Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX
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Solubility of Vitamin E Acetate in Supercritical Carbon Dioxide: Measurement and Correlation Sai Han,† Weifang Wang,† Zhen Jiao,*,†,‡ and Xuebo Wei† †
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, P.R. China Center for Nanobiotechnology, Joint Research Institute of Southeast University and Monash University, Suzhou, Jiangsu 215123, P.R. China
‡
ABSTRACT: Vitamin E acetate (VEA) exhibits promising pharmacological action in the pharmaceutical and cosmetic industries. The solubility of VEA in supercritical carbon dioxide is measured to determine the cloud-point pressures in a highpressure variable-volume view cell at various temperatures (i.e., 308.15, 313.15, 318.15, 323.15, and 328.15 K) and pressures (8−15 MPa). The experimental results indicate that the solubility of VEA can be improved by increasing pressure and reducing temperature in the experimental range. Furthermore, four density-based semiempirical models are employed to correlate and predict the experimental results, including Chrastil, Kumar and Johnston (K-J), Bartle, and Mendez-Santiago and Teja (MST) equations. The theoretical calculation results are consistent with the determinatal results. The relevant AARD values of Chrastil, K-J, Bartle, and MST are 6.62, 6.83, 6.80, and 7.19%, respectively.
1. INTRODUCTION Supercritical carbon dioxide (SC-CO2) is emerging as an efficient solvent in many chemical fields1−6 with the pressure and temperature separately surpassing their critical values (Pc = 7.38 MPa, Tc = 304.13 K). At the same time, it plays a vital role in the food and pharmaceutical industries due to its amenable characteristics, including nontoxicity, high diffusivity, and relative low viscosity.7−9 Moreover, with the small changes in both temperature and pressure values in the vicinity of critical values, the dissolving capacity of supercritical CO2 can be drastically transformed, leading to excellent performance during extraction or dissolution processes using SC-CO2. Over the past decade, an increasing number of waterinsoluble drugs have been broadly studied by numerous researchers via supercritical fluid (SCF) processes, owing to their relative higher solubilities compared to those of watersoluble drugs.10,11 Thus, obtaining accurate data of their solubility is quite prerequisite and deserves scrupulous investigation. However, it is challenging to precisely determine the solubility data due to dynamics instability and thermodynamics instability in the supercritical CO2 system.3 Thus, it is rather difficult to obtain the phase equilibrium data of the drugs. Considerable attempts have been made to predict phase behaviors of various drugs in supercritical fluids as a way of seeking appropriate mathematical models over the past several years. The most commonly used models mainly refer to semiempirical relationships and thermodynamic equations of state, among which the mixing rule may be employed on the condition of cosolvent existence.12−14 Semiempirical models based on density have been most widely used owing to their unique benefits, such as relative simple calculations and high precision without physical property parameters of drugs. © XXXX American Chemical Society
Furthermore, the parameters of these models are can be conveniently obtained by experimental data with least-squares fit. Deservedly, several kinds of typical models like Chrastil,15 Kumar-Johnston,16 Bartle,17 as well as Mendez-Santiago− Teja18 have been proposed successively. Moreover, some researchers have proposed new models for better predicting the experimental data by way of increasing or changing model parameters on the basis of typical semiempirical models.3,8,19 As an example, Madras et al.19 proposed a new equation with only four parameters and successfully correlated the solubility data of propyphenazone in SC-CO2. Vitamin E acetate (VEA), also known as α-tocopheryl acetate, shows very strong pharmacological action in the pharmaceutical and cosmetic industries.20 As shown in Figure 1, VEA is a sort of multitudinous vitamin E derivative. It is a viscous and oleosus liquid light green in color and poorly soluble in water. Additionally, VEA can effectively control cardiac-cerebral thrombosis and is generally employed in pharmaceutical industries. It can moisturize the skin and effectively inhibit lipid peroxidation, which facilitates its wide application in all kinds of cosmetics.
