Solubility Measurement of Lauric, Palmitic, and Stearic Acids in

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

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Solubility Measurement of Lauric, Palmitic, and Stearic Acids in Ethanol, n‑Propanol, and 2‑Propanol Using Differential Scanning Calorimetry Andrea Briones Gonçalves Bonassoli,† Grazielle Oliveira,† Filipe Hobi Bordón Sosa,† Marlus Pinheiro Rolemberg,‡ Marilsa Aparecida Mota,‡ Rodrigo Corrêa Basso,‡ Luciana Igarashi-Mafra,† and Marcos R. Mafra*,†

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF EXETER on 02/12/19. For personal use only.

†

Department of Chemical Engineering, Federal University of Paraná (UFPR), Polytechnic Center, Jardim das Américas, 81531-990 Curitiba, PR, Brazil ‡ Science and Technology Institute, Federal University of Alfenas, Rodovia José Aurélio Vilela, 11.999, BR 267, 37715−400 Poços de Caldas, MG, Brazil S Supporting Information *

ABSTRACT: The solubility of saturated fatty acids is essential for the design and operation of separation and purification units, since crystallization is the main method used in these cases. In this work, differential scanning calorimetry (DSC) was used to determine the solubility of lauric, palmitic, and stearic acid in alcoholic solvents (ethanol, n-propanol and 2-propanol). Comparison with literature data confirmed the efficacy and precision of the DSC technique. For the three saturated fatty acids evaluated, the solubility was highest in 2-propanol and lowest in ethanol. Activity coefficient models (Margules, Wilson, and nonrandom two-liquids models) were correlated with the experimental data, showing good agreement.

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INTRODUCTION Saturated fatty acids, such as lauric acid (n-dodecanoic acid, C12:0), palmitic acid (n-hexadecanoic acid, C16:0), and stearic acid (n-octadecanoic acid, C18:0), are largely used in the cosmetic, pharmaceutical, and food industries as phase-change materials (PCMs).1−3 Due to their high stability, low toxicity and cost, in addition to their biocompatibility, saturated fatty acids have also been used in the development of drug delivery systems and as a means of taste-masking in these compounds.4−8 In the food industry, lauric, palmitic, and stearic acids can be applied as suppliers of flavor for beverages and foods. Besides, saturated fatty acids can be used in the formulation of mass for ice creams, since they act by holding air and providing a lightness to this product.9 Lauric, palmitic, and stearic acids are the major saturated fatty acids used in those applications. These fatty acids are obtained mainly from coconut oil2 and palm kernel.1,8,10 Since crystallization is the main process used in separation and purification of saturated fatty acids, their solubility data in different solvents are essential to processing design and control.1,2,8,10−13 Despite this, solubility data of these fatty acids in traditional solvents are limited in the literature.2,3,14−16 Traditionally, solubility data are obtained by static methods gravimetric,1,8,13,17−19 or isothermal,20−22 and dynamic methods using a laser monitoring system.3,23,24 Despite the reliability of the obtained data, these techniques are slow and consume a large volume of the sample.25 © XXXX American Chemical Society

Differential scanning calorimetry (DSC) is a well-established analytical technique that has been used in the determination of the physical-chemical properties of systems and in quality control, requiring a small amount of sample (in the milligram range). Concerning solid−liquid transitions, DSC has been applied to obtain equilibrium data, as well as to measure the melting enthalpy and other enthalpy transitions in the polymorphic material.26−31 To the best of our knowledge, the use of DSC for solubility determination was first reported by Young and Schall.32 Solubility data of lauric, palmitic, and stearic acid in some solvents were reported some time ago.2,13,16,33−35 Yang et al.3 determined the solubilities of lauric acid in eight alcohols in the temperature range of 276.17−306.12 K. The solubilities of palmitic and stearic acid were also obtained in ethanol, 2-propanol, acetone, heptane, hexane, and trichloroethylene from 290.0 to 325 K.14,15 Mohan et al.25 evaluated three different DSCs to measure the solubility of some organic compounds. Since the decomposition of the samples is to be avoided, DSC can be used to measure the solubility in wide ranges of temperature and pressure. The authors Special Issue: Latin America Received: November 6, 2018 Accepted: January 24, 2019

