Solubility of Aloe-Emodin in Five Imidazolium-Based Ionic Liquids

Aug 15, 2013 - A simplified dual-parameters equation and λh equation were used to correlate the experimental solubility data of aloe-emodin in invest...
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Solubility of Aloe-Emodin in Five Imidazolium-Based Ionic Liquids Li Ge, Dan Li, Yunfei Long, Jing Su, and Kedi Yang* School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China ABSTRACT: The solubility of aloe-emodin in five imidazolium-based ionic liquids (ILs), 1-decyl-3-methylimidazolium chloride ([C10mim]Cl), 1-hexyl3-methylimidazolium hexafluorophosphate ([C6mim]PF6), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim]BF4), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4), and 1-butyl-3-methylimidazolium bis(methylsulfonyl)amide ([C4mim]NTF2) was measured respectively from (295.55 to 363.55) K at atmospheric pressure using a laser monitoring observation method combined with high performance liquid chromatography (HPLC). A simplified dual-parameters equation and λh equation were used to correlate the experimental solubility data of aloe-emodin in investigated ILs and exhibited good agreement.

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INTRODUCTION Aloe-emodin (1,8-dihydroxy-3-(hydroxymethyl)-9,10-anthracenedione, CASRN: 481-72-1) is an important natural anthraquinone compound widely found in plants of Aloe vera L, Rheum Palmatum L, Cassia obtusifolia L and other species of the asphodelaceae and the polygonaceae families. Its structural formula is given in Figure 1. The aloe-emodin and its extract

compounds since Huddleston et al. reported the extraction of substituted-benzene derivatives using ILs [Bmim]PF6 initially.10−15 As for aloe-emodin, there is no reported solubility data in various ILs to date; data that is important in order to select a proper IL or to simulate and design extraction processes. Imidazole-types ILs are frequently used media in separation and chemical reaction processes. In the present work, the solubility of aloe-emodin in five imidazolium-based ILs has been measured using a laser monitoring observation method combined with high performance liquid chromatography (HPLC). The effects of imidazolium-based IL types and temperature on solubility are discussed, and the solubility data are correlated with two empirical equations.

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Figure 1. Molecular structure of aloe-emodin.

EXPERIMENTAL SECTION Materials. Aloe-emodin (mass fraction > 98.5 %) was purchased from Shanxi Ciyuan Biological Technology Co., Ltd., (Xi’an, China). It was recrystallized twice in ethanol, dried in a vacuum oven at T = 378.5 K for 24 h, and stored in a desiccator before use. The purity of the aloe-emodin crystal was more than 99.8 % mass fraction, determined on Agilent 1100 HPLC system (Agilent, USA). 1-Decyl-3-methylimidazolium chloride ([C10mim]Cl), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim]PF6), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim]BF4), and 1-butyl-3-methylimidazolium bis(methylsulfonyl)amide ([C4mim]NTF2) were purchased from Lanzhou Greenchem ILS, LICP. CAS (Lanzhou, China). These ILs were distilled in a rotary evaporator for 4 h and dried at 398.5 K under vacuum for 24 h to remove the potential residual solvents and water prior to use. The HPLC

has been used widely around the world in medicine, cosmetics and toiletries.1 In recent years, the pharmacological actions of aloe-emodin were reported to include laxative, antibacterial and anticancer, and in particular, it was found to inhibit Nacetyltransferase activity and gene expression in human malignant melanoma cells and to have antiproliferative and pro-apoptotic effects on gastric and tongue cancer cells.2−5 Aloe-emodin is sparingly soluble in water and mainly obtained from plant materials by extraction with different organic solvents or their mixtures. However, because of increased safety requirements for operating personnel and consumers, extraction by organic solvents is faced with a heavy challenge owing to the volatility, flammable and toxicity of these solvents. Therefore the desire to reduce the use of organic solvents in bioactive substances extraction processes has led to the development of alternative solvents, such as ionic liquids (ILs). In the past decade, there has been a great interest in ILs as separation and chemical reaction media because of their unique physical and chemical properties of low vapor pressure and high thermal and chemical stability, etc.6−9 Some studies have revealed the ability of ILs to extract natural organic © 2013 American Chemical Society

