Determining the Solubility of Organic Compounds in Supercritical

Article pubs.acs.org/jced

Determining the Solubility of Organic Compounds in Supercritical Carbon Dioxide Using Supercritical Fluid Chromatography Directly Interfaced to Supercritical Fluid Solubility Apparatus Ben Li, Wei Guo, Wei Song, and Edward D. Ramsey* Sustainable Technology Research Centre, University of Science and Technology Liaoning, Anshan 114051, China ABSTRACT: The solubilities of benzoin and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride in supercritical fluid carbon dioxide have been measured using static supercritical fluid solubility apparatus directly interfaced to supercritical fluid chromatography (SFC). The method involves increasing the pressure of supercritical fluid carbon dioxide in controlled steps to produce a series of saturated sample solutions that are sequentially analyzed by online SFC. The system requires minimal manual sample manipulation stages and facilitates a self-validation check procedure. Solubility data for benzoin was determined at 308 and 318 K through the pressure range of 12−24 MPa using 2 MPa steps. Solubility data for 3,3′,4,4′-benzophenonetetracarboxylic dianhydride was determined at 328 and 343 K through the pressure range 15−27.5 MPa using 2.5 MPa steps. The Mendéz-Santiago and Teja model has been successfully applied to correlate all sets of experimental solubility data. plumbed into the recirculation line.13,14 Since variable sample collection/trapping efficiency and manual off-line sample preparation stages are a source of errors, static solubility systems equipped with recirculating pumps have been coupled directly online with liquid chromatography.15,16 However, the use of online HPLC requires a mobile phase with a high organic solvent content capable of completely dissolving injected carbon dioxide to prevent UV/vis detector instability problems.17 We have previously reported the development of a two-valve interface used to connect a supercritical fluid reaction (SFR) system directly online with liquid chromatography to monitor the progress of an esterification reaction conducted in SFCO2.18 The design of the interface enables the direct substitution of HPLC with SFC to monitor a range of supercritical fluid based processes. The use of SFC is a logical alternative to HPLC due to the immediate compatibility of the SFC mobile phase with injected samples of SF-CO2 solutions. Accordingly, this report describes the flexible use of the interface to couple a supercritical solubility vessel (SSV) directly online to SFC to rapidly obtain SF-CO2 solubility data. The static SSV-SFC method involves increasing the sample loaded vessel in controlled pressure steps to produce a series of progressively more saturated sample SF-CO2 solutions for online SFC solubility measurements. This study describes the use of the SSV-SFC method to obtain SF-CO2 solubility data for benzoin and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) as test compounds with the later being an epoxy adhesive curing agent. These compounds were selected to test

1. INTRODUCTION Environmentally benign supercritical fluid carbon dioxide (SFCO2) is considered to be an alternative solvent for green chemistry.1,2 In practice, via density control the solvating strength of SF-CO2 can be regulated to imitate a range of conventional nonpolar organic solvents.3,4 Processes for which SF-CO2 can be used as a replacement solvent include: extraction, purification, fractionation, impregnation, catalysis, reactions, particle engineering, and tissue engineering.5,6 Successful utilization of SF-CO2 is greatly facilitated by the availability of solubility data for the compound(s) of interest. Accordingly, many reports and reviews have appeared in the literature providing SF-CO2 solubility data for a wide range of compounds.7,8 In general, the solubility of compounds in SFCO2 is determined by using either a flow or static procedure.9 Flow methods, also referred to as dynamic or continuous flow methods, involve passing an accurately known volume of SFCO2 through a column or vessel packed with the compound of interest to form a saturated solution which is decompressed at a collection point. Thereafter, a range of off-line analytical techniques can be used to determine the solubility of the collected compound.10 Static methods are often used to determine the supercritical fluid solubility of expensive compounds and involve preparing a saturated solution using an excess of compound loaded into a small volume high pressure cell or vessel. Once a saturated solution is formed, a high-pressure HPLC injection valve can be used to withdraw a small sample from the static solubility cell.11 The withdrawn sample is typically decompressed into a vial containing a suitable solvent for compound collection/trapping prior to offline analysis.12 In order to improve mixing within static solubility systems, extraction vessels have been connected to a recirculating pump with the high-pressure sample valve © XXXX American Chemical Society

Received: January 27, 2016 Accepted: May 13, 2016

A

DOI: 10.1021/acs.jced.6b00081 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Information Concerning Compounds and Reagents

a

chemical name

CAS No.

