Solubility and Thermodynamic Behavior of Syringic Acid in Eight Pure

Article pubs.acs.org/jced

Solubility and Thermodynamic Behavior of Syringic Acid in Eight Pure and Water + Methanol Mixed Solvents Adel Noubigh*,†,‡ and Ali Akermi†,§ †

Department of Chemistry, Faculty of Science, Northern Border University, Arar 91431, Kingdom of Saudi Arabia Institut Préparatoire aux Etudes Scientifiques et Techniques 99/UR/1201 unité de recherches de physico−chimie moléculaire, Université de Carthage, 2070, La Marsa, Tunisia § Faculté des Sciences de Tunis, Laboratoire de Chimie Organique Structurale, Université de Tunis El Manar, 1060, Tunis, Tunisia ‡

ABSTRACT: In this work, the solubility data of syringic acid (SA) in eight mono solvents, including methanol, ethanol, 2-propanol, 1-butanol, 2-butanol, 1-hexanol, ethylene glycol, and ethyl acetate, and water + methanol mixtures were obtained experimentally in temperatures ranging from 283.15 to 323.15 K at 5 K intervals with a gravimetric method under a pressure of 0.1 MPa. It is found that the solubility of SA increases with increasing temperature in all selected pure and binary mixed solvents within the temperature range studied. The mole fraction solubility of SA is found to be highest in methanol and lowest in ethyl acetate. Additionally, three commonly used thermodynamic models, namely, the van’t Hoff equation, the modified Apelblat, and λh models, were applied to correlate the experimental solubility data. It is found that the three empirical thermodynamic models can satisfactorily correlate the solubility of SA in the solvents under investigated temperatures. The excess enthalpy of the solutions was predicted. The experimental data and model parameters would be useful for optimizing the process of purification of syringic acid in industry.

1. INTRODUCTION Syringic acid (IUPAC name: 4-Hydroxy-3,5-dimethoxybenzoic acid, SA, C9H10O5, MW = 198.172 g·mol−1, CAS Registry No. 530-57-4) is a yellow powder solid. Syringic acid is an active compound widely distributed in nature. It is found in date palm fruit (Ajwah, Safawy, and Sukkari)1−3 and other plants (Isatis indigotica and Radix isatidi).4,5 SA has chemical properties that have been shown to facilitate oxidation, polymerization, and condensation reactions. In addition, many bioactivities of SA have been reported, such as the antioxidant,6 antiproliferative,7 hepatoprotective,8 antilipidemic,9 antimicrobial, anti-inflammatory, and anticancer effects.10,11 The chemical structure of SA is shown in Figure 1.

it is necessary to study the experimental data of SA in pure and mixed solvents. In the present study, the solubility of SA in methanol, ethanol, 2-propanol, 1-butanol, 2-butanol, 1-hexanol, ethylene glycol, and ethyl acetate are measured at temperatures ranging from 283.15 to 323.15 K at atmospheric 0.1 MPa. Additionally, three models are Table 1. Provenance and Purity of the Materials Used in this Work mass fraction purity (%)

CAS no.

source

syringic acid

≥97.0

530-57-4

Figure 1. Chemical structure of syringic acid.

vanillic acid ethanol methanol propan-2-ol butan-1-ol butan-2-ol hexan-1-ol ethylene glycol ethyl acetate water

≥97.0 121-34-6 ≥99.4 64-17-5 ≥99.0 67-56-1 ≥99.0 67-63-0 ≥98.0 71-36-3 ≥99.8 78-92-2 ≥99.0 111-27-3 ≥99.5 107-21-1 ≥99.5 141-78-6 made from Maxima ultra UK) machine

Shanghai Yuanye Biotechnology Co. Ltd. Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Scharlau, Spain Labachemi, India Labachemi, India Scharlau, Spain Scharlau, Spain pure water (Elga-Prima Corp,

The solubility of SA in the organic solvents can provide basic data for the esterification, purification, and separation of the reaction mixture. However, there are no SA solubility data in pure and mixed solvents in open literature. For this contribution,

Received: April 10, 2017 Accepted: July 24, 2017 Published: August 8, 2017

© 2017 American Chemical Society

chemicals

3274

DOI: 10.1021/acs.jced.7b00333 J. Chem. Eng. Data 2017, 62, 3274−3283

Journal of Chemical & Engineering Data

Article

used to correlate the experimental results, and some thermodynamic properties are calculated.

2. EXPERIMENTAL SECTION 2.1. Materials. A white crystalline powder of SA was supplied by Shanghai Yuanye Biotechnology Co. Ltd. The mass fraction was higher than 0.97. All solvents used for experiments were analytical reagent grade, and their mass fraction purities were higher than 99%. Freshly prepared deionized doubly distilled water (Maxima Ultra Pure Water, Elga-Prima Corp, UK) was used throughout this study. In this work there was no treatment before use of all the materials. Meanwhile, more details about the purity of solvents are listed in Table 1. 2.2. Differential Scanning Calorimetry. As the basic thermal characteristics, the melting temperature Tm and heat of fusion ΔfusH of syringic acid were measured by a differential scanning calorimeter (DSC 131 evo, Setaram). The apparatus was calibrated with a 0.999999 mole fraction purity indium and tin sample under a nitrogen atmosphere. About 5 mg of SA was

Figure 2. Mole fraction solubility data for vanillic acid in 1-butanol: ●, experimental values; ◇, literature values (Zhang et al., 2016).

