Measuring the Solubility of Anthracene and Chrysene in Supercritical

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Measuring the Solubility of Anthracene and Chrysene in Supercritical Fluid Carbon Dioxide Using Static Solubility Apparatus Directly Interfaced Online to Supercritical Fluid Chromatography Ben Li, Wei Guo, and Edward D. Ramsey* Sustainable Technology Research Centre, University of Science and Technology Liaoning, 185 Qianshan Road, Anshan 114051, China ABSTRACT: A continuously stirred high pressure solubility vessel has been directly interfaced online to a supercritical fluid chromatography (SFC) system using a single valve recirculation interface. The recirculation pump built into the interface maintains a constant flow of fresh supercritical fluid carbon dioxide solution from the solubility vessel through the interface sample injection valve in a return flow circuit. A series of progressively more concentrated anthracene and chrysene solutions were separately prepared by incrementing the pressure within the solubility vessel in controlled steps. Prior to performing online SFC solubility measurements for each polyaromatic hydrocarbon, in situ checks were made to establish that fully saturated solutions were formed at each different pressure step. The method also involves the utilization of a self-validation experimental procedure to check the accuracy of solubility measurements. For anthracene, solubility data was obtained at 313, 323, 333, and 343 K through the pressure range 10−28 MPa using 2 MPa steps. For chrysene, solubility data was obtained at 313, 323, 333, and 343 K through the pressure range 10−27.5 MPa using 2.5 MPa steps. All sets of experimental solubility data were correlated according to the density based models of Méndez-Santiago Teja and Bartle.

1. INTRODUCTION The use of supercritical fluid carbon dioxide (SF-CO2) as an alternative “green” environmentally benign solvent to replace a range of conventional organic solvents has attracted considerable attention. A diverse range of applications and processes involving the use of SF-CO2 have been reported1−6 that include extraction, fractionation, purification, impregnation, cleaning, dyeing, synthetic reactions, polymerization, catalysis, tissue engineering, advanced material manufacture, and particle engineering. Fundamental to the design and successful implementation of SF-CO2 based processes, knowledge of relevant compound(s) solubility in SF-CO2 is of key importance. This has resulted in the construction of SF-CO2 solubility data compilations.7−9 A wide range of different equipment types and various analytical techniques have been used to obtain SF-CO2 solubility data. Despite this, the vast majority of experimental procedures to perform SF-CO2 solubility measurements broadly fall into only two categories, namely, dynamic or static methods.10 Dynamic methods involve the continuous flow of fresh SFCO2 through a saturation column loaded with a vast excess of the compound whose solubility is to be measured. Thereafter, the resultant SF-CO2 solution is decompressed using a calibrated back pressure regulator that typically discharges the sample now entrained in decompressed gaseous carbon dioxide through a trap containing a suitable organic solvent. After collection, the sample solution formed in the trap is then volumetrically prepared for off line quantitative analysis. In addition, the volume of decompressed carbon dioxide to provide the sample must also be taken into account using © XXXX American Chemical Society

dynamic methods, and this frequently involves the additional requirement for an accurate gas flow meter. Apart from complexity, a fundamental problem associated with dynamic methods is reliance on the assumption that a compound will very rapidly form a saturated SF-CO2 solution during the flow of SF-CO2 through the saturation column. However, various compounds only slowly establish dissolution equilibrium in SFCO2.11,12 Consequently, dynamic SF-CO2 solubility measurements may provide low solubility values.13,14 Static methods involve dissolving a compound within a sealed fixed volume solubility vessel that is supplied with just sufficient SF-CO2 to reach the experimental target pressure. After sufficient contact time has lapsed such that equilibrium dissolution has been established, small volume samples can be withdrawn from the pressurized vessel with minimum disruption to perform solubility measurements. This typically involves the use of a HPLC injection valve that discharges a small aliquot of SF-CO2 sample solution taken from the static solubility vessel into a solvent trap prior to volumetric sample preparation and off line quantitative analysis. It is well established that reduction in sample collection and subsequent manual sample preparation stages decrease the two major sources of errors associated with final analytical measurements.15 This is particularly the case with samples obtained from SF-CO2 apparatus for off line analysis since quantitative trapping of compounds may be difficult to achieve Received: September 27, 2017 Accepted: January 24, 2018

A

DOI: 10.1021/acs.jced.7b00857 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

during the SF-CO2 decompression stage.16 As a result of these factors, several reports have described the use of static SF-CO2 solubility apparatus equipped with online spectroscopic17 or chromatographic12 techniques aimed at providing more rapid and reliable measurements. The first use of a recirculating interface connected to a static SF-CO2 solubility vessel involved the use of online HPLC to obtain solubility data.18 However, the alternative use of online SFC interfaced to static solubility SF-CO2 apparatus is more logical. This is due to the immediate compatibility of direct injections of SF-CO2 sample solution taken from the static solubility vessel with the composition and pressure of the SFC chromatographic mobile phase.13 Consequently, a two valve interface that was initially developed to monitor the progress of an esterification reaction in SF-CO2 using online HPLC19 was subsequently used without modification in studies to perform SF-CO2 solubility measurements using online SFC.20,21 Although the two valve interface has proved highly effective, its operation involves the use of a relatively complex sequence of synchronized valve settings. This has led to the introduction of a simplified single valve recirculating interface, initially used to couple a continuously stirred solubility vessel (SSV) with online SFC to obtain solubility data for two basic drugs in SFCO2.22 A subsequent comparative study using the single valve recirculation interface to couple either online conventional bore HPLC or online SFC to a reaction cell to monitor the progress of a photochemical reaction in SF-CO2 has demonstrated significant advantages of using online SFC.23 These advantages include (i) elimination of injected sample solvent/chromatography mobile phase compatibility issues, (ii) an improvement in online SFC chromatographic performance compare to off line SFC performance due to “like being injected into like” in terms of both sample solvent composition and pressure, (iii) potentially faster analysis times, and (iv) potentially enhanced sensitivity relative to online HPLC. In this study, we report the use of the integrated SSV-SFC system to measure the solubility of the two polyaromatic hydrocarbons (PAHs) anthracene and chrysene in SF-CO2. The solubility of anthracene in SF-CO2 has been very extensively studied by various workers using a wide variety of methods making this particular PAH an ideal compound to compare the performance of the SSV-SFC method. In contrast, very little SF-CO2 solubility data for chrysene is currently available. Consequently, for different reasons these two PAH compounds were selected to further test the capability of the SSV-SFC system using the single valve recirculation interface. The sets of experimental results obtained for both PAH compounds in SF-CO2 were used to further evaluate the applicability of the Méndez-Santiago Teja24 model that has been previously successfully applied to SF-CO2 solubility data obtained for a range of compound types using the SSV-SFC method.20−22 For this study, the treatment of SSV-SFC solubility data has also been extended to also evaluate the applicability of the Bartle25 solubility model.

