Solubility of Aspartame in Water, Methanol, Ethanol and Different

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Solubility of Aspartame in Water, Methanol, Ethanol and Different Binary Mixtures in the Temperature Range of (278.15 to 333.15) K Yixin Leng* and Huichen Qi School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, PR China ABSTRACT: The solubility of aspartame in water, methanol, ethanol, and their binary mixtures (methanol + water and ethanol + water) was measured in the temperature range from 278.15 K to 333.15 K under atmospheric pressure. The results indicate that the solubility of aspartame in the selected solvents increases with increasing temperature and is higher in water than in pure alcohols. However, the solubility is higher in the binary mixtures than in water and it increases with an increase of the mass fraction of alcohols. The experimental data were correlated by the modified Apelblat model, and thus calculated values are in good agreement with the experimental data in the temperature range studied. The enthalpy and entropy of solution were evaluated using the van’t Hoff equation. The dissolution enthalpy and entropy of aspartame in pure solvents predicted from the solubility data are less than that in mixed solvents. larger size and good solid−liquid separability.3,4 To the best of our knowledge, no quantitative data at all from the literature are known from processes where organic cosolvents are used; therefore it was necessary to acquire such information, in our opinion. In the present work, the solubility of aspartame in water, methanol, ethanol, methanol + water, and ethanol + water, respectively, between 278.15 K and 333.15 K, was measured at atmospheric pressure (101.3 KPa). Analysis of aspartame was done with an UV spectrophotometer [UV-mini1240, Shimadzu, Japan]. The experimental data were correlated by a leastsquares fitting method using the modified Apelblat equation.

1. INTRODUCTION Aspartame (N-L- α-aspartyl-L-phenylalanine 1-methyl ester, CAS Registry No. 22839-47-0; molecular mass, 294.31; Figure 1 shows the molecular structure), in the form of needle-like

Figure 1. The chemical structure of aspartame.

2. EXPERIMENTAL SECTION 2.1. Materials. Aspartame (purity ≥ 99.9 %) was supplied by Changmao Biochemical Engineering Co., Ltd. All solvents of analytical reagent grade with their mass fraction purities higher than 0.997, were bought from Shanghai Zhengxin Chemical Reagent Co., Ltd., and used in experiments without farther purification. Deionized water used in the experiments. 2.2. Apparatus and Procedures. The solubility of aspartame in water, methanol, ethanol, and their binary mixtures (methanol + water and ethanol + water) was determined in the temperature range of (278.15 to 333.15) K using the isothermal method.5 Binary mixtures (methanol + water and ethanol + water) were prepared by weighing the constitutional parts (with uncertainty of ± 0.1%) before their mixing. To find the suitable time for the equilibrium, the test experiments were carried out over 0.5, 1, 2, 3, 4, and 5 h,

crystals is usually obtained in industrial crystallizers of the stirred-tank type,1 and is about 200 times as sweet as sugar. It has an agreeable taste and low calorie content and has found its use in foods, beverages, and pharmaceuticals.2 In industrial production, aspartame is obtained in high quality and purity by crystallization refinement. Thus, the investigation of the solubility of aspartame in different solvents is very important for choosing the proper solvent and for optimizing the crystallization conditions, and water, methanol + water, or ethanol + water binary mixtures are commonly used in this process. The solubility data of aspartame in water in the temperature range of (283.15 to 358.15) K are available in the literature.1 It is well-known that the solubility of aspartame in water is quite low, but it can be increased by adding such cosolvents as methanol and ethanol. However, no quantitative data are available in the patent of this process. When a mixed solvent is used over 313.15 K, the equipment productivity can be improved as the solubility of aspartame in the mixed solvent is higher than in water alone, and such crystals obtained have © 2014 American Chemical Society

Received: December 26, 2013 Accepted: April 21, 2014 Published: April 28, 2014 1549

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Table 1. Mole Fraction Solubility of Aspartame (x1) in Pure Solventsa T/K