Table 2. Mole Fraction Solubility xexp of Vanillic Acid in 1-Butanol and the Results from the Literature xlit at Temperature T and Pressure p = 0.1 MPaa T (K) 102 xexp 102 xlit

293.15

298.15 0.663 0.665

0.660

303.15 0.671 0.672

308.15 0.677 0.679

313.15 0.696 0.695

318.15 0.750 0.749

a

Standard uncertainty u is u(T) = 0.01 K. The relative standard uncertainty u is ur(p) = 0.05, ur(xexp) = 0.02. xexp is the experimental solubility data of vanillic acid.

Table 3. Experimental and Calculated Mole Fraction Solubility of SA in Different Solvents within the Temperature Range from T = 283.15 to 323.15 K under 0.1 MPaa van’t Hoff equation T/K

100 xexptl SA

100 xcalcd SA

100 RDb

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.370 1.499 1.729 1.903 2.085 2.378 2.714 2.984 3.311

1.334 1.503 1.701 1.918 2.153 2.408 2.684 2.981 3.300

2.610 −0.273 1.598 −0.789 −3.244 −1.244 1.128 0.120 0.331

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.152 1.252 1.449 1.689 1.885 2.168 2.506 2.793 3.155

1.115 1.276 1.469 1.683 1.919 2.179 2.464 2.776 3.116

3.234 −1.928 −1.387 0.363 −1.799 −0.491 1.643 0.594 1.236

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.401 0.455 0.545 0.627 0.752 0.853 0.971 1.151 1.312

0.394 0.461 0.543 0.636 0.741 0.858 0.990 1.137 1.300

1.693 −1.459 0.421 −1.465 1.456 −0.644 −1.971 1.198 0.932

λh equation

Apelblat equation 100 xcalcd SA

100 RDb

100 xcalcd SA

100 RDb

1.365 1.519 1.700 1.900 2.119 2.359 2.621 2.908 3.222

0.372 −1.317 1.659 0.149 −1.607 0.824 3.419 2.538 2.690

1.335 1.505 1.704 1.921 2.156 2.411 2.685 2.979 3.294

2.552 −0.392 1.443 −0.960 −3.404 −1.358 1.087 0.176 0.514

1.124 1.278 1.464 1.673 1.908 2.172 2.466 2.795 3.162

2.469 −2.058 −1.050 0.913 −1.242 −0.156 1.566 −0.079 −0.200

1.111 1.274 1.469 1.685 1.924 2.186 2.473 2.785 3.125

3.554 −1.779 −1.408 0.210 −2.055 −0.806 1.310 0.271 0.965

0.399 0.462 0.541 0.631 0.735 0.853 0.988 1.141 1.316

0.667 −1.661 0.832 −0.713 2.253 −0.008 −1.743 0.801 −0.293

0.394 0.461 0.543 0.636 0.741 0.859 0.990 1.136 1.298

1.747 −1.449 0.396 −1.510 1.410 −0.678 −1.975 1.243 1.045

Methanol

Ethanol

Propan-2-ol

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Table 3. continued van’t Hoff equation T/K

100

xexptl SA

100

xcalcd SA

λh equation

Apelblat equation b

100 RD

100

xcalcd SA

b

100 RD

100

xcalcd SA

100 RDb

Butan-1-ol 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.320 0.375 0.441 0.518 0.624 0.735 0.832 0.980 1.162

0.314 0.373 0.444 0.527 0.621 0.728 0.850 0.987 1.140

1.765 0.528 −0.767 −1.751 0.466 0.895 −2.093 −0.690 1.810

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.188 0.223 0.290 0.357 0.432 0.503 0.604 0.737 0.921

0.187 0.229 0.284 0.349 0.426 0.516 0.622 0.746 0.889

0.713 −2.750 2.152 2.240 1.509 −2.637 −3.039 −1.243 3.558

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.241 0.281 0.330 0.420 0.480 0.580 0.702 0.852 1.012

0.232 0.280 0.341 0.413 0.497 0.594 0.707 0.836 0.983

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.812 0.951 1.130 1.241 1.459 1.699 2.034 2.347 2.691

0.800 0.937 1.103 1.291 1.503 1.743 2.010 2.308 2.640

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.070 0.084 0.100 0.125 0.149 0.180 0.221 0.260 0.323

