Article pubs.acs.org/jced
Solubility of 4‑Aminosalicylic Acid in Supercritical Carbon Dioxide and Subcritical 1,1,1,2-Tetrafluoroethane Jun-su Jin,*,† Xing Fan,† Haifei Zhang,‡ Yi-wei Wang,† and Ze-ting Zhang† †
Beijing Key Laboratory of Membrane Science and Technology, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom ABSTRACT: The equilibrium solubilities of 4-aminosalicylic acid (PAS) in supercritical carbon dioxide (SCCO2) and subcritical 1,1,1,2-tetrafluoroethane (R134a) are crucial to the process of supercritical and subcritical extraction. After reliability tests of the experimental apparatus were carried out and confirmed, measurements of the solubilities of PAS in SCCO2 and subcritical R134a were implemented from (11.0 to 21.0) MPa at temperatures of (308 to 328) K by the dynamic method in SCCO2 and from (5.0 to 15.0) MPa at temperatures of (308 to 328) K by the static method in subcritical R134a, respectively. It was found that the solubility of PAS is much higher in subcritical R134a than in SCCO2 under the same operational conditions (temperature and pressure). The solubility enhancement factor (δ) was calculated and analyzed. Four density-based models (Chrastil, K-J, M-S-T, and S-S) were used to correlate the PAS solubility data in SCCO2 and subcritical R134a. The calculated results showed that the reasonable agreement was obtained with the experimental solubility data.
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INTRODUCTION Supercritical and subcritical fluids have many advantages of liquids or gases. Among these, supercritical carbon dioxide (SCCO2) has been studied widely because of its inherent innocuity, low cost, and environmentally benign nature and especially because its critical conditions are easier to reach (Tc = 304.2 K, Pc = 7.38 MPa).1 In the last three decades, the solubilities of many low-volatility and polar substances in SCCO2 have been reported and reviewed.2−6 Nevertheless, the difficulty for the wide applications of SCCO2 is its weak dissolving capability, especially for polar and/or highmolecular-weight solutes such as pharmaceuticals. This disadvantage results in the limitation of its applications in reaction, separation, and material preparation processes. Fortunately, polar solutes can easily dissolve in fluorohydrocarbon solvents, such as 1,1,1,2-tetrafluoroethane (R134a).7 R134a is apyrous, nonexplosive, innocuous, non-ozonedepleting, nonirritating, noncorrosive, colorless, and tasteless, and it hardly contributes to global warming.8 The critical pressure of R134a is 4.06 MPa, which is much lower than that of SCCO2 (7.38 MPa). Meanwhile, its dipole moment (2.1 D) is much higher than that of SCCO2 (0 D),9 which implies that it may have a greater capability to dissolve polar solutes at lower operating pressures, which may lead to reduce experimental and operational costs in subcritical R134a extraction, purification, and pharmaceutical particle preparation processes. Moreover, its boiling point is 246.9 K at atmospheric pressure, which means R134a can leave negligible solvent residues in the solutes because of its high volatility.10 Actually, R134a, as the most widely used low-temperature and environmentally benign refrigerant, has become a very effective and safe replacement for © 2014 American Chemical Society
dichlorodifluoromethane (R12) because of its good comprehensive performance. However, as an efficient replacement for supercritical fluids, whether and why subcritical R134a has a stronger dissolving capability for many pharmaceuticals compared with SCCO2 has rarely been reported.11,12 Thus, measurements of solubility in subcritical R134a should be performed to analyze how the solvent influences the solubility of various solutes and to provide basic data for determining the operating conditions and industrial process design. p-Aminosalicylic acid (PAS) is one of the mostly used pharmaceuticals for multi-drug-resistant Mycobacterium tuberculosis. It can help other drugs be absorbed by the body as soon as possible, especially isoniazid (INH) and streptomycin (STR);13 what’s more, it is also an important raw material for synthesizing cocrystals of PAS with INH, which are a firstline drug for the treatment tuberculosis that leads to therapeutic advantages.14 Supercritical fluid technologies are being continuously developed in the pharmaceutical industry to obtain ultrapure products and also are popularly favored in drug manufacturing processes because of their abilities to reduce the particle size distribution and control the morphology without contaminating the active ingredient.15 Generally, experimental techniques are either dynamic or static methods. In previous works, most of the measurements of the solubility of solids in SCCO216−19 and subcritical or supercritical R134a extraction7,11 have employed dynamic and Received: March 21, 2014 Accepted: May 10, 2014 Published: May 20, 2014 2095