Received: January 9, 2013 Accepted: July 29, 2013 Published: August 15, 2013 2405

dx.doi.org/10.1021/je400028k | J. Chem. Eng. Data 2013, 58, 2405−2409

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Table 1. Ionic Liquids Used for the Solubility Determination of Aloe-Emodin

purity of the ILs used in this experiment were higher than 99.5 %, determined on Agilent 1100 HPLC system according to the method of Jiang et al.16 with slight modification, and their water content was less than 1000 ppm (Table 1), measured by Karl Fisher titration method on V20 titrator (Mettler Toledo, Switzerland). Apparatus and Procedure. A purpose-made glass tube (10 mL) with a Teflon-coated magnetic stirrer and a glass jacket (about 500 mL) was used to prepare a saturated solution of aloe emodin in ILs. The excess solid solute (aloe-emodin, about 10 mg) and ILs (about 7 mL) were accurately weighed respectively on Sartorius type BS 112 analytical balance with accuracy of ± 0.1 mg, and put into the test tube. Then the test tube was sealed up by a Teflon stopper with a precise thermometer (± 0.01 K), and the inlet/outlet of jacket was connected to a thermostatic water circulator with temperature accuracy of ± 0.01 K. The mixture was stirred and heated to expected temperature value. The dissolution equilibrium of solute in ILs was monitored by a laser detection system composed of a laser generator and a laser receiver. The test tube was placed between the laser generator and receiver to enable the laser beam to pass through from one side to another. At the beginning, the laser through the tube was sheltered by the suspended solid solute in ILs, and the laser power received was low. As more and more solid solute dissolved, the received laser power strengthened. When the received power reached the maximum value and remained unchanged, the dissolution of the sample in ILs reached solid−liquid equilibrium. After that, the undissolved solid and solution were allowed to settle for elutriation, and than the clear saturated solution of 100 μL were withdrawn by preheated glass syringe. The glass syringe with saturated solution was weighed on a Sartorius type 1712 analytical balance with an uncertainty of ± 0.01 mg. The saturated solution was injected into the volumetric flask (5 mL) immediately to prevent precipitating. Subsequently, the glass syringe with the remaining solution was weighed, therefore the mass of saturated solution put into volumetric flask could be

calculated. The solution of sample was diluted to the mark with methanol for HPLC analysis. Chromatographic Conditions. The solubility of aloeemodin was determined on an Agilent 1100 HPLC system. The analysis was carried out on a Zorbax SB-C18 column (150 mm × 4.6 mm, 5.0 μm) by an external standard method. A mobile phase composed of methanol and 0.2 % phosphoric acid aqueous solution, with volume fraction of 90:10, was used to run the separation at a flow rate of 1.0 mL·min−1. The injected volumes of sample and reference standard solution were 10 μL. The detective wavelength was set at 254 nm. All chromatograph procedures were performed at room temperature. The average relative uncertainty of HPLC analysis of the aloeemodin was less than 1.8% (n = 5).

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RESULTS AND DISCUSSION The mole fraction solubility data of aloe-emodin in five imidazolium-based ILs are presented in Table 2 and more visually expressed in Figure 2. In this work, each measurement was repeated three times at the same condition, and the reported solubility data are the average values. According to the model of molecular thermodynamics for the solubility of solid in liquid, the temperature (T) dependence of solubility (x) can be correlated with the following simplified dual parameters equation17 ln x = A +

B T

(1)

where x is the mole fraction solubility of aloe-emodin, T is an absolute temperature (K), and A and B are the empirical parameters. The two parameters A and B were obtained by least-squares fit from the experimental data and listed in Table 3 together with the root-mean-square deviations (rmsd). The rmsd is defined as ⎡1 rmsd = ⎢ ⎢⎣ n 2406

n

∑ (x n=1

cal

⎤1/2

exp 2 ⎥

−x )

⎥⎦

(2)

dx.doi.org/10.1021/je400028k | J. Chem. Eng. Data 2013, 58, 2405−2409

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Table 2. Mole Fraction Solubility xa of Aloe-Emodin in Five Imidazolium-Based ILs at Temerature T and Pressure p = 0.1 MPab T/K

