67, 2911−2917. (9) Yoon, S. H.; Miyawaki, O.; Park, K. H.; Nakamura, K. Transesterification between triolein and ethylbehenate by immobilized lipase in supercritical carbon dioxide. J. Ferment. Bioeng. 1996, 82, 334−340. (10) Sovová, H.; Zarevúcka, M.; Vacek, M.; Stránský, K. Solubility of two vegetable oils in supercritical CO2. J. Supercrit. Fluids 2001, 20, 15−28. (11) Martín, A.; Cocero, M. J. Micronization processes with supercritical fluids: Fundamentals and mechanisms. Adv. Drug Delivery Rev. 2008, 60, 339−350. (12) Nalawade, S. P.; Picchioni, F.; Janssen, L. P. B. M. Supercritical carbon dioxide as a green solvent for processing polymer melts: Processing aspects and applications. Prog. Polym. Sci. 2006, 31, 19−43. (13) Temelli, F. Perspectives on supercritical fluid processing of fats and oils. J. Supercrit. Fluids 2009, 47, 583−590. (14) Ilić, L.; Škerget, M.; Knez-Hrnčič, M.; Knez, Ž . Phase behavior of sunflower oil and soybean oil in propane and sulphur hexafluoride. J. Supercrit. Fluids 2009, 51, 109−114. (15) Knez, Ž .; Ilić, L.; Škerget, M.; Kotnik, P. High-pressure solubility data for palm oil-SF6 and coconut oil-SF6 systems. J. Chem. Eng. Data 2010, 55, 5829−5833. (16) Borch-Jensen, C.; Mollerup, J. Phase equilibria of carbon dioxide and tricaprylin. J. Supercrit. Fluids 1997, 10, 87−93. (17) Tang, Z.; Du, Z.; Min, E.; Gao, L.; Jiang, T.; Han, B. Phase equilibria of methanol-triolein system at elevated temperature and pressure. Fluid Phase Equilib. 2006, 239, 8−11. (18) Straver, E. J. M.; de Roo, J. L.; Peters, C. J.; de Swaan Arons, J. Phase behaviour of the binary system propane and tristearin. J. Supercrit. Fluids 1998, 11, 139−150.

4. CONCLUSIONS In the present research, phase equilibrium behaviors of glycerol tristearate and glycerol trioleate in systems with CO2 and SF6 have been investigated at three different temperatures. The aim was to ascertain if and how the phase equilibria of saturated and unsaturated fats in CO2 and SF6 differ. The high-pressure variable-volume cell was used as experimental equipment for phase equilibria determination. In general results show that CO2 is more soluble in glycerol tristearate and glycerol trioleate than SF6. The solubility of CO2 and SF6 in glycerol tristearate and glycerol trioleate increases with increasing temperature at the same pressure conditions. The solubility of CO2 and SF6 in glycerol tristearate is higher than in glycerol trioleate which indicates that both fluids are more soluble in saturated fats than in unsaturated fats. The results are in agreement with our previous findings that the solubility of SF6 in triglycerides generally increases with increasing temperature and with the mass percent of saturated fatty acids in oils.15 The solubility of both triglycerides in CO2 is similar and is generally lower than in SF6. The results obtained for the glycerol trioleate-SF6 system at 343 K show that the solubility of glycerol trioleate in SF6 is somewhat higher than the solubility of glycerol tristearate. Therefore, the solubility of triacylglycerols in SF6 is higher for triglycerides with a higher content of unsaturated fatty acids which is in agreement with the results of our previous research.15 For both triglycerides in SF6, the inversion of two-phase system was observed at all temperatures investigated at low pressures, in the range from (5.7 to 9.9) MPa, which is a 3609



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(19) Bottini, S. B.; Fornari, T.; Brignole, E. A. Phase equilibrium modelling of triglycerides with near critical solvents. Fluid Phase Equilib. 1999, 158−160, 211−218. (20) Glišić, S. B.; Skala, D. U. Phase transition at subcritical and supercritical conditions of triglycerides methanolysis. J. Supercrit. Fluids 2010, 54, 71−80. (21) Cismondi, M.; Mollerup, J.; Brignole, E. A.; Zabaloy, M. S. Modeling the high-pressure phase equilibria of carbon dioxidetriglyceride systems: A parameterization strategy. Fluid Phase Equilib. 2009, 281, 40−48. (22) Gracia, I.; García, M. T.; Rodríguez, J. F.; Fernández, M. P.; de Lucas, A. Modelling of the phase behaviour for vegetable oils at supercritical conditions. J. Supercrit. Fluids 2009, 48, 189−194. (23) Knez, Ž .; Škerget, M.; Mandžuka, Z. Determination of S−L phase transitions under gas pressure. J. Supercrit. Fluids 2010, 55, 648− 652. (24) Mandžuka, Z.; Knez, Ž . Influence of temperature and pressure during PGSS micronization and storage time on degree of crystallinity and crystal forms of monostearate and tristearate. J. Supercrit. Fluids 2008, 45, 102−111. (25) Knez, Ž .; Škerget, M. Phase equilibria of the vitamins D2, D3 and K3 in binary systems with CO2 and propane. J. Supercrit. Fluids 2001, 20, 131−144. (26) Spilimbergo, S.; Luca, G.; Elvassore, N.; Bertucco, A. Effect of high-pressure gases on phase behaviour of solid lipids. J. Supercrit. Fluids 2006, 38, 289−294. (27) Weber, W.; Petkov, S.; Brunner, G. Vapour-liquid-equilibria and calculations using the Redlich-Kwong-Aspen-equation of state for tristearin, tripalmitin and triolein in CO2 and propane. Fluid Phase Equilib. 1999, 158−160, 695−706.

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Phase Equilibria of Glycerol Tristearate and Glycerol Trioleate in

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