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static methods, respectively. In the present work, reliable apparatuses were set up to accomplish the experiment. As far as we know, the solubilities of PAS in SCCO2 and subcritical R134a have not been reported to date. Such data will contribute to processes for purification of PAS product and preparation of PAS particles with controlled sizes. Considering the limitations of the experimental apparatus, such as the type of pumps and the pressure restriction of the equilibrium cell, and taking the economic costs of extraction20 and the critical properties of the solvents into account, we employed two different methods to measure the solubilities of PAS in SCCO2 and subcritical R134a. The dynamic method, together with gas chromatography (GC) analysis, was used to measure the solubility of PAS in SCCO2 at temperatures and pressures between (308 and 328) K and (11.0 and 21.0) MPa, respectively. The investigation of the solubility of PAS in subcritical R134a was performed at (308, 318, and 328) K and (5.0 to 15.0) MPa using the static method and the same analysis method. The solubility enhancement factor (δ) between the two solvents was defined and analyzed from the viewpoint of solvent density, polarity, and molecular interactions with the solute. Four density-based models, including the Chrastil model, the Kumar and Johnston (K-J) model, the MendezSantiago and Teja (M-S-T) model, and the Sung and Shim (SS) model, were adopted to correlate the experimental data.
Figure 1. Schematic diagram of the dynamic experimental apparatus: 1, CO2 cylinder; 2, compressor; 3, surge flask; 4, pressure regulating valve; 5, preheating band; 6, preheating and mixing cell; 7, normally open valve; 8, high-pressure equilibrium cell; 9, constant-temperature stirred water bath; 10, platinum resistance thermometer; 11, pressure gauge; 12, heating coil; 13, decompression sampling valve; 14, Ushaped tube; 15, rotated flow meter; 16, wet-gas flow meter.
in the cell were 0.05 MPa and 0.1 K, respectively. To maintain experimental reliability, a number of experiments were done to determine the equilibration time, and then all of experimental data were obtained after approximately 30 min. PAS mixed with CO2 was discharged from the cell into two U-shaped containers. The wet-gas flow meter with an indeterminacy of 0.01 L at indoor temperature was used to determine the volume of CO2. The samples collected in the two connected U-shaped tubes were dissolved using anhydrous ethanol and analyzed by GC. In this work, three or more replicate samples were performed to ensure the veracity of each reported datum, the repeatability of which was within ± 5.0 %. Apparatus and Procedure for the Subcritical R134a System. According to the method given by Sherman et al.24 and the specific situation of our laboratory, a modified static method apparatus was implemented to determine the solubility of PAS in subcritical R134a. A schematic illustration of the static apparatus is presented in Figure 2. The main equipment
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EXPERIMENTAL METHODS Materials. The basic information on chemicals used in this work is listed in Table 1, including the molecular formula, CAS number, purity, and source. All of these reagents were without further purification. Table 1. Basic Information on the Reagents Used in This Work compound
formula
CAS no.
massfraction purity
PAS
C7H7NO3
65-49-6
≥98.0
anhydrous ethanol carbon dioxide
C2H6O
64-17-5
≥99.7
CO2
124-38-9
≥99.9
R134a nitrogen
C2H2F4 N2
811-97-2 7727-37-9
≥99.9 ≥99.999
source Aladdin Chemistry Co. Ltd. Beijing Chemical Reagent Factory Beijing Praxair Industrial Gas Co. Ltd. DuPont Company Beijing Praxair Industrial Gas Co. Ltd.