103x1

102[(x1 − xcalcd)/x1]b

T/K

103x1

102[(x1 − xcalcd)/x1]b

308.15 313.15 318.15 323.15 328.15 333.15

1.0236 1.2261 1.5011 1.8239 2.2421 2.7395

0.50 −0.77 −0.098 −0.38 0.31 0.24

308.15 313.15 318.15 323.15 328.15 333.15

1.0654 1.2008 1.3583 1.5506 1.7488 2.0009

0.39 0.022 −0.17 0.38 −0.41 0.10

308.15 313.15 318.15 323.15 328.15 333.15

0.1874 0.2355 0.2906 0.3469 0.4184 0.4928

0.38 0.61 0.72 −0.89 0.030 0.026

Water 278.15 283.15 288.15 293.15 298.15 303.15

0.3376 0.4078 0.4830 0.5807 0.7014 0.8364

−0.74 0.77 −0.075 0.21 0.59 −0.61

278.15 283.15 288.15 293.15 298.15 303.15

0.5316 0.5926 0.6647 0.7395 0.8323 0.9393

−0.031 0.020 0.37 −0.38 −0.24 −0.065

278.15 283.15 288.15 293.15 298.15 303.15

0.031 60 0.045 33 0.061 91 0.084 80 0.1128 0.1445

−0.12 0.74 −0.83 0.14 0.37 −1.21

Methanol

Ethanol

a

Standard uncertainties u are u(T) = 0.01 K and ur(x1) = 0.05. b[(x1 − xcalcd)/x1] is the relative deviation.

and sample solution was measured at the maximum absorption of aspartame at wavelength of 258 nm. The calibration curve for the estimation of aspartame content, was prepared by using the standard solutions in the appropriate concentration range. All solubility experiments were carried out three times to check their reproducibility, and average values are given. The uncertainty of the experimental solubility values, was estimated to be less than 0.05. Though this uncertainty is higher than the one known from literature (0.01),7 it is still low enough to (1) make the conclusions concerning the solubility at different conditions applicable and (2) analyze the data obtained with a help of Apelblat and van’t Hoff equation.

respectively. The solubility data that were measured over 2, 3, 4, and 5 h showed a better agreement, compared with those obtained over 0.5 and 1 h. So the equilibrium time of 2 h was chosen. For each measurement, an excess amount of aspartame was added to a certain mass of solvent in a jacked glass vessel maintained at a desired temperature with continuous stirring. Test tubes were sealed to prevent solvent evaporation. The temperature was controlled by circulating water through the outer jacket from a thermostatically controlled superconstant temperature water bath (type DC-1006, Shanghai Hengping Scientific Instrument Co. Ltd., China) with a fluctuating temperature of ± 0.05 K. After stirring a mixture in a given solvent (or mixture of solvents) for 2 h, the suspended solution was kept then without stirring still for 2 h at the same temperature, to ensure that the suspended solid phase settled on the bottom of the tube. Then, the supernatant liquid was filtered, and the filtrate was diluted several times for accurate UV measurements.6 The mole fraction of aspartame (x1) in a saturated solution can be expressed by the following eq 1, while the composition of a solvent mixture can be expressed by eq 2 and eq 3. x1 =

m1/M1 m1/M1 + ∑ mi /Mi

(1)

wA =

mA mA + m2

(2)

wB =

mB mB + m2

(3)

3. RESULTS AND DISCUSSION 3.1. In Pure and Mixed Solvents. The solubility of aspartame in pure solvents (in mole fraction units) is shown in Table 1. To compare the experimental values clearly, the solubility data are graphically plotted also in Figure 2. To verify the uncertainty of the measurement, the solubility of aspartame in water in this work was measured and compared with the literature data.1 Compared with that of the literature data, the relative deviation of the solubility was lower than 0.05. From Table 1 and Figure 2, it is clear that the solubility of aspartame in water is higher than in methanol or ethanol. The solubility of aspartame in ethanol is the lowest one. This is due to the properties of aspartame. The molecular structure of aspartame is shown in Figure 1. As it can be seen, there is a hydrophilic aspartyl side and a highly hydrophobic phenylalanine side. In aqueous and alcoholic solutions, an isoelectric band of minimun solubility is formed.8 The polarity of the solvent is in the following order: water > methanol > ethanol. Thus, the solubilities increase with the increasing polarity of the solvent. So, to some extent, we can draw the conclusion that the polarity of solvents is regarded as the key factor to determine the solubility. The solubility of aspartame in binary solvent mixtures (methanol + water and ethanol + water) has been investigated

where m1, m2, mA, mB, and mi, represent the mass of the aspartame, water, methanol, ethanol, and the according solvent, respectively; M1, M2, MA, MB, and Mi represent the molecular mass of the aspartame, water, methanol, ethanol, and according solvent, respectively. 2.3. Sample Analysis. To determine the aspartame concentration in the solution, the absorbance of the standard 1550