Table 1. Information Concerning Compounds and Reagents chemical name

CAS No.

source

anthracene chrysene methanol ethanol

120-12-7 218-1-9 67-56-1 64-17-5

acetonitrile carbon dioxide

75-5-8 124-38-9

Aladdin Accustandard Sigma-Aldrich Beijing Chemical Works Sigma-Aldrich Airichem

a GC is gas chromatography. chromatography.

b

mole fraction purity

analysis method

≥0.99 ≥0.99 ≥0.999 ≥0.999

GCa HPLCb GCa GCa

≥0.999 ≥0.999

GCa GCa

HPLC is high-performance liquid

magnetic stir bar drive. The specification of this oven is ±0.3 K. Saturated SF-CO2 solutions of anthracene and chrysene were separately prepared using a stir bar speed of 600 rpm at various oven temperatures. For each series of solubility measurements performed at a fixed temperature, the SSV was loaded one time with either 150 mg of anthracene powder or 20 mg of chrysene powder. A Gilson 307 pump (Middleton, WI) whose pump head was cooled to 253 K using an Anachem pump head refrigeration unit (Luton, Bedfordshire, U.K.) was used to supply liquid carbon dioxide to the SSV for all SF-CO2 solubility studies. A PTX 500 series pressure transmitter (Druck, Leicester, U.K.) was used to calibrate the Gilson 307 pump pressure transducer. Estimated accuracy for the SSV pressure was ±0.1 MPa. A cylinder containing high-purity liquid carbon dioxide equipped with a liquid draw-off tube was connected to the pump. After a complete series of anthracene and chrysene SSV-SFC solubility studies were completed, the SSV and interface were vented via a solvent wash exhaust system.19 A schematic of the experimental system has been previously published.22 2.3. SSV−SFC Interface. The interface used to connect the SSV directly online to SFC was constructed using a Rheodyne 7010 valve (Cotati, CA) fitted with a 5 μL sample loop. This sample injection valve was mounted inside a Gilson 831 oven that had been modified to house a Micropump model GAHX21 (Micropump Inc., WA) recirculation pump. The combined volume of the recirculation pump and tubing used to construct the interface was 5.7 mL. The recirculation pump was operated at 2000 rpm to provide a flow rate sufficiently fast to ensure fresh SF-CO2 solution drawn from the SSV flowed continuously through the SFC sample injection valve in the return flow circuit back to the SSV. For SSV-SFC, 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 9 MPa was obtained. After a thermal equilibration period of 30 min, a second linked pump program delivered liquid carbon dioxide at 8 mL min−1 until the first SSV target pressure for solubility studies was attained. Thereafter, the SSV liquid carbon dioxide supply pump maintained the target pressure in the static SSV and recirculation interface to perform solubility measurements by delivering very short flow pulses if required. For both anthracene and chrysene, samples of the SF-CO2 solutions produced at each target pressure were analyzed at fixed time intervals to confirm that dissolution equilibrium had been established before solubility measurements were performed. After sets of anthracene or chrysene solubility measurements were completed at the first target pressure, the pressure of the SSV was incremented in steps to produce a series of

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. The specifications of the compounds used without further purification in these studies are listed in Table 1. 2.2. Solubility Apparatus. A Jasco EV-3 50 mL high pressure vessel (Hachioji, Japan) equipped with a magnetic stir bar served as the SSV. The SSV was housed within a Jasco SCFSro oven whose design includes an integral variable speed B



Article

assessed by comparison of the experimental concentration value with the correct dissolution value if the compound was completely dissolved. The self-check method has now been modified to include an additional stage. Once a check is completed at a specific test pressure, the pressure within the SSV is increased and online SFC measurements are performed at the second above test pressure value. This was to confirm that the test compound peak integral value did not increase to verify complete dissolution of the test compound at the lower actual test pressure.