103xexp

307.15 310.85 314.05 317.55 321.25 325.15 334.05 340.35 349.65

0.193 0.222 0.244 0.267 0.284 0.326 0.398 0.483 0.580

300.55 306.35 311.85 317.05 321.35 325.15 331.35 343.35 363.55

0.217 0.250 0.285 0.320 0.353 0.381 0.464 0.549 0.789

103x1cal

103x2cal

0.198 0.220 0.240 0.265 0.292 0.324 0.405 0.472 0.584

0.200 0.221 0.241 0.265 0.291 0.322 0.403 0.470 0.587

0.221 0.254 0.288 0.324 0.356 0.386 0.438 0.554 0.793

0.230 0.260 0.291 0.324 0.353 0.381 0.431 0.543 0.801

0.192 0.208 0.233 0.251 0.271 0.304 0.336 0.431

0.200 0.215 0.238 0.254 0.273 0.304 0.336 0.433

[C10mim]Cl

[C4mim]BF4

Figure 2. Solubility of aloe-emodin in [C4mim]NTF2 (◇), [C6mim]BF4 (▽), [C4mim]BF4 (○), [C10mim]Cl (□), [C6mim]PF6 (△). Solid lines are values calculated by the dual-parameter eq 1 and dash lines are values calculated by the λh eq 3 with the parameters from Table 2.

⎛1 ⎛ 1 − x ⎞⎟ 1 ⎞ ln⎜1 + λ = λh⎜ − ⎟ ⎝ x ⎠ Tm ⎠ ⎝T

(3)

[C6mim]PF6 304.15 308.05 313.55 317.15 321.15 327.25 332.75 347.25 311.35 314.75 318.15 322.25 326.95 333.35 338.15 341.75 352.55 295.55 311.35 324.95 335.65 343.55 352.05 360.75

0.172 0.203 0.246 0.250 0.280 0.315 0.332 0.424

[C6mim]BF4 0.247 0.254 0.282 0.288 0.321 0.327 0.392 0.379 0.462 0.446 0.551 0.553 0.652 0.647 0.706 0.726 1.014 1.009 [C4mim]NTF2 1.013 1.063 1.694 1.666 2.409 2.368 3.058 3.061 3.750 3.662 4.419 4.401 5.194 5.265

where Tm is the melting point temperature of the aloe-emodin. The value of λ is identified as the approximate mean association number of solute molecules, which reflects the nonideality of the solution system, and h estimates the excess mixing enthalpy of solution.19 Both parameters λ and h were obtained from the experimental data by nonlinear least-squares fit and their values are listed in Table 3. From Table 2 and Table 3, it is seen that the calculated solubilities are in good agreement with the experimental values, which indicates that both the dual-parameters equation and the λh equation are appropriate to describe the solubility of aloeemodin in selected imidazolium-based ILs for the present work. A comparison of the fitted rmsd value for each IL (Table 3) shows that the correlated result according to eq 1 is a little better than that of eq 3. Also, as can be seen from Figure 2, the solubility of aloeemodin in hydrophobic ILs, [C10mim]Cl, [C6mim]PF6, [C4mim]NTF2, and hydrophilic ILs [C4mim]BF4, [C6mim]BF4 improves with the increase of temperature over the investigated temperature range, which is similar to the dissolution process of solid solute in common organic solvents. When the temperature deviates from the room temperature, the dependence of the solubility of aloe-emodin on temperature becomes more or less remarkable. Especially, the solubility of aloe-emodin in [C4mim]NTF2 is more sensitive to temperature than that in other investigated ILs in this work. However, from the experimental data, it is clearly observed that the solubility of aloe-emodin is largely governed by the molecular structure of the used ILs. The solubility values of aloe-emodin in [C4mim]NTF2 approximate that of emodin in ethanol,20 and are remarkably higher than that in other ILs at the same temperature. This phenomenon may be majorly attributed to the strong hydrogen bonding between the −OH groups of aloe-emodin and (CF3SO2)2N− anion of [C4mim]NTF2. The π−π interaction and hydrophobic interaction between aloeemodin and IL molecules were not the main dissolution mechanisms.21,22