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demonstrated in Figures 3 and 4. From Tables 2 and 3 and Figures 3 and 4, it can be found that the solubility of aspartame in these mixtures increases with an increase of temperature. From 278.15 K to 293.15 K, it increases slowly, while it increases rapidly after temperature reaches 298.15 K. However, the solubility of aspartame in binary solvents (methanol + water or ethanol + water) increase with increasing the mass fraction of methanol or ethanol, respectively. For the better understanding and comparison, mole fraction solubility as a function of alcohol content wA and wB are presented in Figure 5 and Figure 6, respectively. To describe the temperature dependence of aspartame solubility in water, methanol, ethanol, and the binary systems of methanol + water and ethanol + water more quantitatively, the solubility of aspartame (as a function of temperature) has been fitted by the modified Apelblat equation:9 Figure 2. Solubilities of aspartame in pure solvents between 278.15 K and 333.15 K: ▲, ethanol; ●, methanol; ■, water; □, water in literature.1

ln x1 = A +

B + C ln(T ) T

(4)

where x1 is the mole fraction solubility of aspartame; T is the absolute temperature; and A, B, and C are the parameters and are listed in Tables 4 to 6. Thus, the experimental solubility

in the temperature interval ranging from 278.15 K to 333.15 K. These data are presented in Tables 2 and 3, and graphically

Table 2. Mole Fraction Solubility of Aspartame (X1) in Binary Solvent Mixtures of Methanol + Watera 102[(x1 − xcalcd)/x1]b

T/K

103x1

278.15 283.15 288.15 293.15 298.15 303.15

0.1115 0.1655 0.2420 0.3459 0.4787 0.6567

0.20 −0.22 0.11 0.36 −0.58 −0.49

278.15 283.15 288.15 293.15 298.15 303.15

0.1707 0.2410 0.3326 0.4631 0.6316 0.8659

0.27 0.36 −0.81 −0.09 −0.50 0.50

T/K

103x1

102[(x1 − xcalcd)/x1]b

308.15 313.15 318.15 323.15 328.15 333.15

0.8867 1.1798 1.5368 1.9721 2.4552 3.0817

−0.15 0.43 0.51 0.64 −0.80 −0.045

308.15 313.15 318.15 323.15 328.15 333.15

1.1617 1.5585 2.0133 2.7169 3.5611 4.5012

0.20 0.72 −1.76 0.89 1.32 −1.17

308.15 313.15 318.15 323.15 328.15 333.15

1.6002 2.1084 2.7511 3.5225 4.5022 5.5836

0.72 −0.020 −0.078 −0.45 0.47 −0.18

308.15 313.15 318.15 323.15 328.15 333.15

2.0255 2.6865 3.4345 4.4231 5.5529 6.7705

−0.65 0.23 −0.84 0.44 0.71 −0.48

308.15 313.15 318.15 323.15 328.15 333.15

2.4165 3.2055 4.1378 5.2191 6.6708 8.3197

−0.74 0.19 −0.07 −1.24 0.29 0.66

wAc = 0.1

wAc = 0.2

wAc = 0.3 278.15 283.15 288.15 293.15 298.15 303.15

0.1932 0.2909 0.4233 0.6054 0.8589 1.1702

0.0075 0.36 −0.34 −0.54 0.51 −0.49 wAc = 0.4

278.15 283.15 288.15 293.15 298.15 303.15

0.2589 0.3880 0.5579 0.8074 1.1095 1.5369

−0.043 0.39 −0.88 0.77 −0.57 0.91 wAc = 0.5

278.15 283.15 288.15 293.15 298.15 303.15 a

0.3242 0.4795 0.6909 0.9745 1.3498 1.8249

−0.56 0.10 0.30 0.39 0.56 0.10

Standard uncertainties u are u(T) = 0.01 K and ur(x1) = 0.05. b[(x1 − xcalcd)/x1] is the relative deviation. cwA is the mass fraction of methanol. 1551

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Table 3. Mole Fraction Solubility of Aspartame (x1) in Binary Solvent Mixtures of Ethanol + Watera T/K

