Investigation of Amino AcidâPolymer Aqueous Biphasic Systems - The

Article pubs.acs.org/JPCB

Investigation of Amino Acid−Polymer Aqueous Biphasic Systems Rahmat Sadeghi,* Barzan Hamidi, and Nosaibah Ebrahimi Department of Chemistry, University of Kurdistan, Sanandaj, Iran S Supporting Information *

ABSTRACT: Aiming at gathering further information to evaluate the recently proposed1,2 mechanism of the salt effect in aqueous polymer solutions, experimental vapor−liquid equilibria (VLE), liquid−liquid equilibria (LLE), and volumetric-compressibility measurements were carried out for several polymer−amino acid aqueous systems. The constant water activity lines (obtained through the isopiestic method at 298.15 K) of aqueous polypropylene glycol 400 (PPG400) + alanine or glycine systems, which form aqueous biphasic systems (salting-out effect), have a concave and convex slope, respectively, in the one-phase and two-phase regions. However, all the investigated polyethylene glycols (PEG400, PEG2000, PEG6000, and PEG10000) do not form aqueous biphasic systems with alanine or glycine (salting-in effect) and their constant water activity lines have a convex slope. In the second part of this work, the apparent molar volume and isentropic compressibility of transfer of alanine and glycine from water to aqueous solutions of PEG200, PEG2000, PEG10000, and PPG400 were studied at different temperatures. The third part of this work is concerned with the determination of LLE phase diagrams for several ternary polymer−amino acid aqueous systems containing polymers PPG400 and PPG725 and amino acids alanine, glycine, serine, and proline at different temperatures. On the basis of the obtained cloud point values of aqueous solutions of PPG725 in the absence and presence of various amino acids, it was found that all the investigated amino acids have a salting-out effect on PPG725 in aqueous solutions and entropy is the driving force for biphasic formation.

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INTRODUCTION Aqueous biphasic systems (ABS) have a widespread use in biochemistry and biotechnology for purification, extraction, and enrichment of biomolecules, cells, cell particles,3−5 anions,6 cations,7 etc. These systems are composed of two water-soluble, but mutually incompatible, components (e.g., two polymers, a polymer and a salt, two salts, etc.). The ABS are suitable for purification of biological materials, as the phases contain 60− 90% water, thus reducing the risk of denaturation of labile biomolecules. Both equilibrium phases in the ABS have same components with different concentrations, and therefore, ABS formation is an unusual phenomenon. Although this phenomenon is well documented and has been extensively investigated in the literature, its mechanism at the molecular level is still unclear. Among all the ABS, polymer−polymer,3−5 polymer− salt,3−5 and ionic liquid−solute8 ABS are the most studied ABS. Amino acids have been used as salting-out agents in the ionicliquid-based ABS;8−10 however, as far as we know, the phase behavior of polymer−amino acid ABS has not been studied in the literature, and in this work, we report for the first time the formation of polymer−amino acid ABS. Furthermore, the cooperative effects of cations and anions in affecting water/air surface tension, activity coefficient, solubility, and solvation of model compounds including polypeptides or amides have been studied theoretically by Gao and co-workers,11,12 in order to obtain a more general and simple understanding of the Hofmeister series, but the salting effects of amides on the aqueous solutions of different solutes and in turn the position © 2014 American Chemical Society

of amino acids in the Hofmeister series have rarely been studied. Amino acids are a type of inner salts with a high affinity for water and therefore salting-out aptitude. Although amino acids are weaker salting-out inducing species than inorganic salts, polymer-based ABS formed by the addition of these biomolecules may be more gentle systems for liquid−liquid extraction of biological materials than common polymer− inorganic salt ABS. In other words, the conventional high charged density salts used as salting-out agents in the polymer− salt ABS can be replaced by more biocompatible species such as amino acids to form a new polymer−amino acid ABS, which is of relevance for the application of ABS. In this work, in order to investigate amino acid−polymer ABS and in an attempt to obtain further information to evaluate the recently proposed1,2 mechanism of the salt effect in aqueous polymer solutions, experimental measurements of vapor−liquid equilibria, liquid−liquid equilibria, cloud point values, and volumetric and compressibility properties were performed for ternary aqueous solutions of several amino acids in the presence of a large series of water-soluble polymers (polyethylene glycol 200 (PEG200), polyethylene glycol 400 (PEG400), polyethylene glycol 2000 (PEG2000), polyethylene glycol 6000 (PEG6000), polyethylene glycol 10000 Received: May 31, 2014 Revised: July 21, 2014 Published: August 5, 2014 10285

dx.doi.org/10.1021/jp505383r | J. Phys. Chem. B 2014, 118, 10285−10296

The Journal of Physical Chemistry B

Article

Table 1. Structures, Dissociation Constants, and Isoelectric Points of the Amino Acids Investigated13

had been reached, the manifold assembly was removed from the bath, and each flask was weighed with an analytical balance with a precision of ±1 × 10−4 g. From the weight of each flask after equilibrium and the initial weight of solutes, the mass fraction of each solution was calculated. The osmotic coefficients for the standard NaCl aqueous solutions have been calculated from the correlation given in the literature.14 Density and sound velocity measurements were carried out by an Anton Paar DSA 5000 model high precision vibrating tube digital densimeter and sound velocity measuring device, with automatic viscosity corrections and proportional temperature control that kept the samples at working temperature within ±10−3 K. The calibration of the instrument was made with degassed and bidistilled water and dry air at atmospheric pressure according to the instruction manual of the instrument. The uncertainties of measurements were ±5 × 10−6 g·cm−3 for density and ±10−1 m·s−1 for sound velocity. All the solutions for density and sound velocity measurements were prepared by mass on a Sartorius CP225D balance precisely within ±1 × 10−5 g. The experimental apparatus employed for determination of liquid−liquid phase diagrams (binodal curves) and cloud point values is a glass vessel with an external jacket. A Julabo thermostat with a precision of ±0.05 K was used to circulate water at a certain temperature in the external jacket around the vessel. The turbidity titration method15 was used to obtain the binodal curves at atmospheric pressure and different temperatures. The determination of the binodal curves at each temperature was carried out by repetitive dropwise addition of an aqueous polymer solution with known concentration to an aqueous amino acid solution with known concentration until the detection of a cloudy solution (biphasic region), followed by the dropwise addition of water until the formation of a clear solution (monophasic region). These dropwise additions were

(PEG10000), polypropylene glycol 400 (PPG400), and polypropylene glycol 725 (PPG725)).