progressively more concentrated PAH solutions. At each pressure step, checks were performed to verify that saturated solutions were formed prior to obtaining SF-CO2 solubility data. After complete series of PAH solubility data were acquired at each experimental temperature, the interface was cleaned using ethanol. After each cleaning stage was completed, the performance of the recirculation pump was checked with ethanol using a previously described procedure.22 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 anthracene and chrysene analyses. The SFC operating pressure was regulated using a Jasco BP-2080 Plus back pressure regulator and for each compound was set at 15 MPa with a temperature of 333 K. The SFC system was controlled using Jasco ChromNAV software. All SFC analyses for anthracene and chrysene were performed using a 150 mm × 4.6 mm i.d. phenyl hexyl column obtained from Phenomenex (Macclesfield, Cheshire, U.K.) packed with Luna 3 μm stationary phase. The column was housed within an AutoScience AT-950 oven (Tianjin, China) operated at 323 K, with accuracy ±0.5 K. For both anthracene and chrysene, the mobile phase was carbon dioxide with flow rate at 4 mL min−1. 2.5. Quantitation. The concentrations of anthracene and chrysene in saturated SF-CO2 solutions were determined via the use of external graphs that were constructed using calibration standards. Manual syringe injections of calibration standards of anthracene and chrysene separately prepared in ethanol were performed by modifying the interface plumbing in a manner previously described.22 During the SFC analysis of calibration standards, the interface 7010 sample injection valve remained fitted with the same 5 μL sample loop used for all the SSV-SFC solubility measurements. For anthracene, six calibration standards whose concentrations ranged from 1.8 mg to 60 mg of anthracene dissolved in 100 mL of ethanol were analyzed. Using the anthracene SFC peak integral values obtained for the calibration standards at 220 nm and with a fit through the origin, a linear calibration graph was obtained that provided a correlation coefficient of 0.9975. For chrysene, five calibration standards whose concentrations ranged from 0.3 mg to 5 mg dissolved in 50 mL of ethanol were analyzed. Using the chrysene SFC peak integral values obtained at 265 nm and with a fit through the origin, a linear calibration graph was obtained that provided a correlation coefficient of 0.9969. The concentrations of anthracene and chrysene in saturated SFCO2 solutions were calculated using the calibration graphs. The concentration values obtained for anthracene and chrysene were then converted into their respective mole fraction values using SF-CO2 density values obtained from the National Institute of Standards and Technology (NIST) Web site.26 These calculations took into account the 55.7 mL total volume of the static SSV and interface. 2.6. Determination of SSV-SFC Experimental Accuracy. This was achieved using a self-check validation procedure that has been previously described.22 Briefly, the method involves loading the SSV with an accurate mass of compound close yet below its solubility limit value in 55.7 mL of SF-CO2 that has been experimentally determined using the SSV-SFC method. The level of SSV-SFC method accuracy is then

3. RESULTS AND DISCUSSION 3.1. Anthracene SF-CO2 Solubility Studies. The solubility of anthracene in SF-CO2 was measured at 313, 323, 333, and 343 K through the pressure range from 10 to 28 MPa. At each of the four temperatures, a series of SSV-SFC solubility measurements were performed using 2 MPa increments in SSV pressure. Prior to performing any solubility measurements at any temperature and pressure value, a dissolution check was performed to ensure equilibrium had been established. For anthracene this involved making sets of five replicate online SSV-SFC analyses spaced at 1 h intervals at each pressure value. Three sets of check results were obtained for anthracene during a 240 min continuous stir period that included the 60 min required to complete the online SFC analyses associated with the three dissolution checks. Comparison of the three average values for the sets of anthracene peak integral values indicated that equilibrium had been attained after 1 h since subsequent sets of anthracene peak integral values obtained at longer stir periods did not increase. All checks performed at all of the different experimental conditions confirmed that 1 h continuous stirring was sufficient to establish equilibrium for anthracene at all experimental temperatures and pressures used in these studies. Once it was confirmed that equilibrium had been established, a set of five replicate SSV-SFC sample injections of the anthracene SF-CO2 solution were made to measure solubility at the specific temperature and pressure. A representative chromatogram showing five replicate SSV-SFC analyses to measure the solubility of anthracene in SF-CO2 is shown in Figure 1. The results in Figure 1 demonstrate high reproducibility and stable SSV-SFC system performance. After a complete series of anthracene SF-CO2 solubility measurements had been completed at a fixed temperature through the pressure range 10−28 MPa, the SSV was vented

Figure 1. Chromatogram obtained for a set of five online SSV-SFC injections of a saturated SF-CO2 solution of anthracene prepared at 313 K and 18 MPa. Injections made at 0, 4, 8, 12, 16 min providing anthracene peaks 1−5. Detection at 220 nm. C



Article

and opened for inspection. For all four experimental temperatures, the chamber of the vented SSV was coated with a fine layer of needle like anthracene crystals deposited by a rapid expansion of supercritical solution (RESS) process. Also nondissolved anthracene powder whose morphology had not changed remained in the base of the SSV situated around the magnetic stir bar. These observations confirmed that sufficient anthracene powder had been initially loaded into the SSV to produce a complete series of saturated solutions at all four temperatures and all SF-CO2 pressures at which online SFC solubility measurements were performed. The RSD values for the anthracene peak integration values used for all quantifications using the external calibration graph were all within 4.5%. Table 2 shows the mole fraction (y) SSVSFC solubility results obtained for anthracene.

Figure 3. Comparison of SF-CO2 solubility results obtained at 323 K for anthracene using the static SSV-SFC method with literature data (▲ is current study, □ is Anitescu and Tavlarides,27 △ is Hampson,28 ○ is Lou et al.,30 ◊ is Zerda et al.,31 + is Johnston et al.,32 × is Ekart et al.,33 - is Miller et al.,34 and − is Coutsikos et al.35

Table 2. Meana Anthracene Mole Fraction Solubility Values (y) in SF-CO2 with Temperature Values from 313 to 343 K and Pressure Range from 10 to 28 MPa mole fraction solubility (y × 106) SF-CO2 pressure (MPa)

313 K

323 K

333 K

343 K

10 12 14 16 18 20 22 24 26 28

62.5 (2.7) 71 (1.3) 77.5 (1.7) 86.6 (2.7) 90 (0.8) 95 (0.7) 98.7 (2.4) 103 (2.6) 102 (2.8) 104 (0.9)

67.7 (4.0) 80 (1.3) 93.9 (2.9) 99 (2.0) 105 (2.3) 110 (1.4) 112.2 (2.9) 111.5 (2.1) 113.1 (3.5) 114 (4.3)

41.7 (3.3) 79.5 (1.8) 99 (1.2) 114 (2.6) 121.8 (1.5) 130.2 (2.4) 132.6 (1.4) 139 (3.3) 142 (3.1) 144.2 (3.7)