source

purification method

mole fraction purity

analysis method

benzoin 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) methanol acetonitrile carbon dioxide

119-53-9 241-28-5 67-56-1 75-05-8 124-38-9

Aladdin Aladdin Sigma-Aldrich Sigma-Aldrich Airichem

none none none none none

≥0.995 ≥0.985 ≥0.999 ≥0.999 ≥0.99999

GCa GCa GCa GCa GCa

Gas−liquid chromatography.

dioxide to the SSV for all SF-CO2 solubility studies. A Druck PDCR 910-175 pressure transmitter and Druck DPI 145 digital indicator (Leicester, UK) were used to calibrate the carbon dioxide pump pressure transducer and SSV pressure. Estimated accuracy for the SSV pressure was ±0.1 MPa. The carbon dioxide cylinder equipped with a liquid draw-off tube was connected to the pump. 2.3. SSV−SFC Interface and Injection Techniques. As shown in Figure 2, the interface used to connect the SSV directly online to SFC was constructed using two Rheodyne 7010 valves (Cotati, CA, USA). The valves were mounted within a Gilson 831 oven. The interface is designed to enable a representative sample to be directly withdrawn from within a high pressure vessel for SFC analysis. This is accomplished by the pressurized loading of the sample injection valve loop while SF-CO2 solution rapidly flows to load a withdraw valve loop initially set at atmospheric pressure. For these studies a 15 μL sample valve loop and 350 μL sample withdraw loop were used. Details concerning the procedures used for vessel temperature and pressure regulation for SFR-HPLC studies have been previously published18 and are substantially similar to those used to perform the SSV-SFC solubility measurements. Consequently, this section only provides details of different operational parameters. For SSV-SFC, the linked carbon dioxide pump programs provided an initial flow of liquid carbon dioxide to the SSV at 20 mL·min−1 until a prepressure of 1.0 MPa below the target pressure to perform the first solubility measurement for each compound was obtained. Thereafter, following an equilibration period of 10 min, the second linked pump program delivered liquid carbon dioxide at 8.0 mL·min−1 until the first SSV target pressure was attained. After this, the SSV was continuously stirred for a fixed period before the first set of online SSV-SFC injections where made. Once a small volume sample is withdrawn for SFC analysis, the liquid carbon dioxide pump immediately dispenses more CO2 to very rapidly restore the SSV target pressure. Once sets of

the capability of the SSV-SFC method since they have different molecular masses, different polarities that provide different ranges of SF-CO2 solubility values, and potentially different dissolution rates. Furthermore, BTDA is a moisture sensitive compound and is not amenable to SSV-HPLC using aqueous based mobile phases. The sets of SSV-SFC solubility results obtained for both compounds were used to evaluate the correlation model proposed by Mendéz-Santiago and Teja.19

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. The purity of benzoin and BTDA and all reagents used in these studies is provided in Table 1. The structures of benzoin and BTDA are shown in Figure 1.

Figure 1. Molecular structures of (a) benzoin and (b) BTDA.

2.2. Solubility Apparatus. A Jasco EV-3 100 mL high pressure vessel (Hachioji, Japan) with a magnet stir bar served as the SSV and was used to prepare saturated SF-CO2 solutions of benzoin and BTDA. The SSV was housed within a Jasco SCF-Sro oven whose specification is ±0.3 K. The design of this oven includes an integral variable speed magnetic stir bar drive motor. The SSV was loaded with either 400 mg benzoin powder or 220 mg BTDA powder. Saturated SF-CO2 solutions of both benzoin and BTDA were prepared using a stir bar speed of 600 rpm at various oven temperatures. A Gilson 307 pump (Middleton, WI, USA) whose pump head was cooled to 253.15 K using an Anachem pump head refrigeration unit (Luton, Bedfordshire, UK) was used to supply liquid carbon

Figure 2. Sequence of SSV-SFC interface valve positions required to complete one sample analysis. This derivative figure is a modified version of that first published in ref 18 and is presented with kind permission obtained from Elsevier, 2015. B