Figure 2. Schematic diagram of the static experimental apparatus: 1, R134a cylinder; 2, refrigerating machine; 3, constant-flux pump; 4, entry control valve; 5, surge flask; 6, exit control valve; 7, heating band; 8, equilibrium cell; 9, pressure gauge; 10, thermocouple; 11, heating coil; 12, decompression sampling valve; 13, U-shaped tube; 14, glass surge flask; 15, rotated flow meter; 16, wet-gas flow meter.
Apparatus and Procedure for the SCCO2 System. The dynamic method used to measure the solubility of PAS in SCCO2 has been described in the literature.17,18 A schematic illustration of the experimental apparatus is provided in Figure 1. The reliability of this apparatus was tested in our previous work,21 which shows a comparison of the solubilities of benzoic acid measured at 308 K and pressures ranging from (11.0 to 23.0) MPa in SCCO2 with those of previous literature.22,23 It was obviously found that the solubility data obtained by this apparatus were in good agreement with the published data, indicating that the apparatus used in this work is credible. The major apparatus of the dynamic process is a highpressure equilibrium cell with an available volume of 150 mL. The indeterminacies of the measured pressure and temperature
used in this method includes the following: a high-pressure equilibrium cell (Swiss Nova), a refrigerating machine (Zhengzhou Great Wall S&T Co. Ltd., model OLSB 10/40), a constant-flux pump (Dongtai Yanshan Instrument Factory, model BLH-1040), and a wet-gas flow meter (Changchun Instrument Factory, model LML-2). The effective volume of the equilibrium cell was about 10 mL; both ends of the cell were metal membranes to prevent entrainment, and it could be loaded with (3 to 4) g of solid solute. 2096
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model GC-2014C). When the temperatures of sample injector and detector compartment were both set at 403 K and the column temperature was 533 K, the shape of the chromatographic peak of PAS was desirable. To analyze the mass concentration of PAS, the external-standard method was adopted. A group of standard PAS solutions (injection volume 1 μL), which were made by dissolving PAS in anhydrous ethanol in volumetric flasks, were analyzed to obtain a calibration curve, of which the regression coefficient was greater than 0.9999.
To gurantee the reliability of the solubility data, experiments to determine the equilibrium time and analyze the experimental data repeatability were done, and the results are shown in Figure 3. From Figure 3 it can be seen that when equilibrium
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RESULTS AND DISCUSSION Solubility of PAS in SCCO2. The experimental data for the solubility of PAS in SCCO2, together with CO2 density, are listed in Table 2 and portrayed in Figure 4. The PAS solubility Table 2. Experimental Data for the Solubility of PAS in SCCO2 T/K
P/MPa
ρa/g·L−1
105·yb
308
11.0 13.0 15.0 18.0 21.0 11.0 13.0 15.0 18.0 21.0 11.0 13.0 15.0 18.0 21.0
743.95 785.70 815.06 848.04 873.67 603.15 693.65 741.97 789.24 822.91 414.90 571.33 653.50 723.08 767.88
0.77 0.97 1.10 1.28 1.51 0.66 1.13 1.43 1.93 2.43 0.35 1.15 1.85 2.83 3.59
Figure 3. Determination of the equilibrium time for the static method at 318 K and 9.0 MPa. 318
time was over 50 min, the solubility amplification was quite small. Taking the theoretical optimal equilibrium time and experimental efficiency into consideration, we decided to use an equilibrium time of 60 min. Similarly, three or more repeated samples were measured to ensure the data accuracy, and the reproducibility of each datum was within ± 6.9 %. In addition, the results showed that PAS and subcritical R134a are saturated under all experimental conditions, indicating that the experimental data in this experiment are dependable. Liquid R134a from the inverted cylinder was drawn into the refrigerating machine to ensure that liquid R134a was introduced into the constant-flux pump. This was important for the operation of the pump. R134a was compressed into the surge flask via the entry control valve, and then the exit control valve was used to introduce R134a into the equilibrium cell, which was loaded with PAS and wrapped with a heating band. The temperature of the cell was controlled by the heating band and thermocouple with a precison of 0.1 K, and a highprecision pressure gauge with an accuracy of 0.2 MPa was used to monitor the pressure of the cell. After the solute and solvent phase equilibrium was kept under the experimental conditions for 60 min, the initial value of the wet-gas flow meter was recorded, and the saturated R134a flowed from the top of the cell and through a decompression sampling valve, which was wrapped with a heating coil, to be depressurized to barometric pressure. The function of the heating coil was to avoid blockage of PAS by the precipitated solid solute. The final value of the wet-gas flow meter was recorded when the R134a was completely discharged. The difference between the final value and the initial value of the wet-gas flow meter is the volume of R134a. Finally, PAS was separated from the gaseous R134a and gathered in the two U-shaped tubes. Analytical Method. The PAS gathered in the U-shaped tubes was placed in a volumetric flask (50 mL) with anhydrous ethanol. For the sake of complete dissolution, an ultrasonic bath (Kunshan Ultrasonic Co. Ltd., model KQ-250DE) was employed. Analysis of the quantity of solutes collected from the solubility measurement was carried out using GC (Shimadzu,
328
a ρ is the density of pure CO2; values of ρ at different experimental temperatures and pressures were obtained from the NIST fluid property database. bStandard uncertainties for the measured quantities are the following: u(T) = 0.1 K; u(P) = 0.05 MPa; ur(y) = 0.05.