34.5 (2.1) 83.8 (4.2) 113.7 (4.5) 138.5 (3.5) 145.5 (0.9) 152.8 (2.3) 153.4 (4.0) 155.2 (2.6) 159.5 (2.2) 164.5 (2.0)

from the SSV-SFC solubility data, the data shown in Figures 2 and 3 also demonstrates the existence of relatively large differences in previously published SF-CO2 anthracene solubility values. As we have previously reported,20−22 large differences in published SF-CO2 solubility data for the same compound is not uncommon. This can be confirmed by inspection of a large compilation of SF-CO2 solubility data7 and comparing the various solubility values reported for the same compound. This situation probably arises due to a range of factors that include the use of either dynamic or static methods, the scale and complexity of different types of equipment utilized, sample preparation procedures, human error, equipment calibration, and the wide range of different analysis techniques that have been utilized. Apart from these factors, it is essential to ensure that dissolution equilibrium has been established prior to performing final SF-CO2 solubility measurements, since it is reported that 3−12 h may be required for some compounds to reach equilibrium.11,12,22 In terms of experimental setup the data shown in Figure 2 obtained by Hampson28 and Ashraf-Khorassani et al.18 are of particular interest. Although both these previous studies used small volume static solubility vessels that were not equipped with internal stirring, each of these previous studies used the mixing imparted by a recirculation pump as the means to produce anthracene SF-CO2 solutions. The method adopted by Hampson used sample collection prior to off line UV analysis whereas Ashraf-Khorassani et al. used online HPLC to determine anthracene solubility values. As shown in Figure 2, the results obtained by Hampson and Ashraf-Khorassani et al. provided good correlation with respect to each other. In terms of equipment and methodology despite using a recirculation pump, the SSV-SFC method has some significant differences compared to the static solubility apparatus systems used by Hampson and Ashraf-Khorassani et al. The SSV-SFC uses a larger scale vessel equipped with internal continuous magnetic stirring and also the mixing imparted by a recirculation pump to help establish compound equilibrium. In addition the SSV-SFC method used 1 h to ensure that equilibrium was established for anthracene prior to performing solubility measurements. In comparison, Hampson and Ashraf-Khorassani et al. used 25 min to achieve equilibrium before performing anthracene solubility measurements. Apart from the general factors described in the previous paragraph that can result in different

a

n = 3. Values in parentheses are RSD (%). Density values for SF-CO2 and uncertainty values for the densities obtained from the NIST Web site.26 Standard uncertainties u are u(T) = 0.3 K, u(P) = 0.1 MPa; relative standard uncertainties, ur are ur(p) = 0.05, ur(y) = 0.08.

Comparisons of the SSV-SFC solubility data obtained at two of the experimental temperatures with values previously reported in the literature are shown in Figures 2 and 3. Apart

Figure 2. Comparison of SF-CO2 solubility results obtained at 313 K for anthracene using the static SSV-SFC method with literature data (▲ is current study, + is Ngo et al.,17 × is Ashraf-Khorassani et al.,18 □ is Anitescu and Tavlarides,27 △ is Hampson,28 * is Kwiatkowski et al.,29 and ○ is Lou et al.30 D



Article

SF-CO2 solubility values being obtained for the same compound, the two stage mixing utilized by the SSV-SFC method coupled with a relatively long stir period may account for the SSV-SFC method producing the high series of solubility values shown in Figure 2. As shown in Figure 3, the differences between the SSV-SFC solubility data and literature solubility values for anthracene are less apparent at 323 K. The trends shown in Figure 3 are also typical when the SSV-SFC solubility results obtained for anthracene are compared with previously reported literature values obtained at 333 K27,28,30,31 and 343 K.30−32 At 323, 333, and 343 K, the SSV-SFC method initially provided higher anthracene SF-CO2 solubility values at the lower SF-CO2 experimental pressures used in these studies. However, at higher SF-CO2 pressures the SSV-SFC solubility values obtained at 323, 333, and 343 K become bracketed by SFCO2 solubility values previously reported for anthracene in the literature. A factor that may attribute to the higher SSV-SFC solubility values of anthracene measured at 323, 333, and 343 K at the lower SF-CO2 pressures is the two stage mixing and longer stir times employed by the SSV-SFC method. Efficient mixing for longer times would serve to promote establishment of equilibrium particularly at the reduced SF-CO2 solvating strengths associated with lower pressures. This tentative explanation would be consistent with the complete series of higher anthracene SF-CO2 solubility values obtained at 313 K when establishment of equilibrium dissolution might be more difficult to achieve throughout the lowest series of SF-CO2 solvating strengths used in the SSV-SFC studies. 3.2. Chrysene SF-CO2 Solubility Studies. The solubility of chrysene in SF-CO2 was measured at 313, 323, 333, and 343 K through the pressure range from 10 to 27.5 MPa. At each of the four temperatures a series of SSV-SFC solubility measurements were performed using 2.5 MPa increments in SSV pressure. Using the same procedure previously described for anthracene it was determined that a longer continuous stir period of 2 h was required for chrysene to reach dissolution equilibrium at all experimental temperatures and pressures used in this study. After this stir period at each temperature and pressure, SSV-SFC solubility measurements were performed. Once a set of solubility measurements had been obtained the pressure of the SSV was increased by 2.5 MPa and this process was repeated until a complete series of SSV-SFC solubility measurements of chrysene in SF-CO2 had been obtained at each experimental temperature. Figure 4 shows a set of five replicate SSV-SFC analyses obtained to determine the SF-CO2 solubility of chrysene. As for anthracene, inspection of the chromatograms shown in Figure 4 confirms stable and reproducible SSV-SFC performance. After a complete series of chrysene solubility measurements were obtained at each of the four experimental temperatures, the SSV was vented and opened for visual inspection. As with anthracene, a fine layer of chrysene crystals were evenly deposited on the side wall of the SSV due to RESS occurring during venting. Chrysene powder with unchanged morphology was observed at the SSV base. These observations served to confirm that sufficient chrysene had been loaded at the outset to produce complete series of saturated SF-CO2 solutions at all temperatures and pressures used to perform solubility measurements. The RSD values for the chrysene peak integration values used for all quantifications using the external calibration graph