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CO2 solubility concentrations for BTDA were determined, and these values were converted into mole fractions using SF-CO2 density values obtained from NIST. 2.6. Determination of SSV-SFC Experimental Accuracy. This was achieved using a self-check validation procedure. Once SF-CO2 solubility results were obtained, the SSV was accurately loaded with a mass of compound 90−95% less than that experimentally determined to produce a saturated solution at a specific temperature and pressure. Thereafter, the concentration of the compound in the subsaturated solution was experimentally determined and compared with the correct complete dissolution value to assess the level of experimental accuracy. For both benzoin and BTDA, two self-check accuracy tests were performed at each experimental temperature using two different pressures at each temperature. Using this procedure and accepting the lowest accuracy values obtained for both benzoin and BTDA, the experimental accuracy for benzoin was determined as 95.3 ± 2.1%, whereas for BTDA the experimental accuracy was determined to be 96.2 ± 3.1%. The values obtained from the experimental accuracy checks were used to derive the relative SF-CO2 solubility mole fraction uncertainty values for benzoin and BTDA.

benzoin or BTDA solubility measurements were completed at the first target pressure, the pressure of the SSV was incremented in steps to produce a series of progressively more saturated compound solutions for further online SSVSFC solubility measurements. Cleaning of the SSV-SFC interface was performed using ethyl acetate according to the previously published method.18 In order to construct external calibration graphs, the SSV was disconnected from the interface and a loading line fitted with a manual syringe injection port was fitted to port 6 of the sample injection valve shown in Figure 2. An open exhaust line was fitted to port 3 of the sample draw through valve to replace the blank fitting. Thereafter using a syringe, standard solutions of benzoin and BTDA could be manually injected into the sample injection valve loop for SFC analysis. 2.4. Supercritical Fluid Chromatography. SFC was performed using a Jasco PU-2080-CO2 Plus pump linked to a Jasco PU-2080 Plus HPLC pump. Solvent mixing was performed using a Jasco HG-1580-32 dynamic mixer. Detection was performed using a Jasco MD-2018 Plus diode-array detector set to operate through the range from 200 to 300 nm for both benzoin and BTDA analyses. The SFC operating pressure was regulated using a Jasco BP-2080 Plus back pressure regulator and for each compound was set at 17.5 MPa with a temperature of 333 K. The SFC system was controlled using Jasco ChromNAV software. All SFC analyses of benzoin and BTDA used a 150 mm × 4.6 mm i.d. phenyl−hexyl column obtained from Phenomenex (Macclesfield, Cheshire, UK) packed with Luna 3 μm stationary phase. The column was housed within an Auto-Science AT-950 oven (Tianjin, China) operated 323 K, with accuracy ±0.5 K. For benzoin the mobile phase was carbon dioxide−methanol (90:10 v/v) with a flow rate of 3 mL·min−1, whereas for BTDA the mobile phase was carbon dioxide−acetonitrile (90:10 v/v) with a flow rate of 3 mL·min−1. 2.5. Quantitation. The concentrations of benzoin and BTDA in saturated SF-CO2 solutions involved the construction of external SFC calibration graphs. The concentrations of benzoin in SF-CO2 solutions were determined using a series of standard ethanolic solutions of benzoin whose concentrations ranged from 55 to 440 mg benzoin in 100 mL of ethanol. Injections were performed using the interface sample injection valve fitted with the same 15 μL sample loop used for SSV-SFC analyses using the manual syringe loading technique. Using benzoin peak integral values obtained at 285 nm and with the origin considered a data point, a linear calibration graph was obtained that provided a correlation coefficient of 0.9999. The concentrations of benzoin in the saturated SF-CO2 solutions were then calculated. These values were then converted into benzoin mole fractions using SF-CO2 density values obtained from the National Institute of Standards and Technology Web site (http://webbook.nist.gov/cgi/fluid.cgi?ID= C124389&Action=Page). The uncertainty values for the NIST density values are specified as 0.03−0.05% for the experimental temperature and pressure ranges used in these studies. The concentrations of BTDA in saturated SF-CO2 solutions were determined using the same general procedure described for benzoin. Calibration standards were prepared using 2.5−7.5 mg BTDA dissolved in 250 mL of acetonitrile. Using the SFC peak integral values obtained for the calibration standards at 215 nm and considering the origin to be a data point, a linear calibration graph was obtained for BTDA that provided a correlation coefficient of 0.9999. Thereafter, the SF-