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EXPERIMENTAL SECTION Materials. PPG400 and PPG725 were obtained, respectively, from Fluka and Aldrich. L-Proline (>99.0 w/w) was obtained from BDH. PEGs, NaCl (>99.5% w/w), glycine (>99.7% w/w), S(+)-alanine (>99% w/w), L-serine (>99% w/ w), and S(+)-isoleucine (>99% w/w) were obtained from Merck. The PPGs, PEGs, and amino acids were used without further purification. NaCl was dried in an electrical oven at about 383.15 K for 24 h prior to use. Double-distilled and deionized water was used. The chemical structure and isoelectric point of the amino acids under study have been summarized in Table 1.13 Experimental Procedures. The details of the isopiestic method used in this work are similar to the one used previously.1 In this method, different solutions with only one common solvent, when connected through the vapor space, approach equilibrium by transferring solvent mass by distillation. At equilibrium, the chemical potential of solvent (and also solvent activity) in each of the solutions in the isopiestic system is identical. From the solvent activity of one or more standard solutions, the activity of solvent for each solution within the isopiestic system can be known. The apparatus used consisted of a multileg manifold attached to round-bottom flasks. Two flasks contained the standard pure NaCl solution, one flask contained the pure polymer solution, one flask contained the pure amino acid solution, two or three flasks contained the polymer + amino acid solutions, and the central flask was used as a water reservoir. The apparatus was held in a constant-temperature bath at least 5 days (depending on the solute concentration) for equilibrium. The temperature was controlled at 298.15 K to within ±0.05 K. After equilibrium 10286



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Figure 1. Plot of the mass fraction of polymer, wp, against the molality of alanine, ma, for constant water activity curves of polymer (p) + alanine (a) + H2O (w) systems at T = 298.15 K: (a) PEG400 (p) + alanine (a) + water (w): ○, aw = 0.9765; △, aw = 0.9812; □, aw = 0.9853; ◇, aw = 0.9869; ●, aw = 0.989; ▲, aw = 0.9906; ■, aw = 0.9923; ..., calculated by eq 2. (b) PEG2000 (p) + alanine (a) + water (w): ○, aw = 0.9695; △, aw = 0.9756; □, aw = 0.9788; ◇, aw = 0.9824; ●, aw = 0.9853; ▲, aw = 0.9888; ■, aw = 0.9904; ..., calculated by eq 2. (c) PEG10000 (p) + alanine (a) + water (w): ○, aw = 0.9695; △, aw = 0.9756; □, aw = 0.9825; ◇, aw = 0.9854; ●, aw = 0.988; ▲, aw = 0.9888; ■, aw = 0.9923; ..., calculated by eq 2. (d) PPG400 (p) + alanine (a) + water (w): ○, aw = 0.9765; △, aw = 0.9821; □, aw = 0.9844; ◇, aw = 0.9862; ●, aw = 0.9893; ▲, aw = 0.9921; ■, aw = 0.9455; ◆, binodal curve for PPG400 + alanine + water; ..., calculated by eq 2.

ance of clouding was considered as the cloud point temperature (TC). The samples for cloud point measurements were prepared on a Sartorius CP124S balance precisely within ±1 × 10−4 g.

carried out under continuous stirring. The composition of the mixture was determined by mass using an analytical balance with a precision of ±1·10−4 g. The pH of the mixtures was measured with a pH meter (Metrohm model 691) with a precision of 0.01. The cloud point temperatures for solutions of PPG725 in water and in aqueous amino acid solutions with a certain molality of the investigated amino acids were determined by visual observation. The temperature of the stirred sample in a stoppered glass vessel was slowly increased by increasing the temperature of circulated water around the vessel until the sample clouded or got turbid. The sample temperature was then slowly decreased until the turbidity was vanished. The heating−cooling cycle was repeated three times for a given sample, to check the reproducibility of the measurements. The mean value of the temperature for appearance and disappear-

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RESULTS AND DISCUSSION In this work, three parts of experimental measurements have been carried out on several ternary polymer−amino acid aqueous solutions at different temperatures: (i) isopiestic equilibrium measurements for the ternary {PEG400, PEG2000, PEG10000, PPG400 + amino acid (alanine or glycine) + water} systems at T = 298.15 K; (ii) volumetric and compressibility property measurements for amino acid (alanine and glycine) in water and in aqueous solutions of 0.03 w/w PEG200, PEG2000, PEG10000, and PPG400 at T = 288.15, 293.15, 298.15, 303.15, and 308.15 K; and (iii) liquid−liquid equilibria 10287



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Table 2. Isopiestic Equilibrium Concentrations, Water Activities, and the Experimental Deviations Δ0 from eq 2 for Several Polymer (p) + Amino Acid (a) + H2O (w) Solutions at 298.15 K wp 0.0902 0.0651 0.0276 0.1197 0.0702 0.1818 0.1438 0.0544 0.1123 0.0345 0.1217 0.0540 0.1545 0.0790 0.0409 0.1932 0.1071 0.0586 0.1196 0.0736 0.0402 0.1963 0.0950 0.1182 0.0528 0.1208 0.0706 0.1384 0.0809 0.1542 0.0572 0.2252 0.0923 0 0.2322 0.0984 a

ma (mol·kg−1)

aw

PEG400 (p) + Alanine (a) + H2O (w) 0.2626 0.0190 0.3580 0.5042 0.2783 0.0238 0.4783 PEG2000 (p) + Alanine (a) + H2O (w) 0.3494 0.0252 0.4556 0.6972 1.2087 0.0506 1.4759 0.0506 0.359 0.0194 0.512 0.0194 PEG10000 (p) + Alanine (a) + H2O (w) 0.2110 0.0154 0.3312 0.3887 0.2780 0.0207 0.4757 0.5780 0.7407 0.0356 0.8428 0.9322 0.6611 0.0411 1.0143 0.0411 0.4090 0.0194 0.5182 0.0194 PPG400 (p) + Alanine (a) + H2O (w) 0.1259 0.0158 0.2721 0.1478 0.0184 0.3164 0.2413 0.0238 0.5390 0.3743 0.0397 0.8558 1.2306 0.3840 0.0429 0.8967

Δ0a

ma (mol·kg−1)

wp

aw

Δ0a

0.1468 0.0736 0.0296 0.2549 0.1562 0.0878

PEG400 (p) + Glycine (a) + H2O (w) 0.1919 0.0189 0.3401 0.4444 0.2521 0.0243 0.4518 PEG2000 (p) + Glycine (a) + H2O (w) 0.2871 0.0238 0.4690 0.5593 0.6527 0.0498 1.1101 1.4314

0.0407 0.0456 0.0180 0.0689 0.0523 0.0432

0.1349 0.0939 0.0657 0.0989 0.0867 0.0663 0.1018 0.0579 0.0411 0.0813 0.0591 0.1154 0.0533

0.1336 0.0653 0.0276 0.1700 0.0896 0.0449 0.1916 0.1208 0.0633 0.2416 0.1534 0.0812

PEG10000 (p) + Glycine (a) + H2O (w) 0.2714 0.0153 0.3918 0.4387 0.3627 0.0210 0.5517 0.6403 0.4709 0.0365 0.6985 0.8611 0.5735 0.0414 0.9581 1.2221

0.0916 0.0665 0.0127 0.0814 0.0643 0.0323 0.0713 0.0796 0.0641 0.0506 0.0613 0.0363