Figure 1. Chemical structure of vitamin E acetate. Received: June 15, 2017 Accepted: September 29, 2017
A
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 1. Properties of the Chemicals material
molecular formula
source
mass fraction purity
CAS
chemical name
vitamin E acetate (VEA) palmitic acid carbon dioxide
C31H52O3 C16H32O2 CO2
Shanghai Macklin Biochemical Company Nantong hyles pharmaceutical Company Nanjing Shangyuan Gas Company
0.96 0.98 0.999
52225-20-4 57-10-3 124-38-9
α-tocopheryl acetate hexadecanoic acid
However, few literature studies have demonstrated the solubility data of VEA in supercritical CO2. Belhadj-Ahmed et al.21 measured the solubility of α-tocopheryl acetate in SC-CO2 with organic solvent (ethanol) as cosolvent at 313 K and pressures ranging from 10 to 20 MPa by the static method, and the mixed solution was analyzed by UV spectrophotometry. Unfortunately, the solubility data of VEA without organic solvent was not involved, and only one temperature point was not adequate for the process design using SC-CO 2 . Accordingly, it was definitely rewarding to further study its solubility in the supercritical system. It is worth mentioning that some researchers have explored the solubility of the vitamin E (VE) in supercritical systems by an analytical method with UV spectrometry;22,23 however, the addition of ester function in the VEA molecular structure obviously leads to a different solubility. Additionally, the solubility data of VEA in SC-CO2 is of potential interest for both the pharmaceutical and cosmetic industries. For example, the data of VEA in SC-CO2 can provide meaningful information for supercritical extraction of VEA from natural plants. It also provides valid data for VEA liposome preparation by supercritical progress. In this paper, the static cloud-point method is employed to measure the solubility of VEA in SC-CO2 at temperatures of 308.15, 313.15, 318.15, 323.15, and 328.15 K and pressures of 8−15 MPa. Even though the cloud-point method appears to be limited to a range of compounds poorly soluble in pure SCCO2, many researchers have still used it to measure the solubility of different solutes in the supercritical system.2,12,24,25 Although the supercritical fluid can be swept successively through the visible cell by using the dynamic method, it can only provide information on the light phase.26 Thus, the static cloud-point method is relatively convenient in this work. In this study, the effects of temperature and pressure on solubility are discussed in detail. In addition, the experimental data are correlated with the semiempirical methods including Chrastil, Kumar-Johnston, Bartle, and Mendez-Santiago−Teja, which demonstrate their possibility to predict the solubility data of the studied system.
Figure 2. Schematic diagram of the experimental apparatus for VEA solubility in SC-CO2: (1) cooling tank, (2) piston pump, (3) heating apparatus, (4) water bath, (5) high-pressure variable-volume view cell, (6) visible window, and (7) magnetic stirrer.
Scientific Apparatus Company) whose precision of temperature control is ±0.1 °C, and a high-pressure cell whose volume is 68 mL with a visible window. In addition, a stirrer is settled on the bottom of the cell, and its revolving speed can be adjusted to a desired value. In this work, the procedure for measuring the cloud point of VEA was operated as follows. First, a certain quality of VEA (e.g., 0.1004, 0.1508, 0.2007, 0.2516, and 0.2898 g) was loaded in the clean view cell, and the circumambient air was replaced by carbon dioxide. Simultaneously, the cell was put into a water bath heated by the constant temperature device, and the pure CO2 was allowed to cool in the upper part of the plunger pump surrounded firmly by plenty of ice. After cooling, the liquid CO2 was pressured into the cell by slowly driving a moving piston inside the pump. The stirring apparatus was initiated when the experimental pressure and temperature achieved the expected value. During the process, it was not difficult to observe the solution change from cloudy to clear with the solute from insoluble to completely soluble through the visual window.24,27 Then, the view cell was separated from the gas intake system via a ball valve, and the SC-CO2 solution was stablized for several minutes. Eventually, the pressure inside the cell was decreased very slowly and carefully controlled by gradually increasing the volume of the cell via a screw rotating upward until the cloud point was observed. The cloud point demonstrated that the solute in this system started to dissolve out, and the pressure at this point was called the cloud-point pressure. Notably, each experimental point was repeated at least three times to obtain its average values and verify the reliability of this procedure. After every experiment, the system was vented, cleaned using ethanol, and thoroughly dried. The experimental temperature was adjusted from 308.15 to 328.15 K with an increment of 5 K. The amount of CO2 pumped in the cell can be determined by the pressure and temperature along with the volume of the cell at the cloud point, and the solubility of VEA in SC-CO2 can be calculated by means of the known amount of drugs and CO2 in the cell.