0.068 0.083 0.103 0.125 0.152 0.183 0.220 0.262 0.311

0.319 0.374 0.443 0.523 0.616 0.723 0.846 0.989 1.152

0.322 0.059 −0.488 −1.021 1.334 1.650 −1.682 −0.884 0.814

0.315 0.373 0.445 0.527 0.621 0.729 0.850 0.986 1.138

1.675 0.425 −0.871 −1.843 0.403 0.874 −2.054 −0.575 2.015

0.189 0.230 0.282 0.345 0.421 0.512 0.620 0.749 0.902

−0.722 −3.075 2.646 3.175 2.557 −1.760 −2.686 −1.705 2.083

0.194 0.235 0.288 0.350 0.423 0.508 0.607 0.720 0.849

−2.981 −5.480 0.631 1.764 2.047 −1.045 −0.433 2.285 7.822

2.751 0.081 −2.954 2.485 −2.565 −1.668 −0.478 1.352 1.380

0.227 0.276 0.339 0.412 0.498 0.599 0.715 0.850 1.003

5.903 1.760 −2.587 1.893 −3.818 −3.196 −1.950 0.243 0.887

0.332 1.292 2.782 −3.273 −2.229 −1.940 1.385 1.253 0.727

0.791 0.929 1.098 1.290 1.507 1.752 2.025 2.330 2.667

2.642 2.355 2.837 −3.942 −3.298 −3.111 0.428 0.733 0.893

1.337 1.044 −2.103 0.405 −0.715 −1.126 0.578 −1.424 2.170

0.065 0.081 0.100 0.124 0.153 0.186 0.226 0.273 0.327

7.117 4.628 −0.499 0.289 −2.169 −3.573 −2.461 −4.838 −1.125

Butan-2-ol

Hexan-1-ol 4.024 0.235 0.374 0.281 −3.424 0.340 1.640 0.410 −3.545 0.492 −2.424 0.590 −0.730 0.705 1.846 0.840 2.849 0.998 Ethylene Glycol 1.431 0.809 1.546 0.939 2.425 1.098 −4.009 1.282 −3.040 1.492 −2.576 1.732 1.164 2.006 1.639 2.317 1.919 2.672 Ethyl Acetate 2.717 0.069 1.365 0.084 −2.582 0.102 −0.500 0.124 −1.732 0.151 −1.930 0.182 0.294 0.219 −0.921 0.264 3.662 0.316

a

The standard uncertainty u is u(T) = 0.01 K. The relative standard uncertainty u is ur(p) = 0.05, ur(x) = 0.02. xexptl SA is the experimental solubility data b exptl calcd exptl of syringic acid. xcalcd SA is the calculated solubility. RD = (xSA − xSA )/xSA .

and heats of fusion were compared with the reported values. The experiment was carried out in triplicate and repeated at least twice. The standard uncertainty for Tm is 0.5 K. The relative standard uncertainty of ΔfusH was estimated to be 0.02. 2.3. Solubility Measurement. The gravimetric method was used in this work to measure the solubility of SA in methanol, ethanol, 2-propanol, 1-butanol, 2-butanol, 1-hexanol,

placed in hermetically sealed aluminum crucibles and heated under a dry nitrogen atmosphere, while using an empty crucible as reference. Initial estimates of the melting temperatures were obtained using a 3 K·min−1 heating rate over a larger temperature range of 350−550 K. Subsequently, several runs at a rate of 1 K· min−1 around the expected Tm were averaged. To ascertain the precision of the instrument, the obtained melting temperatures 3276



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Table 4. Experimental and Calculated Mole Fraction Solubility of SA in Mixtures of Water + Methanol at Temperature T and Pressure p = 0.1 MPaa van’t Hoff equation T/K

100 xexptl SA

100 xcalcd SA

100 RDb

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.012 0.013 0.016 0.019 0.022 0.026 0.031 0.036 0.044

0.011 0.013 0.016 0.019 0.023 0.027 0.031 0.036 0.042

5.551 −1.885 −0.153 −1.980 −3.162 −1.696 −0.451 0.087 4.398

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.038 0.043 0.051 0.058 0.067 0.079 0.093 0.107 0.125

0.037 0.043 0.051 0.060 0.069 0.080 0.093 0.106 0.122

2.625 −0.612 0.415 −1.808 −2.944 −1.105 0.397 0.512 2.809

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.100 0.112 0.133 0.152 0.172 0.200 0.230 0.261 0.300

0.098 0.113 0.132 0.153 0.176 0.201 0.230 0.261 0.295

1.778 −1.196 1.201 −0.595 −2.013 −0.596 0.011 −0.090 1.636

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.231 0.270 0.301 0.329 0.369 0.424 0.481 0.540 0.631

0.230 0.261 0.298 0.338 0.382 0.431 0.484 0.541 0.603

0.659 3.470 0.983 −2.859 −3.578 −1.736 −0.535 −0.232 4.401

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.421 0.480 0.538 0.585 0.657 0.751 0.848 0.944 1.112