3. RESULTS AND DISCUSSION 3.1. Benzoin Solubility Determination. The SF-CO2 solubility of benzoin was measured at 308 and 313 K through the pressure range 12−24 MPa with sets of SSV-SFC solubility data being acquired using incremental 2 MPa steps of SF-CO2 pressure. For each SSV temperature, the SSV was continuously stirred for 45 min after each target pressure was initially attained. An intrinsic advantage of static solubility apparatus equipped with online analysis capability is that for any selected pressure and temperature values, sets of data can be very conveniently acquired at controlled time intervals to ensure that equilibrium conditions have been established. Hence, for benzoin after the first period of 45 min continuous stirring at any temperature and target pressure, a set of five replicate injections of the resultant benzoin SF-CO2 solution were made. After the first set of SSV-SFC data was acquired, a further two 45 min stir periods were allowed at the same temperature and pressure with two further sets of SSV-SFC analyses being performed. The degree of correlation between the three sets of benzoin peak integral values obtained at 285 nm established that a saturated benzoin SF-CO2 solution was obtained at all temperatures and pressures used throughout these studies after an initial 45 min continuous stir time. The ability to check that a saturated SF-CO2 solution has been formed is important since it is documented that some compounds may take several hours to form saturated SF-CO2 solutions.16,20 In these current studies a relatively large volume 100 mL SSV was used. This selection was made to minimize the impact of disrupting system equilibrium due to small transient pressure drops brought about due to small volume samples being withdrawn from the SSV vessel for SFC analysis. Figure 3 shows a typical set of chromatograms obtained for a benzoin data set with the stable peak heights serving to confirm minimum disruption to the equilibrium established in the SSV following each sample injection. After three sets of online SSVSFC analyses were completed at the first SF-CO2 target pressure, the pressure in the SSV was increased by 2 MPa, and the method of obtaining three sets of solubility data for benzoin was repeated. The process was repeated using 2 MPa pressure C



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the mean values of three sets of five replicate injections. The RSD values for the benzoin peak integration values used for all quantifications using the external calibration graph were all within 3%. To the best of our knowledge only one other report has previously provided SF-CO2 solubility data for benzoin.21 The previous data was obtained using a flow method with a solvent collection stage followed by offline sample preparation and spectroscopic analysis to determine the quantity of collected benzoin. A comparison of the results obtained using the static SSV-SFC method and the flow method are shown in Figure 4. For both experimental temperatures, the static SSV-

Figure 3. Chromatograms obtained for five online SSV-SFC injections of a saturated benzoin solution prepared using SF-CO2 at 16 MPa and 318 K. Injections made at 0, 2.5, 5, 7.5, and 10 min. Detection at 285 nm with peak integration baseline is shown.

steps until a complete series of benzoin SF-CO2 solubility measurements were obtained. After a complete series of benzoin SF-CO2 solubility studies were completed at each temperature, the SSV was vented and opened for inspection. For all experimental temperatures and pressures used, a layer of needle-like benzoin crystals formed by rapid expansion of supercritical solution were deposited on the vessel chamber wall. Also nondissolved benzoin powder whose morphology had not changed remained in the base of the SSV chamber. These observations confirmed that sufficient benzoin had been loaded into the SSV at the outset of each SSV-SFC solubility study to form a full series of saturated benzoin SFCO2 solutions at all experimental temperatures and pressures used. The mole fraction (y) solubility results obtained for benzoin are summarized in Table 2 that also includes the concentration values of dissolved benzoin. Solubility results are presented as

Figure 4. Comparison of the SF-CO2 solubility results obtained for benzoin using the static SSV-SFC method with those obtained using a flow method by Cheng et al.21

SFC method provided consistently higher SF-CO2 solubility results compared to the flow method used by Cheng et al., whose completely different methodology would rely upon the rapid formation and efficient collection of a saturated benzoin solution. It has been previously reported that flow methods often tend to underestimate equilibrium solubilities due to nonachievement of equilibrium conditions and that this can give rise to low solubility values.22 Consequently, this may be a major factor contributing to the different solubility results shown in Figure 4. Selection of an appropriate SF-CO2 flow rate for a compound is important to obtain accurate SF-CO2 solubility results using flow methods.23 Inspection and comparison of a large compilation8 of solubility SF-CO2 data sets for a large number of different compounds indicates that relatively large differences in experimentally determined SF-CO2 solubility values for the same compound is not uncommon. Presumably, these differences arise due to the use of a wide range of methods and different analytical techniques used to obtain SF-CO2 solubility data. Using the experimental accuracy self-check procedure described in the section 2.6, two self-check measurements were performed to assess the accuracy of the benzoin solubility data determined at 318 K. The checks involved loading the SSV with 78 and 150 mg of benzoin, using stir periods of 45 min with the SF-CO2 pressures set at 14 and 24 MPa respectively. The masses of dissolved benzoin were then determined using online SFC. The accuracy for determining the mass of dissolved benzoin using the SSVSFC method was 98.1 ± 2.8% for the 78 mg benzoin sample and 95.3 ± 2.1% for the 150 mg benzoin sample. For both samples after solubility checks were completed, it was observed that no benzoin powder remained in the base of the vented