2. EXPERIMENTAL SECTION 2.1. Materials. The characteristics of the chemicals applied in this experiment are listed in Table 1. VEA was further purified by vacuum distillation method before measurement. 2.2. Apparatus and Procedures. During this process, the measurement of solubility was conducted through a highpressure setup and a variable-volume view cell. The experimental equipment was utilized to explore the cloudpressure of VEA in SC-CO2 with a common static method. The relevant schematic diagram is shown in Figure 2. The apparatus of solubility measurement primarily consists of a CO2 steel cylinder, a single piston pump with a display-control device (Beijing Weixing Factory), which can control the flow rate of carbon dioxide and display the instantaneous pressure in the pump, a digital pressure indicator (Fujian Shangrun Precise Instrument Company, the specification is ±0.01 MPa accuracy), a constant temperature device (Beijing Changliu B
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION 3.1. Solubility Determination. Verifying the reliability and repeatability is quite prerequisite during this process. Hence, the solubility of palmitic acid in SC-CO2 at 313.15 K is measured to verify the reliability of this system, and the results are shown in Table 2 and Figure 3. The results are comparable
Table 3. Cloud-Point Pressures of VEA (0.2516 g) at Temperatures of 308.15−328.15 Ka T/K
Table 2. Comparison of the Solubility Data of Palmitic Acid in SC-CO2a this work T/K
P/MPa
313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15
8.68 9.44 10.03 10.4 10.85 10.9 12.13 14.72
ref 28 4
P/MPa
10 y
P/MPa
10 y
± ± ± ± ± ± ± ±
8 9.1 9.1 10 11 15.1 15.1 20.1 24.8
0.00496 0.242 0.348 1.33 2.67 5.54 5.62 7.59 9.95
9.04 9.06 9.11 9.51 9.56 10.4 12.2 12.3 15.1 16.6
1.36 1.38 1.27 2.52 2.26 3.94 4.34 4.53 5.58 5.74
0.02 0.02 0.04 0.06 0.08 0.08 0.12 0.21
313.15
318.15
323.15
328.15
8.79 8.81 8.79 8.80 0.89%
10.18 10.19 10.20 10.19 0.67%
11.51 11.50 11.52 11.51 0.67%
12.70 12.73 12.71 12.71 1.11%
13.83 13.84 13.85 13.84 0.67%
Standard uncertainties u are u(T) = 0.1 K and u(P) = 0.01 MPa. Each experimental data point is an average value of at least three replicate 100 measurements. bAD (%) = n ∑ (P − P ̅ ), where n means the number of measurement data points.