−0.0150 −0.0178 −0.0240 −0.0266 −0.0447 −0.0550 −0.0579 −0.0431 0 −0.1314 −0.0454

0.1052 0.0647 0.1180 0.0746 0.1326 0.0512 0.1906 0.1220 0.0563 0.2122 0.0867

PPG400 (p) + Glycine (a) + H2O (w) 0.1842 0.0151 0.2941 0.2102 0.0187 0.3441 0.2984 0.0243 0.5528 0.3740 0.0336 0.6044 0.8933 0.5347 0.0425 1.0039

0.0052 0.006 0.0131 0.0048 0.0032

0.0917 0.0518 0.0211 0.1168 0.0671

0.0681 0.0641 0.0444 0.0821 0.0270 0.0846 0.0656

0.0148 0.0207 0.0067 0.0126 0.0044

−0.0472 −0.0495 −0.0771 −0.0629 −0.0853 −0.0703 −0.0779 −0.0818 −0.0243 −0.1422 −0.1093

See the equation below: wp m Δ0 = s0 + 0 − 1 ms wp

electrolyte solutions under isopiestic equilibrium, has the following form:

phase diagram determination for the ternary {PPG400 + alanine, glycine, serine, proline + water} and {PPG725 + alanine, glycine, serine, proline + water} systems in the 298.15− 318.15 K temperature range, and the cloud point measurements for PPG725 in water and in aqueous solutions of 0.2 mol·kg−1 amino acids including proline and isoleucine as a function of PPG725 concentration. The measured vapor−liquid equilibria, phase diagram, cloud point, density, and sound velocity data are presented in the Supporting Information of this manuscript. Vapor−Liquid Equilibria Properties. The Zdanovskii− Stokes−Robinson (ZSR) rule, proposed by Zdanovskii,16 which is a well-known empirical linear isopiestic relation (LIR) between the molalities of different ternary and binary aqueous

∑ i

mi mi0

=1

⎞ ⎛ m ⎜a w = constant and 0 ≤ 0i ≤ 1⎟ mi ⎠ ⎝

(1)

where mi is the molality of solute i in the ternary solution and m0i is the molality of solute i in the binary solution of equal aw. Stokes and Robinson17 theoretically derived this equation for isopiestic mixed nonelectrolyte aqueous solutions from the semi-ideal hydration model. According to the ZSR rule, different aqueous solutions under isopiestic equilibrium exhibit no net effective interaction when mixed; that is, changes of hydration between the dissolved components on mixing are 10288



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the interaction of each solute with water becomes less favorable in the presence of the other solute, and therefore, these systems show the positive deviation from the LIR. As can be seen from Table 2 and Figures 2 and 3, at the same water activity, these

apparently absent. In this work, the following form of the LIR for the ternary aqueous polymer (p)−solute (s) solutions is used: ms ms0

+

wp wp0

=1

⎞ ⎛ w m ⎜a w = constant, 0 ≤ s ≤ 1, and 0 ≤ p ≤ 1⎟ ⎟ ⎜ ms0 wp0 ⎠ ⎝

(2)

where w is the mass fraction. In fact, for the aqueous ternary solutions obeying the LIR (eqs 1 or 2), the constituent binary solutions mix ideally under isopiestic equilibrium and we can conclude that the solute−solvent interactions in the ternary solution are the same as those in the binary solutions. The positive deviation from the LIR shows that the activities of water in a ternary solution in isopiestic equilibrium with certain binary solute 1 + water and solute 2 + water solutions are larger than those we expect in the case of a semi-ideal solution. This behavior shows that the solute 1−water interaction becomes less favorable in the presence of solute 2, and therefore, more free water molecules would be available with respect to the semi-ideal behavior in which the solute−solvent interactions in the ternary solution are the same as those in the binary solutions. Therefore, the concentrations of solutes in a ternary solution which is in isopiestic equilibrium with certain binary solutions are larger than those we expect in the case of a semiideal solution. On the other hand, in the case of negative deviation from the LIR, the interaction of solute 1 with water becomes more stable in the presence of solute 2 and therefore in these ternary systems less free water molecules would be available with respect to the semi-ideal behavior and then the activities of water in the ternary solutions are smaller than those we expect in the case of a semi-ideal solution. Figure 1 shows the experimental constant water activity lines of different polymer + alanine + H2O systems along with the results of eq 2 (dotted lines) at 298.15 K. The similar behavior was obtained for the polymer + glycine + H2O systems. As can be seen, at 298.15 K, for ternary aqueous mixtures of PEG in the presence of both of the investigated amino acids, the positive deviations from the LIR were obtained and this positive deviation increases by increasing the PEG molecular weight. However, for the ternary systems PPG400 + alanine or glycine + water, which can form ABS, the constant water activity lines in the one-phase and two-phase areas, respectively, show the negative and positive deviation from the LIR. In Figure 1d, the constant water activity lines along with the binodal curve of the PPG400 + alanine + water system have been shown. As an example, Table 2 shows the experimental deviations from eq 2 for some of the measured isopiestic data. The amino and carboxyl groups of alanine or glycine dissociate in aqueous solutions and become, respectively, negatively and positively charged or a zwitterion (+NH3−(CH3CH)−COO− or +NH3−CH2−COO−). The neutral dipolar species or zwitterions are the predominant species in the isoelectric solution of amino acids. In acidic solutions, cationic amino acid species become predominant, while anionic amino acid species become predominant in the basic solutions. In the ternary PEG + amino acid aqueous systems which are completely miscible (salting-in effect), as a consequence of preferential amino acid−PEG interactions (cationic portion of amino acid and polyether oxygen of polymer or dispersion interaction between the hydrophobic portions of two solutes),

Figure 2. Plot of wp/w0p against ma/m0a for constant water activity curves of polymer + amino acid + H2O systems at 298.15 K: ○, PPG400 + glycine (aw = 0.9748); △, PPG400 + glycine (aw = 0.9859); □, PPG400 + alanine (aw = 0.9913); ●, PEG10000 + alanine (aw = 0.9697); ▲, PEG10000 + alanine (aw = 0.9757); ■, PEG10000 + alanine (aw = 0.9911); ×, PEG10000 + glycine (aw = 0.9912); ..., calculated by eq 2.