0.414 0.468 0.532 0.603 0.679 0.763 0.853 0.951 1.057

1.648 2.338 1.014 −3.075 −3.366 −1.575 −0.617 −0.786 5.003

283.15 288.15 293.15 298.15 303.15 308.15 313.15

0.631 0.721 0.804 0.888 0.988 1.123 1.266

0.624 0.705 0.800 0.904 1.018 1.142 1.276

1.176 2.325 0.474 −1.864 −3.034 −1.663 −0.789

λh equation

Apelblat equation 100 xcalcd SA

100 RDb

100 xcalcd SA

100 RDb

0.011 0.014 0.016 0.019 0.022 0.027 0.031 0.037 0.043

4.383 −2.185 0.239 −1.201 −2.287 −1.023 −0.227 −0.371 3.088

0.011 0.013 0.016 0.019 0.023 0.027 0.031 0.036 0.042

5.491 −1.933 −0.182 −1.992 −3.157 −1.674 −0.411 0.144 4.470

0.037 0.043 0.051 0.059 0.069 0.080 0.093 0.107 0.124

1.842 −0.637 0.932 −1.012 −2.137 −0.556 0.468 −0.104 1.333

0.037 0.043 0.051 0.059 0.069 0.080 0.093 0.107 0.122

2.880 −0.419 0.535 −1.755 −2.957 −1.178 0.268 0.329 2.582

0.099 0.114 0.131 0.152 0.174 0.200 0.229 0.262 0.298

0.746 −1.450 1.532 0.069 −1.257 0.005 0.251 −0.409 0.585

0.098 0.113 0.132 0.153 0.176 0.201 0.230 0.261 0.295

1.781 −1.200 1.191 −0.606 −2.024 −0.602 0.012 −0.078 1.663

0.232 0.261 0.297 0.336 0.380 0.429 0.483 0.543 0.609

−0.166 3.317 1.307 −2.257 −2.926 −1.251 −0.397 −0.612 3.386

0.230 0.261 0.298 0.338 0.383 0.431 0.484 0.541 0.603

0.678 3.472 0.974 −2.875 −3.596 −1.749 −0.537 −0.218 4.436

0.418 0.469 0.531 0.599 0.675 0.759 0.852 0.954 1.067

0.821 2.163 1.312 −2.500 −2.736 −1.099 −0.471 −1.137 4.048

0.414 0.468 0.532 0.603 0.679 0.763 0.853 0.951 1.056

1.675 2.340 0.997 −3.103 −3.395 −1.595 −0.619 −0.760 5.063

0.629 0.706 0.798 0.899 1.012 1.137 1.274

0.365 2.162 0.779 −1.294 −2.407 −1.189 −0.643

0.623 0.705 0.800 0.905 1.019 1.142 1.276

1.225 2.334 0.453 −1.904 −3.078 −1.696 −0.797

xM = 0.1

xM = 0.2

xM = 0.3

xM = 0.4

xM = 0.5

xM = 0.6

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Table 4. continued van’t Hoff equation T/K

100

xexptl SA

100

xcalcd SA

λh equation

Apelblat equation b

100 RD

100

xcalcd SA

b

100 RD

100

xcalcd SA

100 RDb

xM = 0.6 318.15 323.15

1.410 1.654

1.421 1.577

−0.807 4.633

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.881 0.980 1.133 1.213 1.341 1.512 1.690 1.902 2.211

0.868 0.975 1.101 1.237 1.385 1.546 1.718 1.904 2.103

1.535 0.523 2.861 −1.970 −3.285 −2.189 −1.668 −0.132 4.895

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.095 1.215 1.407 1.522 1.678 1.885 2.109 2.426 2.691

1.070 1.206 1.365 1.538 1.727 1.932 2.153 2.391 2.647

2.193 0.714 3.010 −1.073 −2.932 −2.474 −2.102 1.430 1.627

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.185 1.312 1.552 1.715 1.868 2.093 2.340 2.673 3.022