Figure 4. Experimental data for the solubility of PAS in SCCO2 as a function of pressure at various temperatures: ■, 308 K; ●, 318 K; ▲, 328 K. 2097
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is expressed as mole fraction (y), and its range is 3.50·10−6 to 3.59·10−5 at (308 to 328) K and (11.0 to 21.0) MPa. It can be clearly observed from Figure 4 that the solubility of PAS is enhanced with increasing pressure at a given temperature. It can be reasoned that the density of SCCO2 increases with pressure, which enhances its capability to dissolve PAS. In addition, the stronger interactions between PAS and CO2 molecules contribute to a mean distance decrease with increasing pressure. The influence of temperature is more complex, however, because of the effect on the vapor pressure of the solute, the solvent density, and the intermolecular interaction forces. Consequently, the combination of these effects of temperature leads to a crossover pressure region, which can be seen in Figure 4 about (11.5 to 13.0) MPa. The trends in the solubility change with pressure are distinctly different before and after the crossover region: when the experimental pressure is lower than the crossover region, the solubility of PAS decreases with increasing temperature; however, when experimental pressure is higher than the crossover region, the opposite trend is observed. Solubility of PAS in Subcritical R134a. The experimental data for the molar solubility of PAS in subcritical R134a, ranging from 1.41·10−5 to 4.21·10−5 at (308 to 328) K and (5.0 to 15.0) MPa, are listed in Table 3 along with the density of
Figure 5. Experimental data for the solubility of PAS in subcritical R134a as a function of pressure at various temperatures: ■, 308 K; ●, 318 K; ▲, 328 K.
Table 4. Solubilities of PAS in SCCO2 and Subcritical R134a at the Same Temperature and Pressure ρ/g·L−1
Table 3. Experimental Data for the Solubility of PAS in Subcritical R134a T/K
P/MPa
ρa/g·L−1
105·yb
308
5.0 7.0 9.0 11.0 13.0 15.0 5.0 7.0 9.0 11.0 13.0 15.0 5.0 7.0 9.0 11.0 13.0 15.0
1194.8 1206.1 1216.5 1226.2 1235.1 1243.6 1156.6 1170.2 1182.4 1193.6 1203.9 1213.4 1115.2 1131.8 1146.4 1159.5 1171.4 1182.3
1.71 1.92 2.17 2.45 2.83 3.09 1.59 2.07 2.44 2.71 3.01 3.40 1.41 2.21 2.80 3.23 3.64 4.21
318
328
105·y
T/K
P/MPa
R134a
SCCO2
R134a
SCCO2
δ
308
11.0 13.0 15.0 11.0 13.0 15.0 11.0 13.0 15.0
1226.2 1235.1 1243.6 1193.6 1203.9 1213.4 1159.5 1171.4 1182.3
745.54 786.89 815.06 605.92 695.25 741.97 417.06 573.33 653.50
2.45 2.83 3.09 2.71 3.01 3.40 3.23 3.64 4.21
0.77 0.97 1.10 0.66 1.13 1.43 0.35 1.15 1.85
3.18 2.92 2.81 4.11 2.66 2.38 9.23 3.17 2.28
318
328
enhancement factor (δ) was calculated according to eq 1, and the values are listed in Table 4. y δ = in R134a yin SCCO (1) 2 where yin R134a is the solubility of PAS in subcritical R134a and yin SCCO2 is the solubility of PAS in SCCO2 at the same operational condition (temperature and pressure). As shown in Table 4, the values of δ range from 2.28 to 9.23, from which one can clearly conclude that the ability of subcritical R134a to dissolve PAS is better than that of SCCO2. The reasons for this phenomenon are multiple. On the one hand, as is known to all, R134a is a polar molecule (dipole moment = 2.1 D), while CO2 is a nonpolar molecule. According to the principle of “similar compatible”, the solubility of the polar solute PAS in subcritical R134a is expected to be higher than that in