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DOI: 10.1021/acs.jced.8b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. List of Chemicals Compounds Used in This Work name

structure

CAS no.

source

mass purity (%)

lauric acid palmitic acid stearic acid ethanol n-propanol 2-propanol

CH3(CH2)10COOH CH3(CH2)12COOH CH3(CH2)16COOH CH3CH2OH CH3CH2CH2OH CH3CHOHCH3

143-07-7 57-10-3 57-11-4 64-17-5 71-23-8 67-63-0

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Panreac Synth Panreac

>99 >99 >99 >98 >98 >98

Figure 1. DSC curves with three thermal cycles initially performed for stearic acid (x1 = 0.1000) + ethanol and lauric acid (x1 = 1).

Table 2. Description of Heat Treatment Used To Determine the Solubility of Fatty Acids (Lauric, Palmitic, and Stearic) in Alcohols (Ethanol, 2-Propanol, and n-Propanol) by DSC heating

cooling

heating

system

T0a to Tmaxb

isotherm

Tmaxb to Tminc

isotherm

Tminc to Tmaxb

lauric acid + solvent palmitic acid + solvent stearic acid + solvent

298.15 to 333.15 K 298.15 to 343.15 K 298.15 to 348.15 K

10 min 10 min 10 min

333.15 to 248.15 K 343.15 to 263.15 K 348.15 to 263.15 K

10 min 10 min 10 min

248.15 to 333.15 K 263.15 to 343.15 K 263.15 to 348.15 K

a

T0 = initial temperature in Kelvin bTmax = maximum temperature in Kelvin cTmin = minimum temperature in Kelvin.

indicated that DSC is a suitable method to measure solubility, since an appropriate pretreatment of the sample and an optimized heating rate is used. They conclude that DSC may yet fail to give accurate results for compounds with low solubilities. However, it has been found that additional studies to analyze broad ranges of concentrations and new methodologies for the determination of fatty acid solubilities are required. In this work, the DSC method was used for the measurement of the solubility

of lauric acid, palmitic acid, and stearic acid in pure solvents (ethanol, n-propanol, and 2-propanol). The experimental data were correlated using activity coefficient models: the Margules, Wilson, and nonrandom two-liquids (NRTL) models.

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MATERIALS AND METHODS Chemicals. The lauric acid, palmitic acid, and stearic acids used in this work were purchased from Sigma-Aldrich

B



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White Martins at a flow rate of 20 mL min−1. The proposed methodology was verified by comparison with literature data, both for pure components and for mixtures. In all experiments, a hermetic stainless-steel high-pressure capsule (DSC pan) with 30 μL capacity was used. The other operating conditions were the same as those described in the Solubility Measurement subsection. Sample Preparation. The samples used in this work were prepared by gravimetry, directly in the DSC pans. This procedure was necessary to avoid problems of homogeneity of the samples caused by crystallization during their preparation. An ultra-microbalance (AD-6 Autobalance, PerkinElmer) was used, with an accuracy of 0.006 mg. Around 15 mg of sample (fatty acid and solvent) was used in all experiments. To avoid losses during sample preparation, the system components were weighed in inverse order of volatility. Also, the ambient temperature was set to a maximum of 293.15 K. In the sequence, the DSC pans were quickly sealed with gold-plated copper seals. The maximum standard uncertainties in the molar fraction (u(x)) for all components was estimated to be 0.0054, being around 0.0014 in most of the experiments. Solubility Measurement. The DSC technique was also used to determine the solubility of lauric acid, palmitic acid, and stearic acid in pure solvents (ethanol, 2-propanol, and n-propanol). The DSC pan containing the desired sample was put in the furnace (DSC 8500, PerkinElmer), and the DSC curves were obtained using Pyris software. An empty pan was used as a reference. The temperature was stabilized at 298.15 K and then increased at a rate of 2 K min−1 to Tmax (maximum temperature). The system was held at this temperature for 10 min to promote complete solubilization. Then, the sample was cooled at a rate of 2 K min−1 to Tmin (minimum temperature) to crystallize the solute. The system was maintained at this temperature for 10 min. Finally, the sample was heated at 2 K min−1 to Tmax. The solubility temperature of the solute was defined as the peak