Table 2. Meana Solubility Values of Benzoin in SF-CO2 (y, Mole Fraction of Benzoin) With Concentration Values (C), at Temperatures (T), Pressures (P), and Densitiesb (ρ) T (K)

P (MPa)

ρ (kg·m−3)

benzoin solubility (y × 106)

C (mg·100 mL−1)

308

12.0 14.0 16.0 18.0 20.0 22.0 24.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

768.42 802.49 828.10 848.87 866.48 881.85 895.54 659.73 721.83 761.07 790.18 813.52 833.12 850.10

180.63 200.99 217.71 240.37 248.73 260.94 268.94 202.99 238.15 270.45 298.10 327.65 353.75 390.36

66.8 77.6 86.8 98.2 103.6 110.8 115.6 64.3 82.7 99.0 113.4 128.3 141.9 159.7

318

Average of three sets of five replicate injections. bDensity values for SF-CO2 and uncertainty values for the densities obtained from the National Institute of Standards and Technology Web site (http:// webbook.nist.gov/cgi/fluid.cgi?ID=C124389&Action=Page). Standard uncertainties u are u(T) = 0.3 K; u(P) = 0.1 MPa; relative standard uncertainties, ur are ur(p) = 0.03−0.05%; ur(y) = 5.6%; ur(C) = 5.6%. a

D



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Table 3. Meana Solubility Values of BTDA in SF-CO2 (y, Mole Fraction of BTDA) at Temperatures (T), Pressures (P), and Densitiesb (ρ)

SSV, indicating that complete dissolution had been achieved. The self-check results serve to confirm the validity of the complete series of SF-CO2 results obtained for benzoin at 318 K using the SSV-SFC method. Similar self-validation check results were obtained for benzoin at 308 K that provided experimental accuracy values greater than 95.3 ± 2.1%. 3.2. BTDA Solubility Determination. The SF-CO2 solubility of BTDA was measured at 328 and 343 K through the pressure range 15−27.5 MPa with sets of solubility data being acquired using incremental 2.5 MPa steps of SF-CO2 pressure. The same general methodology used to obtain SSVSFC solubility data for benzoin was used also used for BTDA. However, for this compound the SFC mobile phase was modified using acetonitrile since BTDA, which is moisturesensitive, also rapidly reacted with methanol modified SF-CO2 giving rise to a severely distorted peak shape. At any specific experimental temperature and pressure values, only two sets of SSV-SFC analyses were performed for BTDA at 60 min time intervals to check that equilibrium had been established. For these sets of analyses, triplicate injections of the SF-CO2 solutions of BTDA were made. A comparison of sets of BTDA peak integral values at 215 nm confirmed that BTDA provided a saturated SF-CO2 solution after the first 60 min stirred solvating period for all experimental temperatures and pressures used. A typical set of SSV-SFC chromatograms obtained for BTDA is shown in Figure 5. At the end of

T = 328 K −3

T = 343 K

P (MPa)

ρ (kg·m )

y (10 )

ρ(kg·m−3)

y (106)

15.0 17.5 20.0 22.5 25.0 27.5

654.94 714.91 755.52 786.36 811.37 832.49

1.26 1.57 2.0 2.54 3.08 3.76

507.24 599.31 660.04 703.78 737.67 765.29

0.81 1.05 1.71 2.17 2.97 3.94

6

a

Average of two sets of three replicate injections. bDensity values for SF-CO2 and uncertainty values for the densities obtained from the National Institute of Standards and Technology Web site (http:// webbook.nist.gov/cgi/fluid.cgi?ID=C124389&Action=Page). Standard uncertainties u are u(T) = 0.3 K; u(P) = 0.1 MPa; relative standard uncertainties, ur are ur(p) = 0.03−0.05%; ur(y) = 6.2%.