Figure 3. Plot of wp/w0p against ma/m0a for constant water activity curves of polymer + amino acid + H2O systems at 298.15 K: ○, PPG400 + alanine (aw = 0.9894); △, PPG400 + glycine (aw = 0.9892); □, PEG400 + alanine (aw = 0.9853); ◇, PEG2000 + alanine (aw = 0.9854); ●, PEG10000 + alanine (aw = 0.9855); ..., calculated by eq 2.

positive deviations for ternary systems containing alanine are larger than those containing glycine and increase by increasing PEG molecular weight. In fact, the hydrophobicity of PEG increases by increasing its molecular weight and therefore the preferential amino acid−PEG interactions increase by increasing the hydrophobicity of PEG. Alanine because of a greater proportion of hydrocarbon in its molecule is more hydrophobic than glycine, and therefore, the positive deviations from the LIR for ternary systems containing glycine are smaller than 10289



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Table 3. Vϕ° (cm3·mol−1), bV (cm3·mol−2·kg), Kϕ° (cm3·mol−1·MPa−1), and bK (cm3·mol−2·kg·MPa−1) Values for Glycine and Alanine in Water and in Solvents as PEG200, PEG2000, PEG10000, and PPG400 Solutions of 0.03 w/w at 288.15, 293.15, 298.15, 303.15, and 308.15 K solvent T (K)

quantity

288.15

Vϕ° bV K°ϕ 103bK

293.15


298.15


303.15


308.15


a

H2O

0.03 w/w PEG200

0.03 w/w PEG2000

0.03 w/w PEG10000

0.03 w/w PPG400

59.743a 42.222b 0.701 1.587 −0.0307 −0.0341 4.982 13.211 60.098 42.710 0.667 1.379 −0.0273 −0.0291 4.206 5.382 60.391 43.108 0.650 1.248 −0.0245 −0.0264 3.635 3.737 60.651 43.417 0.633 1.232 −0.0224 −0.0243 3.384 2.299 60.875 43.705 0.623 1.080 −0.0207 −0.0230 3.195 2.095

59.814 42.371 0.681 1.853 −0.0294 −0.0331 5.607 20.116 60.151 42.838 0.662 1.596 −0.0265 −0.0290 4.999 8.880 60.460 43.223 0.631 1.430 −0.0243 −0.0263 4.761 5.678 60.723 43.506 0.611 1.427 −0.0221 −0.0245 4.256 5.357 60.953 43.742 0.592 1.482 −0.0204 −0.0229 3.892 4.318

59.884 42.465 0.648 1.459 −0.0292 −0.0319 5.540 11.615 60.224 42.872 0.624 1.419 −0.0263 −0.0285 4.784 7.294 60.519 43.258 0.608 1.401 −0.0236 −0.0261 3.708 5.764 60.801 43.530 0.555 1.386 −0.0215 −0.0240 3.331 3.214 61.019 43.765 0.555 1.512 −0.0196 −0.0225 2.729 3.286

59.997 42.515 0.502 1.250 −0.0282 −0.0306 4.262 11.575 60.331 42.877 0.492 1.545 −0.0258 −0.0283 4.155 6.119 60.646 43.259 0.447 1.509 −0.0235 −0.0257 3.901 3.937 60.880 43.526 0.461 1.562 −0.0214 −0.0236 3.419 1.880 61.137 43.763 0.405 1.596 −0.0198 −0.0220 3.214 1.630

59.938 42.441 0.395 1.346 −0.0289 −0.0298 5.096 5.628 60.356 42.885 0.196 1.253 −0.026 −0.0285 4.208 8.169 60.646 43.229 0.191 1.243 −0.0237 −0.0265 3.882 6.933 60.890 43.525 0.225 1.213 −0.0212 −0.0243 2.116 5.372 61.106 43.754 0.243 1.216 −0.0194 −0.0229 1.682 4.873

Alanine. bGlycine.

condition, the positive deviation from the LIR increases by decreasing water activity. Although PPG contains a greater proportion of hydrocarbon in its molecule, the side chain methyl groups in PPG hinder bonding between PPG chains and amino acids. In fact, in the case of PPG400 + alanine or glycine + H2O systems which form ABS (salting-out effect), because of the unfavorable polymer−amino acid interactions, the solute 1−water interaction becomes more stable in the presence of solute 2, and therefore, in these ternary systems, less free water molecules would be available with respect to the semi-ideal behavior and then these systems show the negative deviation from the LIR. Since the association of amino acids with PPG is a highly unfavorable process, they exclude themselves from the vicinity of each other due to their preferential hydration and this

those containing alanine. It would be expected that, at the same amino acid concentration, the water activity of the investigated binary amino acid + water systems should be observed in the sequence glycine < alanine. Although this trend was observed experimentally on the basis of amino acid mass fraction, in the plot of aw−m, for dilute solutions (up to molality 0.46 mol· kg−1), the values of aw for both amino acids are similar, and for amino acid molalities higher than 0.46 mol·kg−1 , the experimental water activities of the investigated binary systems were observed in the sequence glycine > alanine. At high concentration, glycine molecules present a tendency for selfassociation18 which may result in a high water activity. However, in the presence of polymer, because of the polymer−glycine interactions, the self-association of glycine decreases. From Figure 2, it can be seen that, under the same 10290



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tigated amino acids computed from eqs 3 and 4 were fitted to the following equation and from which the values of the apparent molar properties at infinite dilution, Yϕ° , were determined.22

exclusion decreases the entropy of system. By increasing the concentration of the solutes (and more decreasing the water activity), the negative deviation from the LIR increases, and therefore, the exclusion will increase and ultimately the system could reach a state where, for entropic reasons, phase formation would become favorable19 and PPG becomes salted out by amino acid. In other words, the repulsive force due to the structural incompatibility of the hydrophilic hydration cosphere of amino acid and hydrophobic hydration cosphere of PPG will cause the phase separation. PPG contains a greater proportion of hydrocarbon in its molecule and also the side chain methyl groups in PPG hinder hydrogen bonding between water molecules and the ether oxygen atoms, and therefore, PPG400 can be more easily salted out by a salting-out-inducing solute to from the polymer-based aqueous two-phase system than PEG. The results show that the negative deviation from the LIR for the PPG + glycine system is larger than that for the PPG + alanine system. For the system containing glycine, the exclusion of amino acid from the near surface region of the PPG in solution increases, which leads to a larger negative deviation of constant water activity curves from eq 4 relatively to the system containing alanine, in agreement with liquid−liquid equilibria phase behavior (in the third part of this work) which shows that the effectiveness of glycine in the salting out of PPG400 is more than that of alanine. Formation of aqueous two-phase systems involves partial dehydration of solutes, and therefore, the interaction of each solute with water (which has been stronger in the presence of other solute) becomes weaker by phase separation and, in the two-phase region, the positive deviation from eq 2 is observed. Volumetric and Compressibility Properties. The following equations were used to calculate the apparent molar volume, Vϕ, and apparent molar isentropic compressibility, Kϕ, of amino acid in aqueous solution from the experimental density and sound velocity data:20,21 M 1000 (d 0 − d ) + s msdd0 d