Figure 4. Chromatogram obtained for a set of five online SSV-SFC injections of a saturated SF-CO2 solution of chrysene prepared at 313 K and 20 MPa. Injections made at 0, 2.5, 5, 7.5, 10 min providing chrysene peaks 1−5. Detection at 265 nm.

were all within 3.1%. Table 3 shows the mole fraction (y) SSVSFC solubility results obtained for chrysene. Table 3. Meana Chrysene Mole Fraction Solubility Values (y) in SF-CO2 with Temperature Values from 313 to 343 K mole fraction solubility (y × 106) SF-CO2 pressure (MPa)

313 K

10 12.5 15 17.5 20 22.5 25 27.5

3.3 (0.5) 4.0(1.6) 4.7 (1.1) 5.2 (1.2) 5.6 (0.5) 6.0 (2.3) 6.4 (0.5) 6.9 (0.2)

323 K 4.1 4.8 5.7 6.6 7.5 8.1 8.6 9.1

(0.9) (0.2) (0.4) (2.1) (1.1) (1.5) (0.3) (2.0)

333 K

343 K

3.3 (2.2) 5.8 (2.4) 7.3 (0.9) 7.9 (1.0) 9.1 (2.3) 10.2 (1.9) 11.1 (2.9) 11.6 (1.6)

2.4 (3.1) 5.5 (2.9) 7.9 (0.9) 9.7 (1.2) 11.2 (1.3) 12.2 (0.7) 13.2 (0.5) 13.5 (0.9)

a

n = 3. Values in parentheses are RSD (%). Density values for SF-CO2 and uncertainty values for the densities obtained from the NIST Web site.26 Standard uncertainties u are u(T) = 0.3 K, u(P) = 0.1 MPa; relative standard uncertainties, ur are ur(p) = 0.05, ur(y) = 0.06.

Unlike anthracene, only a few reports have provided SF-CO2 solubility data for chrysene.30,34 Comparison of the chrysene SF-CO2 results obtained by the SSV-SFC method with those previously published for chrysene are shown in Figure 5. From the somewhat limited comparison it would appear that the SSV-SFC solubility results obtained for chrysene fall within a range that might be expected. 3.3. SSV-SFC Method Accuracy Tests. The accuracy of the SSV-SFC solubility results obtained for anthracene and chrysene were checked using a self-validation method. The method is described in the Experimental Section and has been previously utilized to validate the SSV-SFC solubility results obtained for a range of different compounds.20−22 The results are summarized in Table 4. For each accuracy test, after the test solubility measurement was completed the pressure within the SSV was increased by 2.5 MPa. After a further 1 h stir period at the higher pressure a further analysis was performed. For all anthracene and chrysene test solutions, the peak integration values at the second higher pressures remained stable confirming that complete dissolution of the test samples had occurred at all the first selected test pressure and temperature values. After each of the tests, visual inspection of the vented E



Article

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 (kg m−3), and T is temperature in K. In this model, the solubility of the solid is calculated using multiple linear regression to optimize the parameters A, B, and C which are independent of temperature. The calculations were performed using Athena Visual Studio version 14.0 software applied to the SSV-SFC solubility data shown in Tables 2 and 3. The values shown in Table 5 provide the best fits for the application of the MST model for anthracene and chrysene. Table 5. Values of Constants A, B and C Used in the MST Model Derived from Number of Samples (N) at Temperatures (T) and Pressures (P)a

Figure 5. Comparison of SF-CO2 solubility results obtained for chrysene using the static SSV-SFC method where ⧫ is 313 K, ■ is 323 K, ▲ is 333 K, ● is 343 K. For literature values for Lou et al.:30 ◊ is 313 K, □ is 323 K, ○ is 343 K. For Miller et al.36 × is 313 K.

SSV provided no indication of any remaining nondissolved PAH sample. Inspection of the results shown in Table 4 confirms that the SSV-SFC method provided a high degree of accuracy for measuring the solubility of anthracene and chrysene in SF-CO2. The checks performed for anthracene at 313 K were considered particularly important since at this temperature the SSV-SFC method provides consistently higher solubility values for this PAH as shown in Figure 2 compared with the wide range of previously published values. As far as we are currently aware, the self-assessment accuracy test introduced for the SSV-SFC method is the only procedure currently used for the selfvalidation of experimental SF-CO2 solubility results.

anthracene

chrysene

40 313−343 10−28 −5530.87 1.39 0.94

32 313−343 10−27.5 −6678.8 1.52 8.17

a

Standard uncertainty values u for anthracene are u(A) = 296.94 K, u(B) = 0.057 K m3 kg−1, u(C) = 0.8. Standard uncertainty values u for chrysene are u(A) = 403.58 K, u(B) = 0.076 K m3 kg−1; u(C) = 1.14.

Application of the values shown in Table 5 provides the results shown in Figure 6 that confirm the MST model provides good fits for all experimental SSV-SFC solubility results obtained for both anthracene and chrysene. 4.2. Application of the Bartle Model. The density based solubility correlation proposed by Johnston et al.38 in a derivative form simplified by Bartle et al.25 was used to correlate the experimental results obtained for anthracene and chrysene in this study. The equation for the Bartle model is given below:

4. DENSITY BASED CORRELATION 4.1. Application of the Méndez-Santiago Teja Model. The semiempirical density based model proposed by MéndezSantiago and Teja24 in a form simplified by Hansen et al.37 has been previously used20−22 to successfully correlate SSV-SFC experimental solubility data obtained for: caffeiene, benzoin, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, nifedipine, and quinine. Therefore, the utility of the simplified MéndezSantiago Teja (MST) model was further evaluated by its application to the SSV-SFC solubility results obtained for anthracene and chrysene. The simplified equation is T ln(yP /P0) = A + Bρ + CT

parameter N T/K P/MPa A/K B/K m3 kg−1 C

ln(xP /Pref ) = A + c(ρ − ρref )