Table 3. Comparison between the Melting Temperatures (Tm) and Fusion Enthalpy (ΔfusH) of the Pure Fatty Acids (Lauric, Palmitic, and Stearic) Obtained in This Work Using DSC at Atmospheric Pressure (around 91 kPaa) and Data Reported in the Literature ΔfusH/J g−1a

Tm/Ka this work lauric acid

317.71

palmitic acid

336.13

stearic acid

344.08

literature 28

318.07 316.1541 316.3542 317.4544 336.3027 335.7544 336.3445 336.2046 344.0026 341.6543 344.4046 345.6547

this work

literature

178.36

177.7040 177.0241 174.9043 182.7144 200.3427 203.9844 207.8545 163.9346 210.8026 201.8043 198.9146 220.4047

197.43

213.81

a Standard uncertainties u are u(P) = 10 kPa, u(T) = 0.30 K, and u(ΔfusH) = 0.50 J g−1.

(Steinheim, Germany), and the purity was checked by gas chromatography. Ethanol and 2-propanol were supplied by Panreac (Barcelona, Spain), and n-propanol was from Synth (São Paulo, Brazil). All reagent purities were greater than 98%. Table 1 shows the names, molecular structures, CAS number, and purity of all compounds used in this work. Melting Properties of the Fatty Acids. The melting temperature (Tm) and fusion enthalpy (ΔfusH) of the solutes (lauric, palmitic, and stearic acids) were measured by DSC using a DSC 8500 instrument from PerkinElmer (Shelton, CT, USA). The calorimeter was calibrated using indium (99.999%) certified by PerkinElmer. The calibration was carried out at a heating rate of 2 K min−1, using nitrogen (>99.999%) from

Figure 2. Construction of the solubility curve, for the stearic acid + ethanol system, from the data generated in the DSC; x1 represents the molar fraction of stearic acid in the mixture. C



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additional cycle (cooling and heating) was evaluated, and no significant difference was observed concerning the second cycle, as can be seen in Figure 1 for stearic acid (x1 = 0.1000) + ethanol and lauric acid (x1 = 1). Other rates of heating and cooling were evaluated in preliminary experiments. For a lower rate, 1 K min−1, a large increase in noise in the DSC curves was observed. At higher rates, 5 and 10 K min−1, the melt temperature values of the pure fatty acids presented significant differences from the values in the literature. The standard uncertainties in the temperature measurements using 2 K min−1 (u(T)) was 0.3 K. Minimum and maximum temperatures, Tmin and Tmax, respectively, used for each system are shown in Table 2. The minimum temperature should be low enough for crystallization of the solute, and the maximum temperature Should Be High Enough to Solubilize the Solute without Evaporating the Solvent. To evaluate possible mass losses, the pans with the samples were weighed before and after the DSC runs. At all ranges of molar fraction used in the study, i.e., at low and high concentrations of alcohols, the mass remained the same, demonstrating that there was no loss of solvent during the experiments. Solid−liquid Equilibrium Correlation. On a thermodynamic basis, the solubility of the solute in the liquid (solid− liquid equilibrium) can be represented by eq 1:36,37 É Ä ij x1lγ l yz yz ΔCp ÅÅÅÅ Tm,1 ÑÑÑÑ ΔfusH1 ij Tm,1 j 1 z j z j z − 1zz + lnjj s s zz = − j ÑÑ ÅÅ j x1 γ1 z RTm,1 jk T R ÅÅÅÇ T ÑÑÑÖ { k { ΔCp ij Tm,1 yz zz − lnjjj z R (1) k T { where γi is the activity coefficient of the solute, ΔfusH1 and Tm,1 are the enthalpy of fusion and the melting temperature of the pure solute at the triple point temperature, respectively, R is the universal gas constant, and ΔCp is the difference in solute heat capacity between the solid and liquid at the melting point. Considering that the triple point temperature and melting Tm,1