Vϕ =

Kϕ =

1000(βd0 − β0d) msdd0

+

Yϕ = Y ϕ° + bY ma

where bY is an experimentally determined parameter. The obtained values of V°ϕ, bV, K°ϕ, and bK for glycine and alanine in water and in solvents as PEG200, PEG2000, PEG10000, and PPG400 solutions of 0.03 w/w and at the five temperatures are collectively given in Table 3. At infinite dilution, each solute is surrounded only by the solvent molecules and being infinitely distant with other solutes. Therefore, the obtained Yϕ° values are unaffected by amino acid−amino acid interaction and are a measure only of the amino acid−water or amino acid−polymer interactions. The positive values of bY are attributed to the amino acid−amino acid interactions. As can be seen, because of the higher self-association behavior of glycine, the values of bY for glycine are larger than those for alanine. The transfer apparent molar volumes at infinite dilution, ΔV°ϕ,trs = V°ϕ (in aqueous 0.03 w/w polymer solution) − Vϕ°(in water), of the investigated amino acids from water to aqueous polymer solutions are shown in Figure 4. The volume changes for both

(3)

M sβ d

(4) Figure 4. Temperature dependence of transfer apparent molar volume ° , of alanine (solid lines) and glycine (dashed at infinite dilution, ΔVϕ,trs lines) from water to aqueous polymer solutions: ○, PEG200; △, PEG2000; ×, PEG10000; ◇, PPG400.

where β0 and β are isentropic compressibilities of solvent and solution, respectively. The isentropic compressibility is defined as β=

1 du 2

(6)

° values for the polymer− systems are positive. These ΔVϕ,trs alanine−H2O systems increase with the hydrophobicity of the polymer and are independent of temperature. On the other hand, those of the polymer−glycine−H2O systems for different investigated polymers almost linearly decrease with an increase in temperature and converge at high temperature. As can be seen from Table 3, the obtained K°ϕ have negative values and become less negative by increasing temperature. The negative values of Kϕ° indicate that introduction of amino acid into water or aqueous polymer solution causes a decrease in compressibility which can be explained by the hydrophilic nature of amino acid with two ionic centers in its zwitterionic form. Glycine because of its higher hydrophilic nature has a more negative value of K°ϕ than alanine. The transfer apparent molar isentropic compressibilities at infinite dilution, ΔK°ϕ,trs = Kϕ° (in aqueous 0.03 w/w polymer solution) − Kϕ° (in water), of the investigated amino acids from water to aqueous polymer

(5)

In the above equations, Ms is the molecular mass of amino acid, ms is its molality, d0 and d are the densities of the solvent and the solution, respectively, and u0 and u are the sound velocities of the solvent and the solution, respectively. In order to obtain further information on solute hydration and also to gain a better understanding of different interactions existing in the polymer−amino acid−water systems, a systematic study on volumetric and compressibility properties of different ternary polymer−amino acid aqueous systems containing the amino acids glycine and alanine and the polymers PPG400, PEG200, PEG2000, and PEG10000 were also carried out by accurate measurement of density and sound velocity at various temperatures in the interval from 288.15 to 308.15 K. The values of the apparent molar volume, Vϕ, and apparent molar isentropic compressibility, Kϕ, of the inves10291



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solutions are positive and increase with the molecular weight of the polymer. In the case of PEG + amino acid + water systems, as a consequence of preferential amino acid−PEG interactions, the electrostriction of water in the hydration cosphere of amino acid may be reduced, and therefore, the values of Y°ϕ for the investigated amino acids increased in the presence of PEG and these systems show positive values of ΔYϕ,trs ° . Since the preferential amino acid−PEG interactions increase by increasing the hydrophobicity of PEG, the positive values of ΔY°ϕ,trs often increase by increasing the molecular weight of PEG in agreement with the behavior observed in Figure 3 in which the positive deviation from the LIR increased by increasing PEG molecular weight. In the case of PPG + amino acid + water systems, there are two factors that affect both Y°ϕ values and the salting-out strength of the amino acids: (i) The repulsive force due to the sharing of the hydrophilic hydration cosphere of amino acid and the hydrophobic hydration cosphere of PPG will cause a positive value of ΔY°ϕ,trs 23,24 and an increase in the salting-out strength of the amino acids. (ii) The favorable interaction between the hydrophobic portion of amino acids and the hydrocarbon chain of PPG will also cause a positive value of ΔY°ϕ,trs, but it decreases the salting-out strength of the amino acids. The latter factor for glycine is much weaker than that for alanine. Thus, it is reasonable that the values of ΔY°ϕ,trs for alanine are greater than those for glycine (Figure 4), but the ability of glycine in the salting out of PPG is stronger than that of alanine (see Figures 3 and 5). Liquid−Liquid Equilibria and Cloud Point Properties. In the first part of this work, we observed that distinct pairs of polymers and amino acids can induce phase separation in aqueous media when dissolved at appropriate concentration. In order to obtain further evidence about the possibility of using amino acids in the formation of polymer-based ABS and also to gain a conclusion about the main driving forces that control the phase behavior of these systems, in the present part of this work, liquid−liquid equilibria properties of several ternary polymer−amino acid aqueous systems have been investigated at different temperatures. For the aqueous PPG400 + glycine, PPG400 + alanine, PPG400 + serine, PPG400 + proline, and PPG725 + proline systems, the binodal curves were determined at 298.15, 308.15, and 318.15 K, and for the aqueous PPG725 + glycine, PPG725 + alanine, and PPG725 + serine systems, the binodal curves were determined at 298.15, 303.15, 308.15, 313.15, and 318.15 K. The experimental phase diagrams at three temperatures for the ternary {PPG400 + amino acid + water} and {PPG725 + amino acid + water} systems are presented in parts a and b of Figure 5, respectively. As can be seen, for all the investigated systems, an increase in temperature enhances the biphasic region of the phase diagrams in a ratio depending on the solute concentrations, so that, in the polymer rich region, the temperature dependence of the phase diagrams is considerably greater than that in the amino acid rich region, which may be attributed to the thermosensitivity of PPG. PPG becomes more hydrophobic by increasing temperature,25 and therefore, at higher temperatures, a lower amino acid concentration is required to form an ABS with a certain concentration of polymer, resulting in a binodal curve closer to the axis and a larger biphasic region. It is reasonable to expect that this action should be much stronger at higher concentrations of thermosensitive polymer, and thus, the greater temperature dependence would be observed in the polymer rich region of the phase diagrams in comparison with the amino acid rich region. The expansion of biphasic area by

Figure 5. Binodal curves as the mass fraction of polymer, wp, against the molality of amino acid, ma, for the aqueous (a) PPG400−amino acid and (b) PPG725−amino acid systems at T = 298.15 K (solid lines), T = 308.15 K (dotted lines), and T = 318.15 K (discontinuous line): □, proline; ○, alanine; △, glycine; ×, serine.

