Article pubs.acs.org/jced
Measurement and Correlation for the Solid Solubility of Antioxidants D‑Isoascorbic Acid and Calcium L‑Ascorbate Dihydrate in Supercritical Carbon Dioxide Tzu-Chi Wang* and Ping-Yen Lee Department of Chemical and Materials Engineering and Master Program of Nanomaterials, Chinese Culture University, Taipei, Taiwan, ROC ABSTRACT: The central task of this research is devoted to the measurement of the solubility of two antioxidantsD-isoascorbic acid and calcium L-ascorbate dihydratein supercritical carbon dioxide. The experiment, using a semiflow type apparatus, takes data at various temperatures, over pressures ranging from 12 MPa to 24 MPa. The acquired data are then processed numericallywith the aid of semiempirical equations proposed by Chrastil and by Mendez-Santiago and Tejato find the correlated relationship. The correlated results from both equations yield satisfactory results with an average absolute relative deviation (AARD) under 5 % when optimally fitted parameters are chosen.
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INTRODUCTION Supercritical fluids (SCF), highly compressed gases with properties of both gases and liquids, demonstrate intriguing characteristics. Supercritical fluidsxenon, ethane, and carbon dioxide, to name a fewoffer a slew of exciting chemical possibilities for processes in synthetic and analytical chemistry. In practical applications, supercritical fluids can activate reactions hard to or even impossible to spark off in conventional solvents. Deemed a green technology, supercritical fluid is widely used in various fields, including extraction, reaction, particle formation, and material processing.1,2 This is why the physical properties of supercritical fluid mixtures, such as diffusion coefficient or solubility, are so essential for the effective design and operation of some industrial processes. Among widely used supercritical fluids, supercritical carbon dioxide is an ideal solvent, partly because of its mild temperature ranges when used in extraction process, and partly because of its environmental friendliness. The solubility of various solids in supercritical CO2 have been studied by several authors.3−5 Cortest et al. have their focus on a number of antioxidantsascorbyl palmitate, ascorbic acid, butyl hydroxyl anisole, gallic acid, propyl gallate, dodecyl gallate, and αtocopherol succinate acid, among othersand measure their solubilities in supercritical CO2 under situations of different pressure and temperature.6 However, in light of the tremendous amount of combination of chemical compounds, needed are not only more experimental data for specialty chemicals but also generalized correlation models. In this study, the similar semiflow apparatus are used to take the measurement of solid solubilities of Disoascorbic acid (IUPAC name: (2R)-2-[(1R)-1,2-dihydrox© 2014 American Chemical Society
yethyl]-4,5-dihydroxyfuran-3-one, CAS number: 89-65-6) and calcium L-ascorbate dihydrate (IUPAC name: calcium (5R)-5[(1S)-1,2-dihydroxyethyl]-3-hydroxy-4-oxofuran-2-olate dihydrate, CAS number: 5743-28-2) in supercritical CO2 at different temperatures308.2 K, 318.2 K, and 328.2 Kover the pressure range from 12 MPa to 24 MPa. These two compounds, aside from being nutrient supplements, are antioxidants used in preservation of meat, soups, cakes, and so on. Therefore, the data of the solid solubility of the two compounds in supercritical CO2 will be of great use in food testing and manufacturing. However, there seems to be a lack in literature of such data. This study thus lends itself to provide the essential solid solubility datawhich in this research are correlated using either a semiempirical equation by Chrastil7 or the one by Mendez-Santiago and Teja5for related design and theoretical modeling for research and industrial application.
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EXPERIMENTAL SECTION For fear that impure chemicals would spoil the results, considerable caution is exercised in preparing the relevant compounds. In this study, carbon dioxide is procured, with a minimum purity of 99.5 %, from Chiau-Chung Gas Co. (Taiwan). D-isoascorbic acid (C6H8O6), calcium L-ascorbate dihydrate (C12H14 CaO12·H2O), and salicylic acid (C7H6O3) with a minimum purity of 98 % come from Sigma-Aldrich Co. All of the above chemicals are applied in this research as they are. The physical properties and chemical structure of DReceived: April 18, 2013 Accepted: February 6, 2014 Published: February 24, 2014 613
dx.doi.org/10.1021/je400369n | J. Chem. Eng. Data 2014, 59, 613−617
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Table 1. Physical Properties of D-Isoascorbic Acid and Calcium L-Ascorbate Dihydrate
Figure 1. Schematic diagram of the experimental apparatus. Unit device: 1, CO2 cylinder; 2, cooler; 3, CO2 pump; 4, pressure transducer; 5, preheater; 6, water bath; 7, pre-equilibrium cell; 8, equilibrium cell; 9, pressure transducer; 10, thermocouple; 11, heating tape; 12, solvent cool trap; 13, solvent delivery; 14, air delivery; 15, wet test meter. Valve description: A, two-way needle valve; B, check valve; C, back pressure regulator; D, threeway needle valve.