0.256 0.290 0.328 0.379 0.445 0.551 0.645 0.723 1.012 1.114 1.684 2.351 3.023 3.621 4.384 5.317

a exp x is the experimental value; x1cal and x2cal are the calculated value with dual-parameter and λh equation, respectively. bStandard uncertainties u are u(T) = 0.05 K, ur(x) = 0.02.

where n is the number of experimental points, xexp represents the experimental value of solubility and xcal represents the solubility calculated from eq 1. The λh in eq 3 is another widely used value to describe the relationship between the mole fraction solubility (x) and temperature (T) for solid−liquid phase equilibrium18 2407

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Table 3. Parameters of Dual-Parameter eq 1 and λh eq 3 Correlated According to Experimental Molar Fraction Solubility of Aloe-Emodin in Five Imidazolium-Based ILs λh equation

dual-parameter equation ILs

A

B

105 rmsd

λ

h

105 rmsd

[C10mim]Cl [C4mim]BF4 [C6mim]PF6 [C6mim]BF4 [C4mim]NTF2

0.3908 −1.0379 −2.0448 3.5234 2.0049

−2739.9 −2218.5 −1981.2 −3674.5 −2616.2

0.5802 0.9475 2.4749 1.0541 15.0808

0.003137 0.001594 0.000845 0.01211 0.01857

779349.4 1063095.2 1652577.1 289088.3 120887.8

0.6420 1.3292 2.9345 1.0509 22.3377

in the degree of order of the system due to the alignment disruption of ILs molecules while aloe-emodin were dissolved.

For a real solution, the dissolution enthalpy (ΔHd) and entropy (ΔSd) can be calculated with the van’t Hoff eq 423

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ΔHd ΔSd + (4) RT R where x is the mole fraction solubility. The van’t Hoff plots (ln x versus 1/T) are shown in Figure 3, and are obtained from the

CONCLUSIONS The solubility of aloe-emodin in five pure imidazolium-based ILs, [C4mim]BF4, [C6mim]BF4, [C6mim]PF6, [C10mim]Cl, and [C4mim]NTF2 was determined, respectively, by the laser monitoring observation method combined with HPLC at atmospheric pressure. The solubilities of aloe-emodin in these ILs increased with rising temperature. The IL [C4mim]NTF2 exhibited good dissolubility to aloe-emodin and may be an alternative to organic solvents for aloe-emodin extraction. The experimental data were correlated with the simplified dualparameters equation and λh equation. The calculated solubility values of aloe-emodin showed good agreement with the experimental values. The experimental solubility data and correlation equations in this work may be used as essential data and a reference for the extraction process of aloe-emodin in imidazolium-based ILs.

ln x = −

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0086-771-3233718. Fax: 0086-771-3233718. Funding

Figure 3. The van’t Hoff plots of ln x versus 1/T in [C10mim]Cl(□), [C4mim]BF4(○), [C6mim]BF4(△), [C6mim]PF6 (▽), and [C4mim]NTF2 (◇).

This research work was financially supported by the National Natural Science Foundation of China (No. 21166002). Notes

linear fit of solubility data of aloe-emodin at different temperature. The ΔHd and ΔSd and standard deviation (SD) are shown in Table 4. In this binary system composed of aloeemodin and ILs, because of the intramolecular and intermolecular hydrogen bonding of the aloe-emodin molecules, as well as the intermolecular strong association of ILs molecules, which majorly resulted from the electrostatic action between cations and anions, the interactions between aloeemodin and ILs molecules are not strong enough to destroy the original association in ILs, and thus the dissolution process of aloe-emodin in ILs is expressed as the endothermic process, that is, ΔHd > 0. From Table 4, the positive ΔHd and ΔSd revealed that the dissolution process of aloe-emodin was an entropy-driven process, which is possibly induced by reduction

The authors declare no competing financial interest.

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REFERENCES

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Table 4. Dissolution Enthalpy (ΔHd) and Entropy (ΔSd) of Aloe-Emodin in Five Imidazolium-Based ILs ILs

[C10mim]Cl

[C4mim]BF4

[C6mim]PF6

[C6mim]BF4

[C4mim]NTF2

ΔHd (kJ mol−1) ΔSd (J mol−1K−1) SD

22.87 60.96 0.01979

18.75 49.73 0.0236

17.51 43.63 0.05173

30.89 87.75 0.02586

22.15 75.30 0.02465

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