increasing temperature in the PPG−amino acid ABS is in close agreement with that observed in PPG-salt ABS.1,26,27 Figure 5 also shows that the effectiveness of the amino acids in the salting out of PPG400 and PPG725 follows the order serine ≈ glycine > alanine > proline. The relatively similar results have been obtained for the effectiveness of amino acids in the salting out of 1-butyl-3-methylimidazolium tetrafluoroborate to form ionic liquid−amino acid ABS.9 Recently, Madeira et al.28,29 measured the partition ratio of different amino acids in different polymer−polymer ABS and from which the solute-specific coefficients representing the solute dipole/dipole, hydrogenbonding, and electrostatic interactions with the aqueous environment of the amino acids were determined by multiple linear regression analysis using a modified linear solvation energy relationship. The trend observed here for the salting-out aptitude of amino acids is rather the same as the trend obtained for the solute-specific coefficient Bs. The control of the pH of the PPG−amino acid aqueous systems investigated in this work indicates that the pH value of these solutions is about 5.5 in the whole concentration range, which is close to the isoelectric point of the amino acids under study (see Table 1); therefore, in these systems, amino acids are in their zwitterionic form. 10292



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Figure 5 shows that the salting-out ability of amino acids increases by increasing their hydrophilic nature. In fact, the formation of polymer−amino acid ABS is a direct reflection of the competition between the two solutes for water: the more hydrophobic the side chain of amino acid (salting-out agent), the less the affinity of amino acid for water and thus the less the ability of amino acid to salt out polymer and promote phase separation. Among the amino acids studied in Figure 5, proline because of the greatest hydrocarbon proportion in its structure is the most hydrophobic amino acid, and thus has the weakest salting-out strength. Alanine and serine have the same number of carbon atoms in their structure; however, the presence of a hydroxyl group in the side chain of serine promotes its saltingout aptitude. Glycine because of the smallest hydrocarbon chain in its structure is expected to be the strongest salting-out agent among the amino acids under study, but as can be seen from Figure 5, sometimes the salting-out strength of serine is larger than that of glycine, attributed to the presence of a hydrophilic −OH group at serine. Figure 6 compares the phase behavior of

Figure 7. Binodal curves for the PPG400 (p) + solute (a) + H2O (w) two-phase systems at 318.15 K: ●, Na3Cit;1 ○, Na2CO3;1 △, Na2SO4;1 ▲, NaCl;1 ◆, NaNO3;1 ◇, NaH2PO4;27 ■, Na2HPO4;26 □, Na3PO4;26 -, serine; ×, glycine; +, alanine; ∗, proline.

PPG is a thermosensitive polymer, so a binary PPG + water system becomes turbid when the temperature is increased to a critical value (cloud point temperature). This phenomenon is attributed to the reduction of the hydrophilicity of the polyether chains of PPG, caused by the rise in temperature. The cloud point of the PPG + water system can be enhanced (salting-in effect) or reduced (salting-out effect) by the addition of different solutes to PPG aqueous solutions. Figure 8 shows

Figure 6. Binodal curves as the mass fraction of polymer, wp, against the molality of amino acid, ma, for the aqueous PPG400−amino acid (solid lines) and PPG725−amino acid (dotted lines) systems at T = 298.15 K: □, proline; ○, alanine; △, glycine; ×, serine.

ternary {PPG400 + amino acid + water} and {PPG725 + amino acid + water} systems at 298.15 K. In general, the hydrophobicity increases by increasing the polymer molecular mass; hence, PPG725 with a higher molecular mass requires less amino acid to promote phase separation in comparison with PPG400, resulting in a larger biphasic region for {PPG725 + amino acid + water} systems than {PPG400 + amino acid + water} systems. Figure 6 also shows that the effect of polymer molecular mass on the phase forming ability of polymer−amino acid aqueous systems is significantly higher than the effect of amino acid structure. In Figure 7, the binodal curves for several PPG−sodium salt ABS at 318.15 K have been compared with those of PPG−amino acid ABS obtained in this work. This figure shows that the effectiveness of different solutes in the salting out of aqueous PPG400 solutions follows the order Cit3− > PO43− > SO42− ≈ CO32− > HPO42− > H2PO42− > Cl− > amino acids > NO3−. As can be seen, the position of the amino acids in the Hofmeister series is similar to that of the singly charged anions.

Figure 8. Plot of cloud point temperature, TC, against PPG mass fraction, wp, for PPG725 in water and in aqueous solutions of amino acids: ●, water; ×, aqueous solutions of 0.2 molal isoleucine; ○, aqueous solutions of 0.2 molal proline; △, aqueous solutions of 0.2 molal alanine; +, aqueous solutions of 0.2 molal serine; □, aqueous solutions of 0.2 molal glycine.

the cloud point temperatures, TC, of solutions of PPG725 in water and in aqueous solutions of 0.2 molal amino acids, as a function of polymer concentration. In this figure, the cloud point temperatures of aqueous PPG725 + glycine, PPG725 + alanine, and PPG725 + serine systems were obtained from the binodal data and those of aqueous PPG725 + isoleucine and PPG725 + proline systems were obtained directly from the 10293



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concentration dependence of the cloud point temperature of PPG725 has been shown for four amino acid molalities (0.2, 0.5, 0.8, and 1.1 mol·kg−1). As expected, the reduction of the cloud point temperature of PPG725 aqueous solutions by the addition of amino acids (salting-out effect) becomes more obvious by increasing the concentration of amino acids (saltingout agent). Figures 5b, 8, and 9 show that the effect of the structure of amino acids on the cloud points becomes smaller by increasing temperature. In fact, because of the increasing hydrophobicity of PPG by increasing temperature, the difference in the salting-out ability of amino acids becomes smaller at higher temperatures. With the consideration of clouding as the point of phase separation (or the solubility limit), following Moulik et al.,30 the free energy of phase separation (ΔGm,C) can be calculated by using the relation

cloud point measurements. It can be seen from Figure 8 that the cloud point temperature of PPG725 + amino acid aqueous solutions decreases by increasing polymer concentration and whose values for the investigated system decrease in the order water > isoleucine ≈ proline > alanine > serine ≈ glycine. The effect of the amino acids on the cloud point temperature of PPG725 is in agreement with the order of their phase forming ability observed in Figure 5b. Increasing the hydrophilicity of amino acids lowers the temperature at which phase separation occurs. In other words, amino acids which hydrate strongly evidently induce the dehydration of the polyether group of PPG at a lower temperature. In Figure 9, the polymer

ΔGm,C = RTC ln X p

(7)

where Xp is the mole fraction concentration of PPG at TC. The values of ΔGm,C for solutions of PPG725 in water and in aqueous amino acid solutions are presented in Table 4 with reference to clouding temperatures. The values of ΔGm,C at different temperatures were processed according to the Gibbs− Helmholtz equation (eq 8) to get the enthalpy of phase separation (ΔHm,C) from the slope of the linear plot between ΔGm,c/TC against 1/TC: d

ΔGm,C

( ) = ΔH d( ) TC 1 TC

Figure 9. Plot of cloud point temperature, TC, against PPG mass fraction, wp, for PPG725 in water and in aqueous solutions of the amino acids alanine (discontinuous line), serine (dotted line), and glycine (solid line): ●, water; ○, ma = 0.2 mol·kg−1; △, ma = 0.5 mol· kg−1; ×, ma = 0.8 mol·kg−1; ◇, ma = 1.1 mol·kg−1.

m,C

(8)