(2)

where A = a + b/T, and x is the mole fraction of the solute (taken here to be equal to the ratio of the number of moles of solute divided by the number of moles of SF-CO2), P is pressure, Pref is 0.1 MPa, the experimental pressure of the system in MPa, ρ is the density of carbon dioxide at a certain temperature and pressure, ρref is 700 kg m−3, and T is the temperature in K. In this model, c is assumed to be a constant over the temperature range, and constant A is assumed to be

(1)

Table 4. Summary of Self-Validation Accuracy Test Results for Anthracene and Chrysene

a

SF-CO2 test condition MPa/K

SSV-SFC mole fraction solubility y × 106

equivalent mass of PAH in 55.7 mL of saturated SF-CO2 soln (mg)

test mass of PAH loaded into SSV (mg)

mass of PAH determined in test soln (mg)

accuracya (%)

anthracene 10/313 28/313 10/343 28/343

62.5 104 34.5 164.5

8.91 21.13 1.91 28.44

8.5 20 1.8 28

8.2 18.8 1.9 27.4

96.5 94 94.7 97.9

chrysene 10/313 27.5/313 10/343 27.5/343

3.3 6.9 2.4 13.5

0.6 1.78 0.17 2.97

0.55 1.6 0.15 2.8

0.52 1.54 0.15 2.9

94.5 96.3 100 96.4

Accuracy (%) = 100 − |100 − [(experimental mass/test mass) × 100]|. F



Article

Figure 6. Correlation of the experimental solubility data for (a) anthracene and (b) chrysene in SF-CO2 using the MST model. Experimental temperatures: ◊ 313 K, □ 323 K, △ 333 K, ○ 343 K, where − is the best fit line to the experimental data.

linearly correlated by constants a and b arising from the fugacity of the solute. The solubility of the solid is calculated using multiple linear regression to optimize the parameters a, b, and c, which are independent of temperature. The multiple linear regression calculations for the Bartle model were performed using Athena Visual Studio, version 14.0, software applied to the experimental data shown in Tables 2 and 3. The results obtained are shown in Table 6.

using the values in Tables 2 and 3 obtained for the MST and Bartle models, respectively, are shown as the continuous solid lines in Figure 8. The individual experimental solubility values obtained for anthracene and chrysene are indicated only by the symbols shown in Figure 8. Therefore, a perfect correlation of the model predicted solubility values and the experimental solubility values would result in the solid line perfectly bisecting all the relevant series of individual experimental solubility value symbols at the given temperature values. Inspection of Figure 8 indicates that for anthracene, the MST model provides closer fits for the three sets of SSV-SFC solubility values obtained at 323, 333, and 343 K. However, for chrysene the Bartle model provides closer fits for the three sets of chrysene SSV-SFC solubility values obtained at 313, 323, and 333 K. Overall, although neither model is perfect, the results shown in Figure 8 indicate that selective application of either the MST and Bartle models have generally provided successful correlations to the SSV-SFC solubility data sets obtained for each of the two PAHs. The absolute relative deviation (ARD, %) for the correlated solubility results are calculated as follows according to Ota et al.:39

Table 6. Values of Constants a, b, and c Used in the Bartle Model Derived from Number of Samples (N) at Temperatures (T) and Pressures (P)a parameter

anthracene

chrysene

N T/K P/MPa a b/K c/m3 kg−1

40 313−343 10−28 1.56 −2756.09 0.0023

32 313−343 10−27.5 2.14 −3830.28 0.000155

a

Standard uncertainty values u for anthracene are u(A) = 0.73, u(B) = 239.92 K, u(C) = 0.00015 m3 kg−1. Standard uncertainty values u for chrysene are u(A) = 0.77, u(B) = 251.91 K, u(C) = 0.00016 m3 kg−1.

ARD(%) =

Application of the Bartle model using the values of a and b shown in Table 5, whereby A is expressed as a function of reciprocal temperature, resulted in the linear relationships for anthracene and chrysene shown in Figure 7. The linear relationships confirm that satisfactory a and b values have been determined. 4.3. Comparison of MST and Bartle Solubility Models. The predicted solubility values for anthracene and chrysene

100 N

N

∑ i=1

ln y i ,exp − ln y i ,calc ln y i ,exp

(3)

A more commonly used equation to calculate ARD is ARD(%) =

100 N

N

∑ i=1

y i ,exp − y i ,calc y i ,exp

(4)

For both the equations, N is the total number of data points and where the experimental mole fraction and calculated mole fraction solubility data values calculated using either the MST or Bartle solubility models are denoted with the superscripts exp and calc, respectively. The ARD results calculated applying eqs 3 and 4 to the results obtained for the MST and Bartle models for anthracene and chrysene are summarized in Table 7. For anthracene, as shown in Table 7, the ARD results obtained using eqs 3 and 4 are comparable. However, for reasons that we currently cannot explain the ARD results obtained using eqs 3 and 4 show an almost 2-fold difference in values for chrysene. The ARD values obtained for anthracene and chrysene using eq 4 are within the range previously obtained for several compounds whose solubility results were successfully correlated using only the MST model20−22 and are in general agreement with the ARD values reported by other workers.8,9,12,37

Figure 7. Plot of isotherm intercept A as a function of reciprocal temperature for anthracene (□) and chrysene (△). G



Article

Figure 8. Correlation of the experimental solubility data for anthracene in parts a and b and chrysene in parts c and d in SF-CO2 using the Bartle and MST models. Experimental: blue ◆/◇, 313 K; red ■/□, 323 K; orange ▲/△, 333 K; ●/○, 343 K; −, best fit line for both experimental data.