4
10 y 0.80 1.78 2.88 3.35 4.15 4.16 4.44 5.60
308.15
a
ref 29 4
P/MPa 1 2 3 average AD (%)b
Table 4. Data of Solubility of VEA at Temperatures of 308.15−328.15 Ka T/K
P/MPa
ρb/kg/m3
S/kg/m3
308.15
8.41 8.62 8.79 8.80 9.14 9.64 9.92 10.12 10.19 10.52 10.86 11.13 11.4 11.51 11.87 11.96 12.25 12.58 12.71 13.06 13.03 13.34 13.73 13.84 14.19
597.53 627.74 645.20 646.10 671.57 596.36 622.36 637.24 641.92 661.21 592.76 612.01 628.38 634.41 652.03 582.15 599.65 617.03 623.26 638.62 572.98 589.12 607.18 611.88 625.86
1.82 2.61 3.51 4.42 5.22 1.77 2.57 3.45 4.37 5.13 1.76 2.52 3.38 4.31 5.07 1.73 2.47 3.33 4.26 4.98 1.69 2.43 3.26 4.27 4.88
a
Standard uncertainties u are u(T) = 0.1 K and u(P) = 0.01 MPa; y is the solubility of palmitic acid (mole fraction). Standard uncertainties in the mole-fraction solubilities are reported following ± signs. The values obtained are at least three measurements at each condition.
313.15
318.15
323.15
328.15
104 y/mole fraction 2.83 3.87 5.06 6.37 7.24 2.76 3.84 5.05 6.33 7.22 2.76 3.84 5.01 6.32 7.23 2.76 3.84 5.02 6.36 7.26 2.75 3.84 5.00 6.50 7.26
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.05 0.08 0.10 0.10 0.16 0.08 0.12 0.13 0.16 0.19 0.12 0.15 0.16 0.18 0.23 0.13 0.16 0.18 0.19 0.23 0.15 0.18 0.22 0.23 0.23
a ρ is density of pure CO2. S (kg/m3) is the solubility of VEA. y is the solubility of VEA (mole fraction). Standard uncertainties u are u(T) = 0.1 K and u(P) = 0.01 MPa. Standard uncertainties in the molefraction solubilities are reported following ± signs. bValues provided by NIST.30
Figure 3. Comparison of the solubility data of palmitic acid in SC-CO2 at 313.15 K.
to those in the literature,28,29 which indicates a good reliability of the experimental apparatus. VEA (0.2516 g) was selected to conduct the solubility procedure of the repeatability test. The cloud-point pressures of VEA were measured separately at least three times at temperatures ranging from 308.15 to 328.15 K with an increment of 5 K, and the experimental data are shown in Table 3. The average deviation (AD) of VEA cloud-point pressures at the experimental conditions is less than 1.11%, which demonstrates repetition of the experimental results. The experimental solubility data of VEA in SC-CO2 along with the density of CO2 loaded in the cell at different operation conditions are listed in Table 4. The experimental cloud points are determined as functions of temperature (T) and pressure (P) described before, and each data point was an average value
of at least three measurements. Belhadj-Ahmed et al.19 report the solubility data of VEA in SC-CO2 with an organic solvent as cosolvent. Of course, these data are higher than those determined in pure SC-CO2 in this study. Figure 4 shows the cloud-point pressure−temperature (P-T) isopleths of VEA in SC-CO2 at different weights of VEA placed in the view cell. As shown in Figure 4, each cloud point is the homogeneous phase region with the clear solution, whereas the heterogeneous phase (two-phase) region is the cloudy solution below the point. That is to say, the cloud point represents the boundary of the phase transition, indicating whether the drug of VEA dissolves thoroughly in CO2, which means the relative mass of VEA in CO2 at this point is its solubility.14,24 According to Figure 4, the C
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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temperature increases the distance for CO2 molecules and weakens the intermolecular force between CO2 molecules and VEA. For example, as the temperature changes from 323.15 to 328.15 K at a pressure of ∼13 MPa, y decreases from 7.26 × 10−4 to 2.75 × 10−4. As a consequence, VEA is more soluble in supercritical CO2 at the lower temperature. 3.2. Modeling. Because the measurement of drug solubility in SC-CO2 is technically challenging and time-consuming, four typical semiempirical correlation models, including Chrastil, Kumar and Johnston (K-J), Bartle, and Mendez-Santiago and Teja (MST) are employed to correlate the experimental data in this study. One of the merits of these approaches is that the effects of temperature and pressure are both accounted for with the CO2 density. In addition, the average absolute relative deviation (AARD) calculated by eq 1 is commonly applied to assess the correlation outcome. A lower AARD value represents much better correlation results. Figure 4. Cloud-point pressures at different weights of VEA.