Then, the following equation was used to calculate the entropy of phase separation (ΔSm,C):

Table 4. Free Energy Changes, ΔGm,C, Entropy Changes, ΔSm,C, and Enthalpy Changes, ΔHm,C, for the Clouding Process of PPG725 in Water and in Aqueous Solutions with Certain Molalities (mi) of Amino Acids as a Function of Cloud Point Temperature (TC) ΔGm,C (kJ mol−1) and (ΔSm,C (kJ mol−1·K−1)) −1

mi (mol·kg )

TC = 298.15 K

0 0.2 0.5 0.8 1.1

−16.48 (0.3849) −17.36 (0.3864) −18.34 (0.3803)

0.2 0.5 0.8 1.1

−14.92 −16.05 −16.87 −17.77

(0.3964) (0.3964) (0.4010) (0.3936)

0.2 0.5 0.8 1.1

−15.52 −16.48 −17.33 −18.32

(0.3800) (0.3866) (0.3850) (0.3864)

0.2 0.2

TC = 303.15 K

TC = 308.15 K

TC = 313.15 K

PPG725 in Water −15.62 (0.3837) −18.17 (0.3858) −20.05 (0.3856) PPG725 in Aqueous Solution of Glycine −17.66 (0.3793) −19.06 (0.3777) −21.21(0.3786) −18.52 0.3853) −20.24 (0.3846) −22.27 (0.3849) −19.46 (0.3869) −21.34 (0.3868) −23.45 (0.3873) −20.57 (0.3814) −22.49 (0.3814) −24.41 (0.3815) PPG725 in Aqueous Solution of Alanine −16.81 (0.3961) −18.83 (0.3962) −20.86 (0.3963) −18.01 (0.3963) −19.92 (0.3961) −22.12 (0.3968) −19.19 (0.4020) −21.03 (0.4015) −23.01 (0.4014) −20.23 (0.3952) −22.09 (0.3949) −23.93 (0.3944) PPG725 in Aqueous Solution of Serine −17.54 (0.3804) −18.97 (0.3789) −21.11 (0.3796) −18.37 (0.3865) −20.05 (0.3857) −22.16 (0.3862) −19.37 (0.3854) −21.26 (0.3853) −23.18 (0.3853) −20.43 (0.3870) −22.37 (0.3870) −24.18 (0.3866) PPG725 in Aqueous Solution of Proline −16.41 (0.3681) −18.31 (0.3683) −20.44 (0.3693) PPG725 in Aqueous Solution of Isoleucine −16.37 (0.3790) −18.33 (0.3793) −20.26 (0.3794) 10294

TC = 318.15 K

ΔHm,C (kJ mol−1)

−21.63 (0.3845)

100.7

−23.26 −24.23 −25.02 −25.92

(0.3791) (0.3851) (0.3862) (0.3802)

97.3 98.3 97.8 95.0

−22.81 −23.91 −24.99 −25.76

(0.3962) (0.3961) (0.4013) (0.3940)

103.3 102.1 102.7 99.6

−23.25 −24.26 −25.05 −26.11

(0.3804) (0.3868) (0.3851) (0.3866)

97.8 98.8 97.5 96.9

−22.09 (0.3686)

95.2

−22.18 (0.3794)

98.5


The Journal of Physical Chemistry B ΔSm,C =

Article

ΔHm,C − ΔGm,C TC

increasing temperature, increasing the hydrophilicity of amino acids and increasing the molecular mass of polymer. The results of cloud point measurements show that the addition of amino acids to aqueous solutions of PPG725 causes a decrease in cloud point temperature values (salting-out effect). The calculated values of Gibbs free energy, enthalpy, and entropy of the clouding process indicate that entropy is the driving force for the formation of polymer−amino acid aqueous biphasic systems.

(9)

The calculated energetic parameters reported in Table 4 show that formation of the PPG−amino acid aqueous two-phase systems is an endothermic process (ΔHm,C > 0), but because of an increase in entropy during phase separation (ΔSm,C > 0), the free energies of clouding have negative values (ΔGm,C < 0). Therefore, entropy is the driven force for these processes. Similar results have been obtained for polymer−salt aqueous two-phase formation.19,26,30 The high positive values of ΔSm,C reported in Table 4 suggest the dehydration of polymer during phase separation, which is in agreement with the positive deviation of constant water activity lines from LIR in the biphasic region of polymer−amino acid aqueous systems observed in the first part of this work. As can be seen from Table 4, the values of ΔGm,C become more negative by increasing temperature, amino acid hydrophilicity, and amino acid molality in agreement with behavior observed in Figures 5b, 8, and 9.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables showing the measured vapor−liquid equilibria, phase diagram, cloud point, and density and sound velocity data. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

■

*Phone: +98 871 6624133. Fax: +98 871 6660075. E-mail: [email protected] and [email protected].

CONCLUSIONS The salt effect of different amino acids on several water-soluble polymers in aqueous solutions was investigated by determination of liquid−liquid phase diagrams, cloud point values, and volumetric and compressibility properties at different temperatures and isopiestic determination of vapor−liquid equilibria properties at 298.15 K. Vapor−liquid equilibria behavior of the investigated polymer + amino acid + water systems at 298.15 K show two different salt effects. In the case of the ternary investigated PEG + amino acid + water systems which are not capable of inducing phase separation (salting-in effect), because of the favorable PEG−amino acid interactions, the constant water activity lines have convex slopes and these systems show positive deviations from the linear isopiestic relation in the whole solute concentration region. The second behavior was observed in the case of ternary investigated PPG + amino acid + water systems which can form an aqueous two-phase system (salting-out effect) when an aqueous solution exceeds a specific threshold concentrations of two solutes. In these systems, we can distinguish between two concentration regions. In the onephase area (solute concentration lower than the specific threshold concentrations), the repulsive force due to the structural incompatibility of the hydrophilic hydration cosphere of amino acid and the hydrophobic hydration cosphere of PPG will cause the negative deviation from the LIR. However, in the two-phase area (solute concentration higher than the specific threshold concentrations), partial dehydration of solutes due to the formation of aqueous two-phase systems will cause the positive deviation from the LIR. The volumetric and compressibility behavior of the investigated polymer + alanine or glycine + water systems shows that the positive values of ΔV°ϕ,trs for the ternary polymer + alanine + water systems increase with the hydrophobicity of polymer and are independent of temperature; however, those of the ternary polymer + glycine + water systems for different investigated polymers almost linearly decrease with an increase in temperature and converge at high temperature. On the other hand, the positive values of ΔKϕ,trs ° for both alanine and glycine increase with the molecular weight of polymer. The experimental liquid−liquid equilibrium data for the novel polymer−amino acid aqueous biphasic systems show that the phase forming ability of these systems increases with