103x1

102[(x1 − xcalcd)/x1]b wBc

278.15 283.15 288.15 293.15 298.15 303.15

0.1230 0.1804 0.2617 0.3662 0.5101 0.7059

T/K

103x1

102[(x1 − xcalcd)/x1]b

308.15 313.15 318.15 323.15 328.15 333.15

0.9410 1.2585 1.6647 2.1163 2.7095 3.4813

−0.44 0.13 0.96 −0.89 −0.82 0.74

308.15 313.15 318.15 323.15 328.15 333.15

1.0276 1.4253 1.9793 2.7591 3.8869 5.4126

−0.42 −0.093 −0.083 −0.090 0.60 −0.18

308.15 313.15 318.15 323.15 328.15 333.15

1.5705 2.2223 3.1102 4.3190 6.0561 8.2116

−0.046 0.32 0.16 −0.20 0.92 −0.76

308.15 313.15 318.15 323.15 328.15 333.15

2.1542 2.9990 4.0440 5.3873 7.1802 9.3440

−0.78 0.70 0.23 −0.28 0.34 −0.36

308.15 313.15 318.15 323.15 328.15 333.15

3.0296 4.0420 5.2377 6.8210 8.7040 11.1172

0.44 0.73 −0.56 −0.097 −0.47 0.36

= 0.1

−0.061 −0.14 0.60 −0.48 −0.35 0.71 wBc = 0.2

278.15 283.15 288.15 293.15 298.15 303.15

0.1664 0.2218 0.2998 0.4040 0.5529 0.7434

−0.31 −0.19 0.55 0.24 0.86 −0.89 wBc = 0.3

278.15 283.15 288.15 293.15 298.15 303.15

0.1762 0.2553 0.3712 0.5436 0.7793 1.0997

0.63 −0.57 −0.69 0.58 0.39 −0.73

278.15 283.15 288.15 293.15 298.15 303.15

0.2436 0.3620 0.5330 0.7791 1.1047 1.5720

0.49 −0.36 −0.47 0.30 −0.44 0.59

278.15 283.15 288.15 293.15 298.15 303.15

0.3962 0.5716 0.834 07 1.1761 1.6255 2.2352

0.55 −1.03 0.39 0.13 −0.47 −0.034

wBc = 0.4

wBc = 0.5

a

Standard uncertainties u are u(T) = 0.01 K and ur(x1) = 0.05. b[(x1 − xcalcd)/x1] is the relative deviation. cwB is the mass fraction of ethanol.

Figure 3. Solubilities of aspartame (x1) in methanol (wA) + water binary mixture between 278.15 K and 333.15 K: ■, wA = 0.1; ●, wA = 0.2; ▲, wA = 0.3; ▼, wA = 0.4; ⧫, wA = 0.5.

Figure 4. Solubilities of aspartame (x1) in ethanol (wB) + water binary mixture between 278.15 K and 333.15 K: ■, wB = 0.1; ●, wB = 0.2; ◆, wB = 0.3; ▲, wB = 0.4; ▼, wB = 0.5.

values have been correlated with eq 4 using the least-squares method, and the difference between experimental and

calculated results are presented in Tables 1 to 3. The values of the three parameters, A, B, and C, together with the root1552

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Table 5. Curve-Fitting Parameters of Solubility Curve of Aspartame in Methanol + Water Mixtures in the Temperature Range of (278.15 to 333.15) K WA

A

B

C

105rmsd

0.1 0.2 0.3 0.4 0.5

210.71 −22.03 218.21 241.18 187.44

−14628.68 −4033.13 −15005.04 −15893.50 −13401.73

−29.71 4.95 −30.71 −34.18 −26.19

0.74 2.42 0.93 1.91 2.59

Table 6. Curve-Fitting Parameters of Solubility Curve of Aspartame in Ethanol + Water Mixtures in the Temperature Range of (278.15 to 333.15) K

Figure 5. Solubilities of aspartame (x1) in methanol (wA) + water binary mixture as a function of wA: +, 333.15 K; ●, 328.15 K; ○, 323.15 K; ★, 318.15 K; ◇, 313.15 K; ▲, 308.15 K; ☆, 303.15 K; ■, 298.15 K; △, 293.15 K; ▼, 288.15 K; □, 283.15 K; ◆, 278.15 K.