SCCO2. On the other hand, it may be due to the influence of the solvent density. Within the scope of the experimental conditions, the density of SCCO2 is between (400 and 800) g·L−1, while the density of subcritical R134a ranges from (1100 to 1300) g·L−1. Generally, the solvent with higher density exhibits higher dissolving capability. Moreover, the molecular volume of R134a is much larger than that of SCCO2, so the average distance between R134a and PAS molecules could be less than that between SCCO2 and PAS molecules in the same volume equilibrium cell loaded with the same quantity
ρ is the density of R134a; values of ρ at different experimental temperatures and pressures were obtained from the NIST fluid property database. bStandard uncertainties for the measured quantities are the following: u(T) = 0.1 K; u(P) = 0.2 MPa; ur(y) = 0.069.
a
R134a and are plotted in Figure 5. As shown in Figure 5, the trends in the solubility of PAS in subcritical R134a with changes in pressure and temperature are the same as those in SCCO2.The crossover region, which can be seen in Figure 5, is at about (5.5 to 6.0) MPa. Comparison of the Solubilities of PAS in SCCO2 and Subcritical R134a. To compare the solubilities of PAS in SCCO2 and subcritical R134a under the same conditions, the data are enumerated in Table 4. To clearly express the increment of solubility in R134a clearly, the solubility 2098
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Table 5. Expressions and Results of Correlations of Data for the Solubility of PAS in SCCO2 and Subcritical R134a by Four Density-Based Models model Chrastil K-J
expression a ln S = k 0 ln ρ + 0 + b0 T a1 ln y = k1ρ + + b1 T
M-S-T
T ln(yP) = k 2ρ + a 2T + b2
S-S
⎛ d ⎞ a ln y = ⎜c3 + 3 ⎟ ln ρ + 3 + b3 ⎝ T⎠ T
system
correlation parameters k0 k0 k1 k1 k2 k2 a3
4.827; a0 = −7.036·103; b0 = −12.998 17.260; a0 = −5.684·103; b0 = −107.403 6.260·10−3; a1 = −7.500·103; b1 = 7.822 1.381·10−2; a1 = −5.710·103; b1 = −9.022 2.786; a2 = 22.490; b2 = −1.188·104 10.385; a2 = 30.381; b2 = −2.464·104 −2.973·104; b3 = 54.339; c3 = −6.462; d3 = 3.350·103
3.49 4.55 4.92 4.83 3.91 6.67 2.46
R134a
a3 = −3.293·104; b3 = −22.914; c3 = 4.280; d3 = 3.845·103
4.39
= = = = = = =
of solute, which could cause the molecular interaction between R134a and PAS to be stronger than those between SCCO2 and PAS. As a consequence, the solubility of PAS in subcritical R134a is much higher than that in SCCO2. Correlation with Density-Based Models. Density-based models are most frequently applied to model solubility in SCCO2. The Chrastil, K-J, M-S-T, and S-S equations are common models for binary systems. Good prediction results generated by these models for a solvent density region are usually at pressures ranging from 10.0 MPa to approximately 30.0 MPa. Previous review literature5 elaborated the details of the models used in this work, and the equations for these models that were used to correlate all of the experimental data for the solubility of PAS in SCCO2 and subcritical R134a obtained in this study are shown in Table 5. In the Chrastil model, S is the mass solubility of the solute in subcritical or supercritical solvents, which can be calculated using eq 2:
S=
y·ρ·M2 M1·(1 − y)
AARD/%
SCCO2 R134a SCCO2 R134a SCCO2 R134a SCCO2
Figure 6. Solubility of PAS in SCCO2 correlated using the M-S-T model at temperatures of (308, 318, and 328) K: ■, experimental results; , calculated using the M-S-T model.