AARD (%) =
cloud-point pressure obviously increases with enhanced temperatures at constant VEA addition, which manifests as this system exhibiting typical lower critical solution temperature (LCST) behavior.10,14 This result also reveals that the system requires higher pressure to maintain the solution in homogeneous phase with increasing temperature. The values obtained are at least three measurements at each condition. Figure 5 shows the solubility (y) of VEA in supercritical carbon dioxide as a function of pressure (P) at five different
1 N
∑
|ycal − yexp | |yexp |
× 100 (1)
where N is the total number of experimental data and ycal and yexp present the calculated and experimental results, respectively. Among the four semiempirical relationships mentioned earlier, Chrastil’s relationship15 is generally used to correlate the solubility of the drugs in supercritical fluids (SFs) to density and temperature due to its correctness and simplicity. The specific correlation equation is as eq 2 m ln s = k ln ρ + +n (2) T where s (kg/m3) represents the solubility of VEA in SC-CO2, ρ (kg/m3) is the density of pure CO2 at corresponding pressure and temperature, T is the system temperature, and k, m, and n are adjustable parameters that can be obtained by fitting the least-squares method to experimental data. m can be defined as −ΔH/R, where ΔH is the enthalpy of vaporization and salvation of the solute and R is the gas constant. The molar mass of the solute and solvents will directly influence the value of n. The correlation results can be seen in Figure 6. Another similar semiempirical method based on density is proposed by Kumar and Johnston (K-J),16 which shows a linear relationship between ln y and ρ and T and can be indicated as eq 3 m ln y = kρ + +n (3) T where y (mole solute/mol CO2) is the solubility of VEA in supercritical CO2, ρ means the density of pure CO2, and the parameters k, m, and n are determined by multilinear regression the same as in the Chrastil model. The fitting results can be seen in Figure 7. The third model proposed by Bartle17 is also employed to correlate the experimental solubility data, and the model equation is shown as eq 4
Figure 5. Solubility of VEA at different pressures and temperatures.
temperatures. Apparently, at constant temperature, the solubility of VEA increased as the pressure increased, mainly due to the increasing density of SC-CO2, which results in stronger solvation power of supercritical CO2 and improves the interactions between VEA and carbon dioxide molecules. Additionally, the system temperature is also a critical factor affecting the solubility of VEA in SC-CO2, principally influencing the solute vapor pressure and solvent density. Figure 5 also indicates that the increasing temperature reduces the solubility (y) of VEA in this procedure when the pressure remains constant. This phenomenon is probably because higher
⎛ Py ⎞ m ln⎜ 2 ⎟ = k(ρ − ρref ) + +n T ⎝ Pref ⎠
(4)
where y2 is the mole fraction of solute, Pref is the standard pressure of 0.1 MPa, and ρref is a reference density with a value of 700 kg/m3. The correlation results are shown in Figure 8, D
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 8. Correlation results and measurement data of the solubility of VEA in SC-CO2 using the Bartle model.
Figure 6. Correlation results and measurement data of the solubility of VEA in SC-CO2 using the Chrastil model.
Figure 9. Correlation results and measurement data of the solubility of VEA in SC-CO2 using the MST model.
Figure 7. Correlation results and measurement data of the solubility of VEA in SC-CO2 using the K-J model.