WB

A

B

C

105rmsd

0.1 0.2 0.3 0.4 0.5

132.94 −377.93 −119.53 75.84 127.35

−11125.17 11789.31 −424.56 −8964.25 −10821.45

−18.11 58.07 19.97 −9.23 −17.11

1.24 0.79 2.46 1.57 2.14

Figure 7. The van’t Hoff plots of ln(x1) versus 1/T in pure solvents: ■, methanol; ●, ethanol; ▲, water.

Figure 6. Solubilities of aspartame (x1) in ethanol (wB) + water binary mixture as a function of wB: +, 333.15 K; ●, 328.15 K; ○, 323.15 K; ★, 318.15 K; ◇, 313.15 K; ▲, 308.15 K; ☆, 303.15 K; ■, 298.15 K; △, 293.15 K; ▼, 288.15 K; □, 283.15 K; ◆, 278.15.

Table 4. Curve-Fitting Parameters of Solubility Curve of Aspartame in Pure Solvents in the Temperature Range of (278.15 to 333.15) K solvent

A

B

C

105rmsd

water methanol ethanol

−218.63 −151.49 269.98

6587.53 4642.82 −16553.03

33.22 22.61 −39.24

0.52 0.33 0.13

mean-square relative deviations (rmsd), are listed in Tables 4 to 6. The rmsd is defined as the following: N

rmsd =

∑i = 1 (x1 − xcalcd)2 N

(5)

Figure 8. The van’t Hoff plots of ln(x1) versus 1/T in methanol (wA) + water mixtures: ■, wA = 0.1; ●, wA = 0.2; ▲, wA = 0.3; ▼, wA = 0.4; ⧫, wA = 0.5.

where N is the number of experimental points, and x1 and xcalcd represent the mole fraction solubility of the experiment and that calculated from eq 4, respectively. 1553

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From Tables 7 to 9, it can be found that the course of aspartame dissolving in the solvents in the experimental temperature range is endothermic (ΔHd > 0). This is additionally supported by the results of the solubilities of aspartame in all solvents selected in this work, where they increase with increasing temperature. The dissolution process is endothermic because the interactions between aspartame molecules in the solid and between the solvent molecules are stronger than those between the solvent and aspartame molecules. ΔHd and ΔSd for aspartame dissolving in the binary solvent systems are relatively large. In the binary mixtures, the values of ΔHd increase with the decline in the mass fraction of water and attain a maximum at wA = 0.3 or wB = 0.3, which was similar for the entropy ΔSd. In the pure solvent, the values of ΔHd and ΔSd in water are lower than the ones found for ethanol, and the value in methanol is the lowest. The positive ΔHd and ΔSd for aspartame dissolving in pure solvents and in the binary solvent systems reveal that aspartame dissolves by an entropy-driven process.

Figure 9. The van’t Hoff plots of ln(x1) versus 1/T in ethanol (wB) + water mixtures: ■, wB = 0.1; ●, wB = 0.2; ▲, wB = 0.3; ▼, wB = 0.4; ⧫, wB = 0.5.

4. CONCLUSIONS New data on the solubility of aspartame in the water, methanol, ethanol, and binary mixtures such as ethanol with water and methanol with water from 278.15 K to 333.15 K, have been accumulated and discussed. Solubility increases with an increase of temperature. The solubility of aspartame is higher in pure water than in pure methanol or ethanol in the whole temperature range. However, the solubility of aspartame in methanol + water or ethanol + water binary solvent mixtures increases with an increase in the mass fraction of methanol or ethanol, respectively, exceeding by far its solubility in pure water. The calculated solubility of aspartame has a good conformity with the experimental solubility and thus, the correlation equation used in this work, can be used for solvent selection and model research of the crystallization process of aspartame. In addition, the thermodynamic properties for the solution process including enthalpy and entropy, have been obtained by the van’t Hoff equation. For all the cases studied, both of the values of enthalpy and entropy are positive, which indicated that the process is endothermic and entropy-driven.