through a needle valve, which is wrapped with heating tape to maintain the temperature at 333 K so that D-isoascorbic acid and calcium L-ascorbate dihydrate will not freeze to block the pipeline. The solid part is now separated from the gas and dissolves into a pure water flask. To recover the residual solute in the line, further delivery of water solvent, which in the sampling line is purged by air in the final step, is applied. The relevant data are taken with scrupulous care. The total volume of CO2 flow is measured by a wet test meter (Ritter TG05). The concentration of the water solution in the flask is taken and analyzed by UV−vis (Cary 50 Spectrophotometer) detector. To make sure of the accuracy of the instrument, standard solutions of known concentrations are used to calibrate the UV−vis detector. The sharp absorption peaks of D-isoascorbic acid and calcium L-ascorbate dihydrate are observed in the UV−vis detector at bands of 249 nm and 268.6 nm, respectively. In each round of experiment, the accuracy is confirmed by taking at least three measurements at any given set of temperature and pressure. Furthermore, measurements are repeatedly taken at different effluent CO2 flow rates between 4 L·h−1 and 5 L·h−1 at atmospheric conditions; however, it should be noted that these flow rates do not have any effect on the solid solubility measurements. Above all, these experimental results are all verified as equilibrium data.
isoascorbic acid (C6H8O6) and calcium L-ascorbate dihydrate are listed in Table 1. Aside from the chemicals, the experimental apparatus is also constructed with no less meticulousness. The solid solubility in supercritical CO2 is measured using a semiflow type apparatus, the configuration of which is shown in Figure 1. This whole procedure can be divided into three parts: the feed, the equilibrium, and the analysis. At first, supercritical CO2 is fed into the apparatus, then the equilibrium between the solid and the supercritical fluid is reached, and finally the measured results are analyzed to find the coefficients of the models. From the inception, a cooler lowers the temperature of pure CO2 to 275.2 K, and an HPLC pump (Thermo Separation Product), regulated by a back pressure regulator, compresses the liquefied CO2 to the desired pressure. The compressed CO2 subsequently passes through the preheating coil, a device that submerged in a water bath to keep its temperature stable. The CO2 then flows through two cells: the pre-equilibrium cell and equilibrium one. Each cell is of 75 cm3 in volume and has 5 g of drug sample, packed with glass beads, in it. For the purpose of avoiding any physical entrainment, glass wool or filter is used to plug the cells at their ends. A calibrated thermocouple takes the temperature, with resolution of ± 0.1 K; a Druck pressure transducer (PTX 1400), the pressure, with resolution of ± 0.1 MPa. When leaving the equilibrium cell, the supercritical CO2 lowers its pressure to atmospheric pressure with an expansion 614
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RESULTS AND DISCUSSION
Table 2. Solubility of D-Isoascorbic Acid (2) in Supercritical Carbon Dioxide (1)a
To start with, both the experimental equipment and the procedures are calibrated in advance to confirm their reliability: the solubility of salicylic acid in supercritical carbon dioxide is measured and then compared with that in literature data.8−10 The experimental results are shown in Figure 2, where satisfactory agreement between various measurements can be observed.
T/K
P/MPa
y2
308.2
12 14 16 18 20 22 24 12 14 16 18 20 22 24 12 14 16 18 20 22 24
2.633·10−7 3.294·10−7 3.509·10−7 3.906·10−7 4.391·10−7 4.627·10−7 4.888·10−7 2.152·10−7 2.756·10−7 3.387·10−7 3.922·10−7 4.511·10−7 4.789·10−7 5.018·10−7 1.404·10−7 2.354·10−7 3.006·10−7 3.831·10−7 4.491·10−7 4.898·10−7 5.496·10−7
313.2
318.2
a Standard uncertainties u are u(T) = 0.01 K, u(P) = 0.01 MPa, and the combined expanded uncertainty Uc is Uc(y2) = 1.404·10−8 (0.95 level of confidence).