chrysene in SF-CO2 have been satisfactorily correlated using both the MST and Bartle models with acceptable ARD values. Considering the fits shown in Figure 8, the MST model generally provided better correlation for the SSV-SFC solubility results obtained for anthracene whereas the Bartle model generally provided better correlation for the SSV-SFC solubility results obtained for chrysene. At present there is no standard reference method for measuring the solubility of compounds in SF-CO2. As a consequence, there are often large differences in SF-CO2 solubility values reported for the same compound in the scientific literature. In this study, this is illustrated by the wide variation in the current literature SF-CO2 solubility values of anthracene that were obtained from a relatively large number of published works for this extensively studied PAH. This situation appears to be the outcome of a diverse range of different factors that include use of dynamic and static methods, experimental setups of different scale and complexity, off line and online sample analysis procedures, equipment calibration, mixing methods, and the true establishment of dissolution equilibrium. Therefore, at this present point in time it seems advisible to exercise caution on relying on published SF-CO2 solubility values if a very high level of accuracy is required for a specific application. This could quite conceivably remain the case until an extensively tested and validated standard method for determining the solubility of compounds in SF-CO2 is introduced.

Table 7. Values of ARD Calculated Based on Equation 3 and Equation 4 ARD for Bartle

ARD for MST

component

eq 3

eq 4

eq 3

eq 4

anthracene chrysene

2.17 4.75

9.64 6.76

2.19 8.5

9.62 12.8

5. CONCLUSIONS The SSV-SFC method provides a rapid and convenient means of determining the solubility of anthracene and chrysene in SFCO2. The SSV-SFC procedure significantly simplifies experimental setup, in particular, eliminating the use of ancillary equipment that requires calibration to perform off line sample decompression and trapping followed by manual sample preparation stages prior to final off line analysis. Because of phase composition and pressure considerations, online SFC is the logical chromatography match for coupling to SF-CO2 solubility apparatus. As a consequence, the use of online SFC permits the use of a simplified and easy to operate single valve recirculation interface. Online HPLC can also be used to measure the solubility of compounds in SF-CO2 using a recirculating interface to couple SF-CO2 solubility apparatus. However, the use of online HPLC requires the recirculating interface to be equipped with two valves.18 The additional valve with its own ancillary gas supply is required to purge the sample injection valve sample loop between sample injections to prevent cross-contamination of the SF-CO2 apparatus with HPLC mobile phase. This requirement adds additional complexity to the experimental setup and its operation. In order to assess the accuracy of the SSV-SFC method a self-check validation procedure has been introduced. Satisfactory accuracy check results were obtained for anthracene and chrysene using the SSV-SFC self-validation procedure. The SSV-SFC solubility results obtained for anthracene and

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Edward D. Ramsey: 0000-0002-8217-0905 H



Article

Funding

Reaction Performed in Supercritical Fluid Solution. J. Chromatogr. A 2015, 1388, 141−150. (20) Li, B.; Guo, W.; Song, W.; Ramsey, E. D. Determining the Solubility of Organic Compounds in Supercritical Carbon Dioxide Using Supercritical Fluid Chromatography Directly Interfaced to Supercritical Fluid Solubility Apparatus. J. Chem. Eng. Data 2016, 61, 2128−2134. (21) Li, B.; Guo, W.; Song, W.; Ramsey, E. D. Interfacing Supercritical Fluid Solubility Apparatus with Supercritical Fluid Chromatography Operated with and without On-line Post-column Derivatization: Determining the Solubility of Caffeine and Monensin in Supercritical Carbon Dioxide. J. Supercrit. Fluids 2016, 115, 17−25. (22) Li, B.; Guo, W.; Ramsey, E. D. Determining the Solubility of Nifedipine and Quinine in Supercritical Fluid Carbon Dioxide Using Continuously Stirred Static Solubility Apparatus Interfaced with Online Supercritical Fluid Chromatography. J. Chem. Eng. Data 2017, 62, 1530−1537. (23) Li, B.; Guo, W.; Chi, H. J.; Kimura, M.; Ramsey, E. D. Monitoring the Progress of a Photochemical Reaction Performed in Supercritical Fluid Carbon Dioxide Using a Continuously Stirred Reaction Cell Interfaced to On-line SFC. Chromatographia 2017, 80, 1179−1188. (24) Méndez-Santiago, J.; Teja, A. S. The Solubility of Solids in Supercritical Fluids. Fluid Phase Equilib. 1999, 158−160, 501−511. (25) Bartle, K. D.; Clifford, A. A.; Jafar, S. A.; Shilstone, G. F. Solubilities of Solids and Liquids of Low Volatility in Supercritical Carbon Dioxide. J. Phys. Chem. Ref. Data 1991, 20, 713−756. (26) NIST Chemistry WebBook SRD 69, Thermophysical Properties of Carbon Dioxide, http://webbook.nist.gov/cgi/fluid.cgi?ID= C124389&Action=page, 2017 (accessed September 15, 2017). (27) Anitescu, G.; Tavlarides, L. L. Solubility of Solids in Supercritical FluidsI. New Quasistatic Experimental Method for Polycyclic Aromatic Hydrocarbons (PAHs) + Pure Fluids. J. Supercrit. Fluids 1997, 10, 175−189. (28) 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. (29) Kwiatkowski, J.; Lisicki, Z.; Majewski, W. An Experimental Method for Measuring Solubilities of Solids in Supercritical Fluids. Berichte der Bunsen-Gesellschaft 1984, 88, 865−869. (30) Lou, X.; Janssen, H.-G.; Cramers, C. A. Temperature and Pressure Effects on Solubility in Supercritical Carbon Dioxide and Retention in Supercritical Fluid Chromatography. J. Chromatogr. A 1997, 785, 57−64. (31) Zerda, T. W.; Wiegand, B.; Jonas, J. FTIR Measurements of Solubilities of Anthracene in Supercritical Carbon Dioxide. J. Chem. Eng. Data 1986, 31, 274−277. (32) Johnston, K. P.; Ziger, D. H.; Eckert, C. A. Solubilities of Hydrocarbon Solids in Supercritical Fluids. Ind. Eng. Chem. Fundam. 1982, 21, 191−197. (33) Ekart, M. P.; Bennett, K. L.; Ekart, S. M.; Gurdial, G. S.; Liotta, C. L.; Eckert, C. A. Cosolvent Interactions in Supercritical Fluid Solutions. AIChE J. 1993, 39, 235−248. (34) Miller, D. J.; Hawthorne, S. B. Determination of Solubilities of Organic Solutes in Supercritical CO2 by Online Flame Ionization Detection. Anal. Chem. 1995, 67, 273−279. (35) Coutsikos, P.; Magoulas, K.; Tassios, D. J. Solubilities of Phenols in Supercritical Carbon Dioxide. J. Chem. Eng. Data 1995, 40, 953−958. (36) Miller, D. J.; Hawthorne, S. B.; Clifford, A. A.; Zhu, S. J. Solubility of Polycyclic Aromatic Hydrocarbons in Supercritical Carbon Dioxide from 313 to 523 K and Pressures from 100 bar to 450 bar. J. Chem. Eng. Data 1996, 41, 779−786. (37) Hansen, B. N.; Harvey, A. H.; Coelho, J. A. P.; Palavra, A. M. F.; Bruno, T. J. Solubility of Capsaicin and β-Carotene in Supercritical Carbon Dioxide and in Halocarbons. J. Chem. Eng. Data 2001, 46, 1054−1058.