Teja19 in a form simplified by Hansen et al.24 was used to correlate the experimental results for benzoin and BTDA. The equation is T ln(yP /P0) = A + Bρ + CT

(1)

where P is the experimental pressure of the system in MPa, P0 = 1.0 MPa, y is the mole fraction solubility of the solute, ρ is the density of carbon dioxide, and T is temperature in Kelvin. In this model, the solubility of the solid is calculated using multiple linear regression to optimize the values of the constants A, B, and C which are independent of temperature. These calculations were performed using Athena Visual Studio Version 14.0 software applied to the experimental data shown in Tables 2 and 3. The values given in Table 4 provide the best fits for the applied model for each compound. Table 4. Values of Constants A, B, and C Used in the Mendéz-Santiago and Teja Correlation Model Derived from a Number of Samples (N) at Temperatures (T) and Pressures (P)a

Figure 5. Chromatograms obtained for three online SSV-SFC injections of a saturated BTDA solution prepared using SF-CO2 at 175 MPa and 328 K. Injections made at 0, 2.5, and 5 min. Detection at 215 nm with peak integration baseline is shown.

acquiring each series of SF-CO2 solubility measurements the vented SSV was inspected. As for benzoin it was visually confirmed that sufficient BTDA had been used to produce series of saturated SF-CO2 solutions for all SSV-SFC solubility measurements. Using the experimental accuracy self-check procedure for BTDA, after a 60 min stir period the experimental accuracy for BTDA was determined to be 96.2 ± 3.1% through the temperature and pressure ranges studied. The mole fraction SF-CO2 solubility results obtained for BTDA are summarized in Table 3. Solubility results are presented as the mean values of two sets of three replicate injections. The RSD values for the BTDA peak integration values used for all quantifications using the external calibration graph were within 4%. To the best of our knowledge, these are the first SF-CO2 solubility results reported for this compound. 3.3. Density-Based Correlation. The density based semiempirical model proposed by Mendéz-Santiago and

parameter

BTDA

benzoin

N T (K) P (MPa) A (K) B (K·m3·kg−1) C

12 308.15, 318.15 15.0−27.5 −7206.16 3.0 5.04

14 328.15, 343.15 12.0−24.0 −9199.61 2.38 17.84

a

Standard uncertainty values u for benzoin are u(A) = 418.59 K; u(B) = 0.0957 K·m3·kg−1; u(C) = 1.19. Standard uncertainty values u for BTDA are u(A) = 778.37 K; u(B) = 0.177 K·m3·kg−1; u(C) = 2.09.

Figure 6 illustrates that the Mendéz-Santiago and Teja correlation model provides a good fit for the experimental solubility results obtained for both benzoin and BTDA, respectively. The model has also been previously successfully applied to SF-CO2 data obtained using flow systems19,25 and static systems using off-line analytical procedures12 and also a static recirculation vessel interfaced with online HPLC.16 The absolute average relative standard deviation AARD (%) for the correlated solubility results is calculated as follows: AARD(%) = E

100 N

N

∑ i=1

y exp − y calcd y exp

(2)



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AUTHOR INFORMATION

Corresponding Author

*Address: University of Science and Technology Liaoning, Qianshan Road 185, Hi-Tech Zone, Anshan, 114051, China. Email: [email protected]. Funding