( ) ≈ 0), the heat capacity of the

temperatures are close (ln

T

liquid and solid are similar (ΔCp ≈ 0), and the solid phase is composed by a pure component (xs1γsi = 1), eq 1 becomes ln(x1lγ1l) = −

y ΔfusH1 ij Tm,1 jj − 1zzzz j RTm,1 k T {

(2)

In this work, Margules three-suffix, Wilson, and NRTL models were used to fit the experimental data. Margules Three-Suffix Model. The Margules three-suffix model is calculated and described by eq 3:36 ln γ1 =

(A + 3B)x 2 2 − 4Bx 2 3 RT

(3)

where A and B are the adjustable parameters of the model. Wilson Model. The Wilson model is calculated and described by eq 4: yz ij Λ12 Λ 21 zz ln γ1 = − ln(x1 + x 2 Λ12) + x 2jjj − zz j x1 + x 2 Λ12 x Λ + x 1 21 2{ k

Figure 3. Comparison between the solubility data, (A) lauric acid in ethanol, (B) lauric acid in 2-propanol, and (C) lauric acid in n-propanol, obtained in this study using DSC and data found in literature obtained by different techniques.

(4)

where Λ12 and Λ21 are the adjustable parameters of the model and are related to the molar volume of the pure component and energy of interaction between the molecules.38

temperature obtained during the second heating (see Figure S1 in the Supporting Information). In preliminary tests, an D



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Figure 5. Comparison between the solubility data, (A) stearic acid in ethanol and (B) stearic acid in 2-propanol, obtained in this study using DSC and data found in literature obtained by different techniques. Figure 4. Comparison between the solubility data, (A) palmitic acid in ethanol and (B) palmitic acid in 2-propanol, obtained in this study using DSC and data found in literature obtained by different techniques.

where Texp is the experimental melting temperature and Tcalc is the temperature calculated from eq 2. The root-mean-square deviation (RMSD) and average deviation (AD) were defined by eqs 8 and 9, respectively, where n is the number of experiments:

NRTL Model. The calculation of the activity coefficient through the NRTL model is presented in eq 5: ÅÄÅ ÑÉÑ 2 Å ij ÑÑ yz τ12G12 G21 2Å Å ÑÑ j z ln γ1 = x 2 ÅÅÅτ21jj zzz + ÑÑ 2 j ÅÅ k x1 + x 2G21 { Ñ ( + ) x x G 2 1 12 Ñ (5) ÅÇ ÑÖ

RMSD =

where G12 and G21 are energetic interaction parameters between molecules 1 and 2, calculated by ln G12 = −ατ12 and ln G21 = −ατ21, and τ21 is related to the degree of nonrandomicity of the mixture and can be calculated by eq 6. The parameters, as shown in eq 6, were obtained for the NRTL model, where τ12 and τ21 are energy parameters characteristic of the interaction between molecules 1 and 2:39 τ12 =

−Δg12

and

τ21 =

AD =

■

∑ (Texp − Tcalc)2

n

(8)

∑ (|Texp − Tcalc|) n

(9)

RESULTS AND DISCUSSION Thermodynamic Properties of Pure Fatty Acids. Melting temperature (Tm) and enthalpy of fusion (ΔfusH) of the pure solutes (lauric, palmitic, and stearic acids) obtained in this work by the peak temperature of the DSC curve are shown in Table 3 (see Figure S2 in Supporting Information). One can see that the data obtained in this work present good agreement with the literature, indicating the precision of the DSC method. Determination of Solubility Curves. The solubility curves obtained by DSC are shown in Figure 2 for the stearic acid + ethanol system. The same procedure was adopted for all systems (see Figures S3−S10 in the Supporting Information).

−Δg21

(6) RT RT The parameters for these models were determined by the Excel solver tool by minimization of the objective function (OF) described in eq 7:

OF =

∑ (Texp − Tcalc)2

(7) E


320.28 326.15 330.09 332.33 335.58 337.51 339.08 340.69 342.27 344.08

The melting temperature was determined from the peak temperature for all evaluated systems.25,26,31,48−51 Furthermore, according to Xu et al.52 and Ma et al.,53 the peak temperature is independent of the mass of the sample, which contributes to reducing uncertainties un the measurements. To evaluate the DSC experimental methodology used, the experimental data obtained in this work were compared with the literature data obtained by different techniques: isothermic methods3,8,16 and dynamic methods.2,14,15,34,54−56 For lauric acid in ethanol, for example, it is possible to observe in Figure 3 that the solubility values obtained in this work show good agreement with literature data over the entire composition range. The same occurs for palmitic acid and stearic acid in ethanol, as shown in Figures 4 and 5, respectively. Table 4 shows the experimental data obtained in this work. The solubility of lauric acid in the three solvents is shown in Figure 6. As expected, the solubility of lauric acid is directly