Table 7. Solution Enthalpy and Entropy of Aspartame in Pure Solvents ΔHd (kJ mol−1) ΔSd (J mol−1 K−1) r

methanol

ethanol

water

18.57 3.60 0.9970

38.42 52.80 0.9979

29.22 37.96 0.9974

Table 8. Solution Enthalpy and Entropy of Aspartame in Methanol + Water Mixtures ΔHd (kJ mol−1) ΔSd (J mol−1 K−1) r

wA = 0.1

wA = 0.2

wA = 0.3

wA = 0.4

wA = 0.5

46.50 92.11

46.04 93.28

47.12 98.94

45.75 96.53

45.26 96.51

0.9992

0.9999

0.9991

0.9989

0.9993

From Tables 1 to 3, it is clearly seen that the solubility calculated by the modified Apelblat equation is in a good agreement with the experimental values. 3.2. Dissolution Enthalpy and Entropy. The van’t Hoff equation relates the logarithm of mole fraction of a solute in an ideal solution as a linear function of the reciprocal of the absolute temperature10,11

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0519 86330356. Fax: +86 0519 86330356. E-mail: [email protected].

ΔHd ΔSd + (6) RT R where x1 is the mole fraction solubility, ΔHd is the solution enthalpy, ΔSd is the solution entropy, T is the absolute temperature, and R is the gas constant. The van’t Hoff plots shown in Figures 7 to 9 were obtained from the linear fit of ln(x1) versus 1/T. The solution enthalpy and entropy of aspartame shown in Tables 7 to 9 can be calculated from the slope and the intercept of these plots. ln(x1) = −

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kishimoto, S.; Naruse, M. A Process Development for the Bundling Crystallization of Aspartame. J. Chem. Technol. Biotechnol. 1988, 43, 71−74.

Table 9. Solution Enthalpy and Entropy of Aspartame in Ethanol + Water Mixtures ΔHd (kJ mol−1) ΔSd (J mol−1 K−1) r

wB = 0.1

wB = 0.2

wB = 0.3

wB = 0.4

wB = 0.5

46.70 93.41 0.9997

48.81 101.97 0.9972

54.03 121.90 0.9997

51.20 115.04 0.9999

45.26 96.51 0.9995

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(2) Hsu, W. P.; Koo, K. K.; Myerson, A. S. The Gel-Crystallization of and Aspartame from Aqueous Solutions. Chem. Eng. Commun. 2002, 189, 1079−1090. (3) Suzuki, Kawasaki, Kanagawa. Method of Crystallizing α-L-aspartylL-phenylalanine methyl ester. Eur. Patent 0514940, 1995. (4) Ebisawa, K.; Kawaoka, R. Method of crystallizing aspartame. U.S. Patent 5298648, 1994. (5) Zhou, X. Q.; Fan, J. S.; Li, N.; Du, Z. X.; Ying, H. J.; Wu, J. L.; Xiong, J.; Bai, J. X. Solubility of L-Phenylalanine in Water and Different Binary Mixtures from 288.15 to 318.15 K. Fluid Phase Equilib. 2012, 316, 26−33. (6) Dattatreya, B. S.; Usha, D. V.; Susheelamma, N. S.; et al. Aspartame: Studies on UV Spectral Characteristics. Int. J. Food Sci. Technol. 2003, 38, 67−77. (7) Wang, L.; Dai, L.-y.; Lei, M.; Chen, Y. Solubility of Hexamethylenetetramine in a Pure Water, Methanol, Acetic Acid, and Ethanol + Water Mixture from (299.38 to 340.35) K. J. Chem. Eng. Data 2008, 53, 2907−2909. (8) Needham, T. E., Jr.; Paruta, A. N.; Gerraughty, R. J. Solubility of Amino Acids in Pure Solvent Systems. J. Pharm. Sci. 1971, 60, 565− 567. (9) Thati, J.; Nordstrom, F. L.; Rasmuson, A. C. Solubility of Benzoic Acid in Pure Solvents and Binary Mixtures. J. Chem. Eng. Data 2010, 55, 5124−5127. (10) Li, Z.; Jiang, X.; Lu, J.; Rohani, S. Solubility of L-Phenylalanine in Aqueous Solutions. Chem. Eng. 2010, 43, 810−813. (11) Song, L. C.; Gao, Y.; Gong, J. B. Measurement and Correlation of Solubility of Clopidogrel Hydrogen Sulfate (Metastable Form) In Lower Alcohols. J. Chem. Eng. Data 2011, 56, 2553−2556. L-Phenylalanine

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