1.177 1.328 1.505 1.699 1.910 2.139 2.387 2.654 2.942

0.602 −1.193 3.005 0.942 −2.240 −2.171 −1.998 0.717 2.668

1.426 1.593

−1.156 3.681

1.421 1.576

−0.777 4.713

0.875 0.977 1.097 1.231 1.377 1.539 1.716 1.910 2.123

0.764 0.364 3.143 −1.427 −2.688 −1.736 −1.529 −0.465 3.987

0.867 0.975 1.101 1.238 1.386 1.546 1.718 1.903 2.101

1.592 0.531 2.832 −2.021 3.340 −2.229 −1.676 −0.091 4.997

1.102 1.224 1.367 1.524 1.697 1.886 2.092 2.317 2.562

−0.670 −0.747 2.851 −0.163 −1.136 −0.029 0.798 4.508 4.817

1.079 1.214 1.371 1.542 1.728 1.928 2.144 2.375 2.622

1.375 0.051 2.549 −1.345 −2.984 −2.287 −1.663 2.113 2.581

1.187 1.330 1.501 1.689 1.898 2.128 2.382 2.662 2.969

−0.224 −1.372 3.295 1.495 −1.610 −1.676 −1.818 0.420 1.759

1.111 1.274 1.469 1.685 1.923 2.185 2.472 2.785 3.124

6.229 2.933 5.360 1.769 −2.955 −4.391 −5.654 −4.180 −3.379

xM = 0.7

xM = 0.8

xM = 0.9

a

The standard uncertainty u is u(T) = 0.01 K. The relative standard uncertainty u is ur(p) = 0.05, ur(x) = 0.02. xM is the mole fraction of methanol in the mixed solvents. The relative standard uncertainty of the initial mole fraction of methanol is ur(xM) = 0.005. xexptl SA is the experimental solubility b exptl calcd exptl data of syringic acid. xcalcd SA is the calculated solubility. RD = (xSA − xSA )/xSA .

by circulating water with a thermoelectric controller within ±0.01 K. The supersaturated solution was stirred with a magnetic stirrer at a desired temperature for at least 12 h to reach equilibrium. It was then settled for another 2 h without stirring. Subsequently, the samples of the saturated solutions were taken using plastic syringes coupled with syringe filters (the pore size of filter is 0.2 μm) and evaporated in a vacuum drying oven at 318.15 K. Afterward, the samples were weighted every 2 h until reaching constant weight. All of the masses were measured by an analytical balance (A&D, GR-120, Japan) with the precision of ±0.0001 g. Triplicate measurements were performed under the same conditions, and the average value was used to calculate the experimental molar fraction solubility. Further verification measurements were taken 24 h after the initial mass measurements. All cases that showed negligible change in mass were noted, thus indicating that the samples were completely dry. To validate the experimental method, the solubility of vanillic acid in 1-butanol was measured by using this apparatus and compared with the literature value,15 which was shown in Figure 2 and given in Table 2. The relative standard uncertainty between experimental and literature values are less than 2%. It suggests that the method we used is validated.

Figure 3. Heating DSC curves of syringic acid.

ethylene glycol, and ethyl acetate, and also in the water + methanol mixtures over a temperature ranging from 283.15 to 323.15 K at atmosphere pressure, similar to that described in the literature.12−14 An excess amount of SA was added in a known mass of the selected solvent (25 g) in a double-jacketed reactor controlled 3278



Article

The mole fraction solubility of syringic acid (xexptl SA ) in each pure solvent is calculated by eq 1 and in the aqueous methanol mixtures was calculated using eq 2. exptl xSA

mSA /MSA = mSA /MSA + mS /MS

exptl xSA =

mSA /MSA

mSA /MSA + mW /MW + mM /MM

respectively. Experimental (xexptl SA ) solubility of SA are presented in Tables 3 and 4.

3. RESULTS AND DISCUSSION 3.1. Melting Properties. The melting temperature Tm and the fusion enthalpy ΔfusH of SA are 485.8 K and 33.4 kJ·mol−1, corresponding to the literature data of 480.30 K,16 481.55 K,17 and 33.7 kJ·mol−1,16 respectively. The difference of the melting temperature Tm is not small. The deviation is perhaps due to the differences in equipment, purity, and purification method. The difference of the ΔfusH is mainly due to the distinction of the purity of the SA. The DSC curve is shown in Figure 3.

(1)

(2)

where mSA, mS, mW, and mM represents the mass of SA, solvent, water, and methanol, respectively; mSA, mS, mW, and mM denote the molecular weight of the solute, solvent, water, and methanol,

Figure 4. Solubility of syringic acid in different solvents at evaluated temperatures: ethanol, ▲; ethylene glycol, hexan-1-ol, □; butan-2-ol, ∗ ethyl acetate, ●. Solid lines, values calculated from the van’t Hoff equation.

△;

propan-1-ol, +; butan-1-ol, ■;

Figure 5. Solubility of syringic acid (xSA) with the mole fraction in methanol + water at different temperatures, where xM is moles of methanol. ●, xM = 0.1; ▲, xM = 0.3; △, xM = 0.5; □, xM = 0.7; ■, xM = 0.9; line, calculated values of the λh equation. 3279



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Table 5. Correlation Results of the van’t Hoff Equation, Apelblat Equation, and λh Equation for SA in Different Pure Solvents van’t Hoff equationa 104 RMSDd

solvent

a/K

b

methanol ethanol propan-2-ol butan-1-ol butan-2-ol hexan-1-ol ethylene glycol ethyl acetate

3.063 3.882 4.185 4.738 6.433 5.719 4.897 5.0792

−2092.035 −2375.153 −2755.675 −2976.749 −3604.963 −3341.602 −2756.810 −3506.7496


1.307 1.264 0.891 0.900 2.799 3.402 2.777 2.068 Σ(104RMSD) = 15.408 Apelblat equationb