0.1000 0.2000 0.3000 0.4001 0.5006 0.5999 0.7000 0.8000 0.9000 1.0000 315.69 322.81 327.67 331.90 334.82 337.00 339.14 340.75 342.82 344.08 0.1000 0.2001 0.3017 0.4001 0.5000 0.6000 0.7001 0.8000 0.9001 1.0000 307.27 311.76 317.46 320.97 324.04 330.06 334.39 334.98 335.59 336.13

T/Ka

x1a

ethanol

T/Ka

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0.0682 0.1133 0.2034 0.2844 0.3657 0.5824 0.7899 0.8628 0.9031 1.0000 306.51 316.21 320.95 323.24 326.67 328.41 331.20 333.55 334.95 336.13

Standard uncertainties u are u(P) = 10 kPa, u(T) = 0.30 K, and u(x1) = 0.0054.

0.0623 0.1304 0.2329 0.3479 0.4509 0.5318 0.6814 0.7288 0.8600 1.0000 284.96 293.48 298.68 302.61 306.97 308.84 311.20 312.65 315.61 317.71 0.1299 0.2256 0.3172 0.4070 0.5276 0.5973 0.6950 0.7492 0.8724 1.0000 276.14 286.46 295.65 301.55 305.93 307.86 311.34 313.27 316.02 317.71 0.0797 0.1636 0.2837 0.4104 0.5171 0.5975 0.7108 0.7779 0.8890 1.0000

Figure 6. Experimental mole fraction solubility (x1) of lauric acid in different alcohols: (●) ethanol; (△) n-propanol; (□) 2-propanol. The lines represent the correlation results for the Margules model (blue) to ethanol, (red) to n-propanol, and (black) to 2-propanol.

temperature-dependent for all solvents. For a given temperature, the solubility of lauric acid follows the order: 2-propanol > n-propanol > ethanol. It is possible to observe that at high temperatures there is no significant difference in the solubility of lauric acid in the solvents evaluated. The same behavior is observed for palmitic acid (Figure 7) in the same solvents. In the case of stearic acid, solubility data in ethanol and 2-propanol were obtained, since the previous results showed a similar behavior of the fatty acids in 2-propanol and n-propanol. In these two solvents (ethanol and 2-propanol), the solubility curves of stearic acid presented similar behavior to those of the other two fatty acids (Figure 8). Thermodynamic Modeling. The experimental solubility data were correlated by Margules, Wilson and NRTL activity coefficient models. Table 5 shows the optimized model parameters. All models presented a good correlation with the experimental data, presenting maximum RMSD and AD values of 0.983 and 0.824, respectively. Since all models presented a good correlation with the experimental data, only the Margules model curves are presented in Figures 6−8 for lauric, palmitic, and stearic acids, respectively. Since the solubility behavior of the fatty acids in these cases varies according to the solvent, it was expected that the ideal solution would not be able to

a

0.0890 0.2117 0.3145 0.3782 0.4903 0.5675 0.6796 0.7900 0.8744 1.0000 277.62 288.32 295.59 301.05 305.70 308.05 311.55 312.80 315.92 317.71

0.0890 0.2117 0.3146 0.3782 0.4904 0.5673 0.6796 0.7898 0.8744 1.0000

305.41 314.74 319.42 322.04 327.15 328.87 330.71 331.76 334.48 336.13

T/Ka x1a T/Ka T/Ka T/Ka

2-propanol

x1a

x1a

n-propanol

x1a

ethanol

T/Ka

x1a

2-propanol

n-propanol

x1a

ethanol

T/Ka

x1a

2-propanol

stearic acid palmitic acid lauric acid

Table 4. Experimental Mole Fraction Solubility (x1) of Fatty Acids (Lauric, Palmitic, and Stearic) in Alcohols (2-Propanol, n-Propanol, and Ethanol) Measured by DSC at Atmospheric Pressure (around 91 kPa)


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hypothesis was also evaluated, and showed a large deviation from the experimental data, as can be observed in these figures. The other figures are presented in the Supporting Information (Figures S11−S13).