Figure 2. Comparison of solubility data of salicylic acid in supercritical carbon dioxide.
Table 3. Solubility of Calcium L-Ascorbate Dihydrate (3) in Supercritical Carbon Dioxide (1)a
The isothermal solid solubility data of the two antioxidants (D-isoascorbic acid and calcium L-ascorbate dihydrate) in supercritical CO2 are measured at different pressures, ranging from 12 MPa to 24 MPa. The solid compounds’ mole fractions at equilibrium state in the supercritical phase can be found in Tables 2 and 3. For simplicity’s sake, the subscripts 1, 2, and 3 are used in this article to represent carbon dioxide, Disoascorbic acid, and calcium L-ascorbate dihydrate, respectively. The equilibrium solubilities are satisfactorily reproducible when recurrent measurements at various CO2 flow rates are taken out in the course of experiment. Variation of the flow rate in the range from 4 L·min−1 to 6 L·min−1 was found to have no effect on the observed solubilities. It can be noted that, for each solid compound, the solubility increases in accordance with the pressure along an isotherm. D-isoascorbic acid and calcium L-ascorbate dihydrate have a similar solubility range, from 10−7 to 10−8. Statistically, all of the data points have rather small standard deviations (Tables 2 and 3). The solid solubility measurements’ coefficient of variance, the standard deviation divided by mean value, is below 5 %. These indicators demonstrate that the experimental measurements are highly repeatable. Graphical presentations of the results can be found in Figures 3 and 4. The crossover points in the related figures show the temperature effect on the solid solubility. The crossover pressures for D-isoascorbic acid and calcium L-ascorbate dihydrate are determined at 20 MPa and 12 MPa, respectively. Two semiempirical equationsone by Chrastil,7 the other by Mendez-Santiago and Teja5are usually applied to find the correlation of the solid solubility in supercritical carbon dioxide. The former model shows a linear relationship between the solid
T/K
P/MPa
y3
313.15
12 14 16 18 20 22 24 12 14 16 18 20 22 24 12 14 16 18 20 22 24
3.887·10−8 4.397·10−8 4.87·10−8 5.206·10−8 5.913·10−8 6.223·10−8 6.628·10−8 4.117·10−8 5.185·10−8 6.188·10−8 7.087·10−8 7.915·10−8 8.430·10−8 9.241·10−8 4.452·10−8 6.750·10−8 7.589·10−8 8.777·10−8 1.009·10−7 1.109·10−7 1.262·10−7
318.15
323.15
a
Standard uncertainties u are u(T) = 0.01 K, u(P) = 0.01 MPa, and the combined expanded uncertainty Uc is Uc(y3) = 3.782·10−9 (0.95 level of confidence).
solubility and the density of pure carbon dioxide (component 1), both in their logarithmic form: a ln cj = k ln ρ1 + +b (1) T 615
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The parameters for the two semiempirical equations and the correlation results are shown in Table 4. The average absolute Table 4. Correlated Results of Solid Solubility Data in Supercritical Carbon Dioxide model CO2 (1) + D-isoascorbic acid (2) Chrastil Mendenz-Santiago and Teja CO2 (1) + calcium Lascorbate (3) Chrastil Mendenz-Santiago and Teja
L-ascorbate
n
∑ k=1
k = 3.9, a = −7697.1, b = 10.7 A = −12180.8, B = 105729.4, C = 18.8
2.7 4.8
against the density of supercritical CO2 is nearly a straight line. This kind of result amounts to a guarantee that the parameters obtained by applying the MST model can be accurately extrapolated to other operating conditions.
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CONCLUSION New solid solubility data for two antioxidants of D-isoascorbic acid (at temperatures 308.2 K, 313.2 K, and 318.2 K) and calcium L-ascorbate dihydrate (at temperatures 313.2 K, 318.2 K, and 323.2 K) in supercritical CO2 are presented in this study over the pressure range from 12 MPa to 24 MPa. The two antioxidants have a solubility range between 10−7 to 10−8. The crossover pressures for D-isoascorbic acid and calcium Lascorbate dihydrate are determined at 20 MPa and 12 MPa, respectively. With an average absolute relative deviation in solid solubility below 5 %, both the Chrastil and the Mendez-
(2)
where the subscript k denotes the kth experimental data point for solid solute j. The latter semiempirical equation, developed by MendezSantiago and Teja5 (the MST model), also has three parameters: T ln(yj P) = A + Bρ1 + CT
3.5 2.5
Figure 5. Test of consistency for solubility data of D-isoascorbic acid using the Mendez-Santiago and Teja (MST) model.