A

B

C

−41.387 −48.898 −57.040 −61.110 −73.586 −68.502 −56.314 −72.779

2.056 −9.900 −0.823 −0.981 −1.239 −1.038 −0.822 −1.222

6.567 7.870 9.123 9.804 11.921 11.059 9.120 11.600

102 RADe 0.026 0.163 0.018 1.196 0.056 0.068 2.194 0.041 Σ(102RAD) = 3.762

104 RMSDd

102 RADe

0.879 0.907 1.472 1.129 5.685 3.977 3.665 1.812 Σ(104RMSD) = 19.526

1.620 1.081 0.995 0.917 2.268 1.746 1.691 1.211 Σ(102RAD) = 11.529

λh equationc

a

solvent

λ × 10


11.426 14.828 8.590 9.380 11.040 13.336 19.493 5.766

2

104 RMSD

h 18665.262 16432.651 32308.967 31865.868 31081.218 25881.182 14706.013 64718.377

2.731 1.412 1.841 1.853 12.701 8.629 6.203 13.401 Σ(104RMSD) = 48.771

102 RAD 1.321 1.373 1.272 1.193 0.512 2.471 2.330 2.967 Σ(102RAD) = 13.439

Standard uncertainties for the van’t Hoff model parameters are u(a) = u(b) = 0.02. bStandard uncertainties for the Apelblat equation parameters are

calcd exptl 2 ⎤ )⎦ . − xSA u(A) = u(B) = u(C) = 0.02. cStandard uncertainties for the λh equation parameters are u(λ) = u(h) = 0.02. dRMSD = ⎡⎣ 1n × ∑in= 1 (xSA 1/2

e

RAD =

1 n

n

× ∑i = 1

exptl calcd − xSA xSA exptl xSA

.

3.2. Solubility Data. The mole fraction solubility values of syringic acid (xexptl SA ) in ethanol, methanol, propan-2-ol, butan-1-ol, butan-2-ol, hexan-1-ol, ethylene glycol, and ethyl acetate from 283.15 to 323.15 at atmospheric pressure are listed in Table 3 and presented graphically in Figure 4. It can be clearly seen from Figure 4 and according to Table 3, within the experimental temperatures range, that the solubility of SA in the all studied solvents increases with the increase in temperature and increases in the order: methanol > ethanol > ethylene glycol > propan-2-ol > butan-1-ol > hexan-1-ol > butan-2-ol > ethyl acetate. It is obvious that polarity of solvents is not the only factor that affects the solubility of SA because the polarity order of solvents is ethylene glycol (79) > methanol (76.2) > ethanol (65.4) > butan-1-ol (60.2) > hexan-1-ol (55.9) > butan-2-ol (55.2) > propan-2-ol (54.6) > ethyl acetate (22.8).18 The abnormal solubility behavior may be explained by the intermolecular interaction and the ability of solvent to form a hydrogen bond with the SA molecules caused by the presence of hydrophilic groups (methoxy groups, hydroxyl group) and hydrophobic groups (carbon−carbon double bond, phenyl group) in SA.

The SA solubilities in water + methanol mixtures at the temperatures varying from 283.15 to 323.15 K are listed in Table 4 and shown in Figure 5. From Table 4 and Figure 5, it can be found that the solubility in water + methanol mixtures increases with an increase in temperature and mole fraction of methanol. Analogous results were obtained by our group in the study of vanillic acid solubility in mixed solvents at different temperatures.19 AT xM = 0.1 and T = 313.15 K, the mole fraction solubility of syringic acid xexptl = 3.6 × 10−4. Based on the observed SA dependence on the solvent composition, the solubility in water should be lower than this value. However, the reported solubilities of syringic acid in water at this temperature are x = 9.2 × 10−4 and x = 4.0 × 10−4, respectively.20,16 The variation between the experimental values and literature20,16 values of 313.15 K is large. The difference observed between our results and the results from literature due to the distinction of impurities in the syringic acid, experimental error, and/or analysis method. 3.3. Solubility Modeling. In this paper, the experimental solubility data of SA in eight pure and water + methanol mixed solvents were regressed by the van’t Hoff eq 3, the modified 3280



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Apelblat eq 4, and the λh eq 5. The parameters of these models are of interest in industrial application. On the basis of the theory of liquid−solid phase equilibrium, the relationship between solubility and temperature can be described by the van’t Hoff model.21,22

ln xSA =

a +b T /K

According to the solid−liquid phase equilibrium theory, the changing trends of solubility against temperature in the monosolvent are described by the modified Apelblat equation. This model is first used by Apelblat,23−25 which can give a relatively accurate correlation with three parameters

(3)

Table 6. Parameters of the van’t Hoff Equation, Apelblat Equation, and λh Equation for SA in Mixed Solvents of Water and Methanol

⎡ ⎡ 1 λ(1 − xSA ) ⎤ 1 ⎤ − ln⎢1 + ⎥ = λh⎢ ⎥ xSA (Tm/K) ⎦ ⎣ (T /K) ⎣ ⎦

van’t Hoff equationa 104 RMSDd

a/K

b

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1.632 1.820 2.0453 1.791 2.147 2.484 2.468 2.842 3.021