(2)
where y is the mole-fraction solubility of PAS, M1 and M2 (g· mol−1) are the molar masses of the solvent (CO2 or R134a) and PAS, respectively, and ρ (g·L−1) is the density of subcritical or supercritical fluid. The average absolute relative deviation (AARD) was used to express the precision of these models. The AARD was calculated using eq 3: AARD =
100 % n
n
∑ i=1
|ycalcd, i − yexptl, i | yexptl, i
(3)
where ycalcd,i are the calculated values, yexptl,i are the experimental data, and n is the number of experimental data values. The correlation parameters and corresponding AARDs are tabulated in Table 5. The AARDs show that all four models are reasonably consistent with the experimental data, and the S-S model provides the minimum AARDs among the four models (2.46 % for SCCO2 and 4.39 % for R134a). For the M-S-T model correlation, the mole fraction (y) conforms to a single straight line when T ln(yP) − a2T is plotted versus the solvent density (ρ). Figures 6 and 7 show the comparisons of the experimental data and data calculated using the M-S-T model for the two solvents. The plots clearly show that the solubilities of PAS in SCCO2 and subcritical R134a increase with increasing density. Moreover, it can be seen in Table 5 that the values of k0, k1, and k2 obtained from the correlations using the Chrastil, K-J, and M-S-T models, respectively, which represent the number of
Figure 7. Solubility of PAS in subcritical R134a correlated using the M-S-T model at temperatures of (308, 318, and 328) K: ●, experimental results; , calculated using the M-S-T model.
solvent molecules that associate with one solute molecule,25are greater for the (PAS + R134a) system than for the (PAS + SCCO2) system. With a larger value of k, the molecular interactions between the solute and the solvent are stronger, so the solubility of PAS in subcritical R134a is much higher than that in SCCO2, with solubility enhancement factors (δ) of up to 9.23 at 328 K and 11.0 MPa. 2099
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CONCLUSION The mole-fraction solubilities of PAS in SCCO2 were in the range from 3.50·10−6 to 3.59·10−5 at temperatures and pressures from (308 to 328) K and (11.0 to 21.0) MPa, respectively, while those in subcritical R134a ranged from 1.41· 10−5 to 4.21·10−5 at temperatures and pressures from (308 to 328) K and (5.0 to 15.0) MPa, respectively. The crossover pressure region for the (PAS + SCCO2) system was at about (11.5 to 13.0) MPa, and that for the (PAS + R134a) system was at about (5.5 to 6.0) MPa. The experimental data were compared under the same operational conditions, and the maximum value of the solubility enhancement factor δ is 9.23, which may result from the greater solvent density and polarity of R134a, together with the stronger interactions between solvent and solute molecules. The Chrastil, K-J, M-S-T, and S-S models were used to calculate the solubilities of PAS, and the AARDs of these models were in the range of (2.46−4.92) % in SCCO2 and (4.39−6.67) % in subcritical R134a; moreover, the S-S model had the best relevance for the two systems.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86-10-64434788. Fax: +86-10-64436781. E-mail: jinjs@ mail.buct.edu.cn. Funding
This research was financially supported by funds awarded by the National Natural Science Foundation of China (21176012), the National Natural Science Foundation of Jiangsu Province (BK2012595), and Petrochina Limited Company (2012A2012−01). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the support of this research from the Mass Transfer and Separation Laboratory in Beijing University of Chemical Technology.
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REFERENCES
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