|yjkexp − yjkcal | yjkexp
k = 5.8, a = −2658.6, b = −3.9 A = −8143.9, B = 144781.5, C = 5.5
dihydrate (3) in
where cj is solute j’s concentration in the supercritical fluid (kg· m−3) and ρ1 is the density of the supercritical fluid (pure CO2) (kg·m−3). The association number k and constants a and b are empirically fitted model parameters. The values of these parameters can be obtained by minimizing the following objective function with regression: 100 obj = n
AARD/%
relative deviation for the calculated solid solubilities is shown to be below 5 %, which implies satisfactory accuracy of the experiment results. Figures 3 and 4 show the graphic correlated results of the two semiempirical equations. Literature5 indicates that whether the experimental data is self-consistent or not can be examined by using the MST model. A typical example is shown in Figures 5 and 6, in which the plot of T ln (yjP) − TC
Figure 3. Solubility for D-isoascorbic acid (2) in supercritical carbon dioxide (1).
Figure 4. Solubility for calcium supercritical carbon dioxide (1).
parameters
(3)
By minimizing the same objective function shown in eq 2, the parameters A, B, and C can be optimally found. 616
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and Hexanedioic Acid + Pentanedioic Acid. J. Chem. Eng. Data 2010, 55, 5797−5800. (10) Wilson, G. M. Vapor-Liquid Equilibrium. XI: A New Expression for the Excess Free Energy of Mixing. J. Am. Chem. Soc. 1964, 86, 127−130. (11) Renon, H.; Prausnitz, J. M. Local Compositions in Thermodynaic Excess Functions for Liquid Mixtures. AIChE J. 1968, 14, 135−144. (12) Prausnitz, J. M.; Lichtenthaler, R. N.; Azevedo, E. G. A. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice Hall: Upper Saddle River, NJ, 1999. (13) Rowley, R. L.; Wilding, W. V.; Oscarson, J. L.; Yang, Y.; Zundel, N. A.; Daubert, T. E.; Danner, R. P. DIPPR Data Compilation of Pure Compound Properties; Design Institute for Physical Properties, AIChE: New York, NY, 2003. (14) Ihmels, E. C.; Gmehling, J. Extension and Revision of the Group Contribution Method GCVOL for the Prediction of Pure Compound Liquid Densities. Ind. Eng. Chem. Res. 2003, 42, 408−412. (15) Hefter, G. T.; Tomkins, R. P. T. Experimental Determination of Solubilities; John Wiley & Sons: Chichester, UK, 2003. (16) Gamsjäger, H.; Lorimer, J. W.; Salomon, M.; Shaw, D. G.; Tomkins, R. P. T. The IUPAC-NIST Solubility Data Series: A Guide to Preparation and Use of Compilations and Evaluations. J. Phys. Chem. Ref. Data 2010, 39 (2), 023101. (17) Jeffrey, G. H.; Vogel, A. I. The Dissociation Constants of Organic Acids. Part XI. The Thermodynamic Primary Dissociation Constants of Some Normal Dibasic Acids. J. Chem. Soc. 1935, 21−30. (18) Cingolani, A.; Berchiesi, G. Thermodynamic Properties of Organic Compounds. Note 1. A DSC Study of Phase Transitions in Aliphatic Dicarboxylic Acids. J. Therm. Anal. 1974, 6, 87−90. (19) Wilhoit, R. C.; Shiao, D. Thermochemistry of Biologically Important Compounds. Heats of Combustion of Solid Organic Acids. J. Chem. Eng. Data 1964, 9, 595−599. (20) Othmer, K. Encyclopedia of Chemical Technology; Inter-Science: New York, 1978.
Figure 6. Test of consistency for solubility data of calcium L-ascorbate dihydrate using the Mendez-Santiago and Teja (MST) model.
Santiago and Teja semiempirical equations give satisfactory correlation results for solid solubilities. In regard to the limited amount of antioxidants that are added to the food for preservation, the supercritical fluid technique is a treatment safe enough for those compounds.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +886-2-2861-4011. Notes
The authors declare no competing financial interest.
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REFERENCES
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