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CONCLUSIONS The DSC technique was used to determine the solubility of saturated fatty acids (lauric, palmitic, and stearic acid) in organic solvents (ethanol, n-propanol, and 2-propanol). The experimental data presented good agreement with data from the literature over the entire composition range, proving the efficacy of the technique. The solubility of the fatty acids followed the order: 2-propanol > n-propanol > ethanol. The activity coefficient models (Margules, Wilson, and NRTL) were correlated with the experimental data with a low mean deviation. Therefore, the DSC technique presents a fast and precise alternative for the determination of the solubility of fatty acids.

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Figure 7. Experimental mole fraction solubility (x1) of palmitic acid in different alcohols: (●) ethanol; (△) n-propanol; (□) 2-propanol. The lines represent the correlation results for the Margules model (blue) to ethanol, (red) to n-propanol, and (black) to 2-propanol.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01044. DSC curves and other correlations between experimental solubility data and activity coefficient models (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marcos R. Mafra: 0000-0002-0018-6867 Funding

The authors are grateful for the scholarship provided by CAPES (Coordination for the Improvement of Higher Education Personnel), CNPq (Proc. Num. 310905/2015-0), and the support of the Post-Graduation Program in Food Engineering and the Analytical Center of Federal University of Paraná (Curitiba, Brazil). Notes

Figure 8. Experimental mole fraction solubility (x1) of stearic acid in different alcohols: (●) ethanol and (□) 2-propanol. The lines represent the correlation results for the Margules model (blue) to ethanol and (red) to 2-propanol.

The authors declare no competing financial interest.

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REFERENCES

(1) Maeda, K.; Nomura, Y.; Guzman, L. A.; Hirota, S. Crystallization of fatty acids using binodal regions of two liquid phases. Chem. Eng. Sci. 1998, 53, 1103−1105.

represent such systems, since the ideal solution hypothesis does not predict the influence of solvent type. The ideal solution

Table 5. Parameters of Margules, Wilson, and NRTL Models, as Well as the Correlation Performance, for Fatty Acids (Lauric, Palmitic, and Stearic) in Alcohols (Ethanol, 2-Propanol, and n-Propanol) Margules solvent

a

A/J mol−1

B/J mol−1

NRTLa

Wilson RMSD

AD

ethanol 2-propanol n-propanol

2053.5 1224.2 1355.3

2105.4 1311.5 1785.7

0.804 0.528 0.370

0.691 0.445 0.309

ethanol 2-propanol n-propanol

2273.4 1658.1 1930.1

2437.9 1501.8 1864.8

0.975 0.872 0.693

0.811 0.794 0.579

ethanol 2-propanol

2507.6 1167.6

1290.0 870.9

0.581 0.566

0.459 0.445

Λ12

Λ21

lauric acid 0.5562 0.6794 0.7171 0.7914 0.8736 0.5474 palmitic acid 0.6373 0.5615 0.5404 0.9222 0.5737 0.7716 stearic acid 0.2644 1.2252 0.4917 1.2212

τ21

RMSD

AD

0.3036 0.1987 0.6959

0.6188 0.3511 0.0199

0.851 0.556 0.376

0.727 0.469 0.313

0.812 0.785 0.585

0.5228 −0.0222 0.1734

0.4381 0.6972 0.6081

0.983 0.880 0.709

0.824 0.798 0.592

0.353 0.449

−0.4674 −0.3238

1.5576 0.8186

0.548 0.566

0.396 0.448

RMSD

AD

0.823 0.551 0.364

0.701 0.464 0.298

0.963 0.870 0.696 0.523 0.560

τ12

NRTL model was calculated with α = 0.3, as recommended by Prausnitz et al.36 G



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Solubility Measurement of Lauric, Palmitic, and Stearic Acids in

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