Edward D. Ramsey wishes to express his sincere gratitude to the 1000 Plan for Foreign Experts Program sponsored by the Chinese Central Government for Project Number WQ20122100062 and also for Award GDW20172100109. Ben Li wishes to acknowledge financial support provided by Projects L2015258 and 601009681 to purchase reagents. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Beckman, E. J. Supercritical and Near Critical CO2 in Green Chemical Synthesis and Processing. J. Supercrit. Fluids 2004, 28, 121− 191. (2) Williams, J. R.; Clifford, A. A. Supercritical Fluid Methods and Protocols; Humana Press: Totowa, NJ, 2000. (3) Ramsey, E. D.; Sun, Q. B.; Zhang, Z.; Zhang, C. M.; Guo, W. Mini-review: Green Sustainable Processes Using Supercritical Carbon Dioxide. J. Environ. Sci. 2009, 21, 720−726. (4) Brunner, G. Applications of Supercritical Fluids. Annu. Rev. Chem. Biomol. Eng. 2010, 1, 321−342. (5) Ramsey, E. D.; Guo, W.; Liu, J. Y.; Wu, X. H. Supercritical Fluids, in: Moo-Young, M. Comprehensive Biotechnology, 2nd ed.; Elsevier: Amsterdam, The Netherlands, 2011; pp 1007−1026. (6) Brunner, G. Supercritical Fluids as Solvents and Reaction Media; Elsevier: Amsterdam, The Netherlands, 2004. (7) Gupta, R. B.; Shim, J. J. Solubility in Supercritical Carbon Dioxide; CRC Press: Boca Raton, FL, 2007. (8) Skerget, M.; Knez, Z.; Knez-Hrncic, M. Solubility of Solids in Sub- and Supercritical Fluids: a Review. J. Chem. Eng. Data 2011, 56, 694−719. (9) Knez, Z.; Cor, D.; Knez-Hrncic, M. Solubility of Solids in Suband Supercritical Fluids: A Review 2010−2017. J. Chem. Eng. Data 2017, DOI: 10.1021/acs.jced.7b00778. (10) 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. (11) Karimi Sabet, J.; Ghotbi, C.; Dorkoosh, F.; Striolo, A. Solubilities of Acetaminophen in Supercritical Carbon Dioxide with and without Menthol Cosolvent: Measurement and Correlation. Sci. Iran. 2012, 19, 619−625. (12) 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. (13) Johannsen, M.; Brunner, G. Solubilities of the Xanthines Caffeine, Theophylline and Theobromine in Supercritical Carbon Dioxide. Fluid Phase Equilib. 1994, 95, 215−226. (14) Tsai, W. C.; Ruan, Y.; Rizvi, S. S. H. Solubility Measurement of Methyl Anthranilate in Supercritical Carbon Dioxide Using Dynamic and Static Equilibrium Systems. J. Sci. Food Agric. 2006, 86, 2083− 2091. (15) Analytical Methods Committee AMCTB No. 56. What Causes Most Errors in Chemical Analysis? Anal. Methods 2013, 5, 2914−2915. (16) Turner, C.; Eskilsson, C. S.; Bjorklund, E. Collection in Analytical-scale Supercritical Fluid Extraction. J. Chromatogr. A 2002, 947, 1−22. (17) Ngo, T. T.; Bush, D.; Eckert, C. A.; Liotta, C. L. Spectroscopic Measurement of Solid Solubility in Supercritical Fluids. AIChE J. 2001, 47, 2566−2572. (18) 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. (19) Ramsey, E. D.; Li, B.; Guo, W.; Liu, J. Y. Interfacing Supercritical Fluid Reaction Apparatus with On-line Liquid Chromatography: Monitoring the Progress of A Synthetic Organic I



Article

(38) Johnston, K. P.; Peck, D. G.; Kim, S. Modeling Supercritical Mixtures: How Predictive Is It? Ind. Eng. Chem. Res. 1989, 28, 1115− 1125. (39) Ota, M.; Hashimoto, Y.; Sato, M.; Sato, Y.; Smith, R. L., Jr.; Inomata, H. Solubility of Flavone, 6-Methoxyflavone and Anthracene in Supercritical CO2 with/without a Co-solvent of Ethanol Correlated by Using a Newly Proposed Entropy-based Solubility Parameter. Fluid Phase Equilib. 2016, 425, 65−71.

J


Measuring the Solubility of Anthracene and Chrysene in Supercritical

Recommend Documents