−3038.597 −2756.820 −2543.4261 −2230.383 −2164.356 −2143.729 −2045.296 −2092.036 −2115.595

102 RADe

104 RMSDd

A

B

C

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

−65.840 −59.653 −54.394 −47.798 −46.003 −45.214 −43.045 −40.948 −44.022

−1.053 −0.917 −0.801 −0.564 1.965 1.980 2.026 2.071 1.999

10.053 9.166 8.408 7.390 7.175 7.108 6.782 6.451 7.009

xM

λ × 102

h

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.361 0.793 1.526 2.271 3.754 5.556 6.768 8.610 14.831

841394.926 349233.823 166954.920 98594.265 58036.665 38973.260 30631.743 24234.915 16432.651

4.610 1.397 0.796 4.440 4.435 3.288 4.591 1.161 1.500 Σ(104RMSD) = 26.218 λh equationc 104 RMSD

102 RADe

RD =

1.667 1.002 0.700 1.736 1.810 1.520 1.789 1.746 2.278 Σ(102RAD) = 14.248

RAD =

1 n

n

× ∑i = 1

exptl xSA

exptl xSA

1 n

n

(6) exptl calcd xSA − xSA

∑

exptl xSA

i=1

⎡1 RMSD = ⎢ ⎢⎣ n

⎤1/2

n

∑ i=1

calcd (xSA

(7)

−

exptl 2 ⎥ xSA )

⎥⎦

(8)

exptl where n denotes the number of data points and xcalcd SA and xSA represent the calculated and experimental mole fraction solubility of SA, respectively. The calculated values of RD are presented in Tables 3 and 4, and the values of RAD and RMSD according to the regressed parameters values are listed in Tables 5 and 6. As we can see from Tables 5 and 6, the correlated data are in good agreement with the experimental data due to small RMSDs and RADs. It could be observed that the total RAD obtained by the van’t Hoff equation, modified Apelblat equation, and λh equation are 3.762 × 10−2, 11.529 × 10−2, and 13.439 × 10−2, respectively. From the point of comparison of the total RAD, the best fitting result of the solubility-temperature dependence data is given by the van’t Hoff. We can draw the conclusion that the van’t Hoff equation and the modified Apelblat equation are more suitable for accurate descriptions of the solubility of SA in the tested pure and binary mixed solvents, respectively. 3.4. Excess Enthalpy of Solution. The mean association number of solute molecules in solution may be reflected by the values parameter λ in the λh equation. The parameter h is related to the excess enthalpy of solution, which can provide

102 RAD

7.870 2.162 3.200 1.433 1.514 1.017 6.415 2.059 6.619 2.172 5.175 1.886 6.537 2.145 4.247 1.883 9.341 4.095 Σ(104RMSD) = 50.918 Σ(102RAD) = 18.852

exptl calcd xSA − xSA

exptl calcd xSA − xSA

RAD =

a Standard uncertainties for the van’t Hoff model parameters are u(a) = u(b) = 0.02. bStandard uncertainties for the Apelblat equation parameters are u(A) = u(B) = u(C) = 0.02. cStandard uncertainties for the λh equation parameters are u(λ) = u(h) = 0.02. 1/2 1 n calcd exptl 2 ⎤ d − xSA RMSD = ⎡⎣ n × ∑i = 1 (xSA )⎦ . e

(5)

where λ and h are two equation constants, Tm is the normal melting temperature of the solute which had been measured by differential scanning calorimetry (DSC 131 evo, Setaram). The value of Tm was found to be 485.8 K. The value of λ reflects the nonideality of the solution system, where λh estimates the enthalpy of solution. To evaluate the selected thermodynamic models and find the difference between the measured solubility and calculated ones, the relative deviation (RD), the relative average deviation (RAD), and root-mean-square deviation (RMSD) are used and expressed as eqs 6−8, respectively.

3.930 2.151 1.606 1.470 0.749 1.013 3.175 2.050 3.250 2.158 2.511 1.863 3.169 2.118 2.187 1.951 1.837 1.726 Σ(104RMSD) = 22.414 Σ(102RAD) = 16.5 Apelblat equationb

xM

(4)

where xSA represents the mole fraction solubility of syringic acid, T is the experimental temperature in K, and A, B, and C are the regression curve parameters in the equation which are listed in Tables 5 and 6. To know the relationship between solubility and temperature in the process of solute dissolution, the Buchowski−Ksiazaczak λh equation was adopted. In this study, the solubility data were correlated with the λh equation.26,27

here a and b are the model parameters determined by the experimental solubility data which are listed in Tables 5 and 6, T is the absolute temperature in kelvin, and xSA is the mole fraction syringic acid in organic solvent at temperature T.