Edward D. Ramsey wishes to express his sincere gratitude to the 1000 Plan for Foreign Experts Program sponsored by the Chinese Central Government, Project Number WQ20122100062, who provided funding to help facilitate these studies. Ben Li wishes to acknowledge financial support provided by Liaoning Provincial Government project L2015258 and 601009681 to purchase reagents. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kerton, F. M.; Marriott, R. Alternative Solvents for Green Chemistry, 2nd ed.; RSC Publishing: Cambridge, 2013; pp 115−132. (2) Ramsey, E. D.; Sun, Q. B.; Zhang, Z. Q.; Zhang, C. M.; Gou, W. Mini-Review: Green sustainable processes using supercritical carbon dioxide. J. Environ. Sci. 2009, 21, 720−726. (3) Beckman, E. J. Supercritical and near critical CO2 in green chemical synthesis and processing. J. Supercrit. Fluids 2004, 28, 121− 191. (4) Taylor, L. T. Supercritical Fluid Extraction; John Wiley & Sons: New York, 1996. (5) Ramsey, E. D.; Guo, W.; Liu, J. Y.; Wu, X. H. Supercritical Fluids. In Moo-Young, M., Ed.; Comprehensive Biotechnology, 2nd ed., Vol. 2; Elsevier: Amsterdam, 2011; pp 1007−1026. (6) Brunner, G., Ed. Supercritical Fluids as Solvents and Reaction Media; Elsevier: Amsterdam, 2004. (7) Skerget, M.; Knez, Z.; Knez-Hrncic, M. Solubility of solids in suband supercritical fluids: a review. J. Chem. Eng. Data 2011, 56, 694− 719. (8) Gupta, R. B.; Shim, J. J. Solubility in Supercritical Carbon Dioxide; CRC Press: Boca Raton, 2007. (9) Bristow, S.; Shekunov, B. Y.; York, P. Solubility analysis of drug compounds in supercritical carbon dioxide using static and dynamic extraction systems. Ind. Eng. Chem. Res. 2001, 40, 1732−1739. (10) Sane, A.; Taylor, S.; Sun, Y. P.; Thies, M. C. A semicontinuous flow apparatus for measuring the solubility of opaque solids in supercritical solutions. J. Supercrit. Fluids 2004, 28, 277−285. (11) Burgos-Solorzano, G. I.; Brennecke, J. F.; Stadtherr, M. A. Solubility measurements and modeling of molecules of biological and pharmaceutical interest with supercritical CO2. Fluid Phase Equilib. 2004, 220, 55−68. (12) Hybertson, B. M. Solubility of α-tocopheryl succinate in supercritical carbon dioxide using offline HPLC-MS/MS analysis. J. Chem. Eng. Data 2007, 52, 1123−1127. (13) Hampson, J. W. A recirculating equilibrium procedure for determining organic compound solubility in supercritical fluids. Anthracene in carbon dioxide. J. Chem. Eng. Data 1996, 41, 97−100. (14) Hampson, J. W.; Maxwell, R. J.; Li, S.; Shadwell, R. J. Solubility of three veterinary drugs in supercritical carbon dioxide by a recirculating equilibrium method. J. Chem. Eng. Data 1999, 44, 1222−1225. (15) Ashraf-Khorassani, M.; Combs, M. T.; Taylor, L. T.; Schweighardt, F. K.; Mathias, P. S. Solubility study of sulfamethazine and sulfadimethoxine in supercritical carbon dioxide, fluoroform, and subcritical Freon 134A. J. Chem. Eng. Data 1997, 42, 636−640. (16) Elizalde-Solis, O.; Galicia-Luna, L. A. New apparatus for solubility measurements of solids in carbon dioxide. Ind. Eng. Chem. Res. 2011, 50, 207−212. (17) Ashraf-Khorassani, M.; Barzegar, M.; Yamini, Y. On-line coupling of supercritical fluid extraction with high performance liquid chromatography. J. High Resolut. Chromatogr. 1995, 18, 472−476.

Figure 6. Correlation of the experimental solubility data for: (a) benzoin and (b) BTDA in SF-CO2 by the method of Mendéz-Santiago and Teja. Experimental: ◊, 308 K; □, 318 K; △, 328 K; ○, 343 K; , best fit line to the experimental data.

where N is the number of data points. The experimental mole fraction and calculated mole fraction solubility data (y) according to the simplified Mendéz-Santago and Teja model are denoted with the superscripts exp and calcd, respectively. For benzoin the AARD was 3.51%, whereas for BTDA the AARD was 10.21%. These AARD values compare favorably to those obtained using the Mendéz-Santago and Teja model applied to the SF-CO2 solubility results for four compounds using a static solubility vessel directly interfaced online with HPLC.16

4. CONCLUSIONS The use of SSV-SFC provides an integrated static solubilityanalytical method to obtain SF-CO2 solubility for organic compounds. The current work demonstrates the applicability of the SSV-SFC method for determining the solubility of organic compounds that exhibit low to medium SF-CO2 solubility. The SSV-SFC method is well-suited for obtaining solubility data for light and/or air-sensitive compounds. Additionally unlike online reverse phase HPLC methods, moisture sensitive compounds such as BTDA are amenable to SFC since the SF-CO2 mobile phase can be modified with polar aprotic solvents such as acetonitrile. The SSV-SFC solubility data obtained for both benzoin and BTDA provide close fits to the solubility correlation model proposed by Mendéz-Santiago and Teja. F



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Determining the Solubility of Organic Compounds in Supercritical

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