Table 5. Correlation Parameters and AARD Values of the Four Modelsa
and the values of m and n are calculated from A = m/T + n by plotting value A against 1/T. The last semiempirical model used in this study is put forward by Mendez-Santiago and Teja (MST),18 which can be described as eq 5 T ln(y2 P) = kρ + mT + n
(5)
where y2 is the solute solubility, and P (MPa) and T (K) are the system pressure and temperature, respectively. The plot between T ln(y2P) − mT and ρ gives a straight line for all isotherms, which is generally used to check the consistency of measurement solubility of both liquid and solid drugs at different isotherms. The correlation results can be seen in Figure 9. In addition, parameters and relevant AARD values of four semiempirical density-based models are shown in Table 5. The experimental and calculated results of all four models clearly show an acceptable error range (Figures 6−9 and Table 5). The R-square (0 < R2 < 1) means goodness of fit index, and it indicates that the regression lines correlated by the four
model
k
m
n
AARD (%)
R2
Chrastil K-J Bartle MST
10.95 0.01599 0.01742 5.543
−2947.96 −2949.38 −5446.60 11.9079
−59.98 −8.31 15.56 −8898.14
6.62 6.83 6.80 7.19
0.9529 0.9398 0.9582 0.9480
a
The values k, m, and n are of relevant model coefficients. R2 is the goodness of fit index.
models better match the experimental data when the value is closer to 1. Otherwise, the values are closer to 0, which shows that the regression lines values are a worse match to the experimental results. The tendency of correlation results obtained separately (shown in Figures 6 and 7) by Chrastil and K-J models with average absolute relative deviations (AARDs) of 6.62 and 6.83%, respectively, are very close, which further demonstrates the reliability of the experimental results. E
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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It should be noted that the AARD values of all four models are less than 10%, which indicates that the correlation results can be preferable for predicting the variation trend of solubility in this study. However, these points at 308.15 K are not as good as those at other temperatures, which might be the reason that the properties of the supercritical fluid and the solvating strength of SC-CO2 change dramatically with the small change in the temperature and pressure around its critical point. This phenomenon results in the instability of this supercritical system and the fluctuation of dissolution behavior. As can be seen in Figure 8, the calculated data via Bartle correlation are at a satisfactory level, and its AARD value is 6.80%. Although the AARD value is slightly similar to that of K-J, the fitting results are much better. Although the entire results correlated by MST method are not a strict linear relationship from Figure 9, most of them scatter around a straight line with the AARD value of 7.19%, and the situation of data distribution is similar to the results in the literature.31,32 In conclusion, the four models described in this paper are all applicable for correlation of solubility results of VEA in SC-CO2, and the Chrastil model displays the best accuracy.
REFERENCES
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4. CONCLUSIONS In this study, the solubility of VEA in SC-CO2 is determined at temperatures of 308.15, 313.15, 318.15, 323.15, and 328.15 K and pressures of 8−15 MPa. The solubility values are in the range of 2.76 × 10−4 to 7.26 × 10−4 mole fraction, indicating that VEA has a relatively large solubility in SC-CO2. The results indicate that the cloud-point pressures of VEA in SC-CO2 are increased with increasing temperature. Furthermore, the solubility of VEA is increased by increasing the pressure and reducing the temperature. The experimental results also reveal that the density of CO2 has a direct and main influence on the solubility of VEA. The larger density of CO2 leads to better solubility of VEA. In addition, four typical semiempirical density-based models are proposed to correlate the experimental data. The results calculated by Chrastil, Kumar and Johnston (K-J), Bartle, and Mendez-Santiago and Teja (MST) methods are at an acceptable level with AARD values of 6.62, 6.83, 6.80, and 7.19%, respectively. It is expected that this work can provide effective experimental data for VEA solubility in SC-CO2 for the process design using SC-CO2, such as supercritical extraction and the rapid expansion process of supercritical solution (RESS).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhen Jiao: 0000-0001-5383-5077 Funding
This study was supported by the Natural Science Foundation of Jiangsu Province (BK20130602), Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities. Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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DOI: 10.1021/acs.jced.7b00550 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