xM

B + C ln(T /K) T /K

ln xSA = A +

. 3281



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Table 7. Predicted Values of Excess Enthalpy for SA in Different Studied Solvents at Several Temperatures and p = 0.1 MPaa HE (kJ·mol−1)

a

T/K

ethanol

methanol

propan-2-ol

butan-1-ol

butan-2-ol

hexan-1-ol

ethylene glycol

ethyl acetate

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.189 1.292 1.495 1.743 1.945 2.238 2.586 2.883 3.257

1.668 1.825 2.105 2.317 2.539 2.896 3.305 3.634 4.032

0.944 1.070 1.283 1.474 1.768 2.006 2.284 2.706 3.086

0.741 0.867 1.021 1.199 1.445 1.702 1.927 2.269 2.689

0.423 0.502 0.652 0.803 0.973 1.132 1.359 1.657 2.073

0.438 0.511 0.600 0.764 0.873 1.055 1.275 1.548 1.840

0.722 0.845 1.004 1.103 1.296 1.509 1.807 2.085 2.391

0.354 0.426 0.504 0.629 0.754 0.908 1.114 1.312 1.630

The standard uncertainty u is u(T) = 0.01 K. The relative standard uncertainty u is ur(p) = 0.05.

Table 8. Predicted Values of Excess Enthalpy for SA in Mixed Solvents of Water + Methanol at Several Temperatures and p = 0.1 MPaa HE (kJ·mol−1)

a

T/K

xM = 0.1

xM = 0.2

xM = 0.3

xM = 0.4

xM = 0.5

xM = 0.6

xM = 0.7

xM = 0.8

xM = 0.9

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

0.835 0.920 1.120 1.309 1.531 1.827 2.166 2.536 3.072

1.088 1.232 1.465 1.678 1.933 2.281 2.670 3.070 3.594

0.105 0.118 0.140 0.159 0.181 0.210 0.241 0.274 0.315

1.818 2.124 2.363 2.584 2.904 3.331 3.784 4.245 4.960

1.891 2.154 2.415 2.625 2.951 3.372 3.808 4.237 4.995

1.834 2.097 2.336 2.580 2.872 3.265 3.680 4.097 4.807

1.950 2.169 2.507 2.684 2.968 3.346 3.740 4.207 4.893

1.840 2.041 2.365 2.558 2.820 3.168 3.544 4.077 4.523

1.222 1.354 1.602 1.770 1.928 2.160 2.415 2.759 3.119

The standard uncertainty u is u(T) = 0.01 K. The relative standard uncertainty u is ur(p) = 0.05.

acetate. The van’t Hoff equation, modified Apelblat equation, and λh equation were used to correlate the solubility data. It was found that the van’t Hoff model and the modified Apelblat equation provides the best description of the solubilitytemperature dependence in pure and binary mixed solvents, respectively. Furthermore, the excess enthalpy of the solutions of SA is derived for each solution based on the obtained parameter of the λh equation. The experimental solubility data and equations presented in this study are very useful for optimization of the dissolution, extraction, and purification of SA.

more detailed information on understanding a solution behavior. By using the h values presented in Tables 5 and 6, and enthalpy of fusion ΔfusH of syringic acid,28,29 the excess enthalpy of solution HE can be calculated according to eq 9.30,31 hR = ΔfusH +

HE xSA

(9)

in which xSA represents saturated mole fraction solubility of SA in pure solvents and h denotes the parameter in λh equation. The calculated values of the excess enthalpy are presented in Tables 7 and 8 for different pure and water + methanol mixed solvents, respectively. It is seen from Tables 7 and 8 that the values of HE of dissolution are all positive and increase with the increase in temperature. The positive values of dissolution enthalpy for SA mixing with the studied solvents illustrate that the cross associating intermolecular interactions formed between SA and the solvents are weaker than the self-associating interactions, which result in an endothermic solution process and also in the increasing values of HE with an increase in temperature. A similar behavior can also be found for other systems.31,32

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +216 98 934 601; fax: + 216 71 74 65 51. E-mail address: [email protected]; [email protected] (A. Noubigh). ORCID

Adel Noubigh: 0000-0003-2010-7633 Notes

The authors declare no competing financial interest. Funding

The authors wish to acknowledge the approval and the support of this research study by the grant no. SCI-2016-1-6-F- 4600 from the Deanship of the Scientific Research in Northern Border University (N.B.U.), Arar, KSA.

4. CONCLUSION The solubility data of SA in methanol, ethanol, propan-2-ol, butan-1-ol, butan-2-ol, hexan-1-ol, ethylene glycol, and ethyl acetate and binary mixtures of water + methanol were experimentally determined by a gravimetric method at temperatures ranging from 283.15 to 323.15 K. The solubility of SA in all selected solvents increases with increasing temperature and in the order methanol > ethanol > ethylene glycol > propan-2-ol > butan-1-ol > hexan-1-ol > butan-2-ol > ethyl

■

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Solubility and Thermodynamic Behavior of Syringic Acid in Eight Pure

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