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sadeghi, R.; Jahani, F. Salting-in and Salting-out of Water Soluble Polymers in Aqueous Salt Solutions. J. Phys. Chem. B 2012, 116, 5234−5241. (2) Moradian, T.; Sadeghi, R. Isopiestic Investigations on the Interactions of Water Soluble Polymers with Imidazolium-based Ionic Liquids in Aqueous Solutions. J. Phys. Chem. B 2013, 117, 7710−7717. (3) Albertsson, P. A. Partitioning of Cell Particles and Macromolecules, 3rd ed.; Wiley-Interscience: New York, 1986. (4) Walter, H.; Brooks, D. E.; Fisher, D. Partitioning in Aqueous TwoPhase Systems; Academic Press: New York, 1985. (5) Zaslavsky, B. Y. Aqueous Two-Phase Partitioning, Physical Chemistry and Bioanalytical Applications; Marcel Dekker: New York, 1995. (6) Griffin, S. T.; Dilip, M.; Spear, S. K.; Huddleston, J. G.; Rogers, R. D. The Opposite Effect of Temperature on Polyethylene Glycol-Based Aqueous Biphasic Systems Versus Aqueous Biphasic Extraction Chromatographic Resins. J. Chromatogr. B 2006, 844, 23−31. (7) Rogers, R. D.; Bauer, C. B. Partitioning Behavior of Group 1 and 2 Cations in Poly(ethylene glycol)-Based Aqueous Biphasic Systems. J. Chromatogr. B 1996, 680, 237−241. (8) Freire, M. G.; Claudio, A. F. M.; Araujo, J. M. M.; Coutinho, J. A. P.; Marrucho, I. M.; Lopes, J. N. C.; Rebelo, L. P. N. Aqueous Biphasic Systems: A Boost Brought About by Using Ionic Liquids. Chem. Soc. Rev. 2012, 41, 4966−4995. (9) Zhang, J. M.; Zhang, Y. Q.; Chen, Y. H.; Zhang, S. J. Mutual Coexistence Curve Measurement of Aqueous Biphasic Systems Composed of [bmim][BF4] and Glycine, L-Serine, and L-Proline, Respectively. J. Chem. Eng. Data 2007, 52, 2488−2490. (10) Domínguez-Perez, M.; Tome, L. I. N.; Freire, M. G.; Marrucho, I. M.; Cabeza, O.; Coutinho, J. A. P. (Extraction of Biomolecules using) Aqueous Biphasic Systems formed by Ionic Liquids and Aminoacids. Sep. Purif. Technol. 2010, 72, 85−91. (11) Gao, Y. Q. Simple Theory for Salt Effects on the Solubility of Amide. J. Phys. Chem. B 2012, 116, 9934−9943. (12) Xie, W. J.; Gao, Y. Q. A Simple Theory for the Hofmeister Series. J. Phys. Chem. Lett. 2013, 4, 4247−4252. (13) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: 2004. (14) Clarke, E. C. W.; Glew, D. N. Evaluation of the Thermodynamic Functions for Aqueous Sodium Chloride from Equilibrium and 10295



Article

Calorimetric Measurement below 154 ◦C. J. Phys. Chem. Ref. Data 1985, 14 (2), 489−610. (15) Zafarani-Moattar, M. T.; Sadeghi, R. Liquid-Liquid Equilibria of Aqueous Two-Phase Systems Containing Polyethylene Glycol and Sodium Dihydrogen Phosphate or Disodium Hydrogen Phosphate Experiment and Correlation. Fluid Phase Equilib. 2001, 181 (1−2), 95−112. (16) Zdanovskii, A. B. Regularities in the Property Variations of Mixed Solutions. Tr. Solyanoi Lab. Akad. Nauk SSSR 1936, 6, 5−70. (17) Stokes, R. H.; Robinson, R. A. Interactions in Aqueous Nonelectrolyte Solutions. I. Solute-Solvent Equilibria. J. Phys. Chem. 1966, 70, 2126−2131. (18) Lin, R.; Hu, X.; Ren, X. Homogeneous Enthalpic Interaction of Amino Acids in DMF−H2O Mixed Solvents. Thermochim. Acta 2000, 352−353, 31−37. (19) da Silva, L. H. M.; Loh, W. Calorimetric Investigation of the Formation of Aqueous Two-Phase Systems in Ternary Mixtures of Water, Poly(ethylene oxide) and Electrolytes (Or Dextran). J. Phys. Chem. B 2000, 104, 10069−10073. (20) Millero, F. J. In Water and Aqueous Solutions; Horne, R. A., Ed.; John Wiley & Sons: New York, 1972; pp 519−595. (21) Owen, B. B.; L, S. H. Standard Partial Molal Compressibilities by Ultrasonics. I. Sodium Chloride and Potassium Chloride at 25°C. J. Phys. Chem. 1957, 61, 479−482. (22) Bhat, R.; Ahluwalia, J. C. Partial Molar Heat Capacities and Volumes of Transfer of Some Amino Acids and Peptides from Water to Aqueous Sodium Chloride Solutions at 298.15 K. J. Phys. Chem. 1985, 89, 1099−1105. (23) Sasahara, K.; Uedaira, H. Volume and Compressibility Changes on Mixing Aqueous Solutions of the Amino Acid and Poly(ethylene glycol). Colloid Polym. Sci. 1994, 272, 385−392. (24) Sasahara, K. Temperature Dependence of Volume Changes on Glycine-PEG and L-Alanine-PEG in Aqueous Solution. Colloid Polym. Sci. 1995, 273, 782−786. (25) Sadeghi, R.; Shahebrahimi, Y. Vapor-Liquid Equilibria of Aqueous Polymer Solutions from Vapor Pressure Osmometry and Isopiestic Measurements. J. Chem. Eng. Data 2011, 56, 789−799. (26) Sadeghi, R.; Jamehbozorg, B. The salting-out effect and phase separation in aqueous solutions of sodium phosphate salts and poly(propylene glycol). Fluid Phase Equilib. 2009, 280 (1−2), 68−75. (27) Sadeghi, R.; Jamehbozorg, B. Effect of temperature on the salting-out effect and phase separation in aqueous solutions of sodium di-hydrogen phosphate and poly(propylene glycol). Fluid Phase Equilib. 2008, 271 (1−2), 13−18. (28) Madeira, P. P.; Bessa, A.; Alvares-Ribeiro, L.; Aires-Barros, M. R.; Rodrigues, A. E.; Uversky, V. N.; Zaslavsky, B. Y. Amino Acid/ Water Interactions Study: A New Amino Acid Scale. J. Biomol. Struct. Dyn. 2014, 32, 959−968. (29) Madeira, P. P.; Bessa, A.; Alvares-Ribeiro, L.; Aires-Barros, M. R.; Rodrigues, A. E.; Zaslavsky, B. Y. Analysis of Amino Acid-Water Interactions by Partitioning in Aqueous Two-Phase Systems. I−Amino Acids with Non-polar Side-Chains. J. Chromatogr. A 2013, 1274, 82− 86. (30) Dan, A.; Ghosh, S.; Moulik, S. P. The Solution Behavior pf Poly(vinylpyrrolidone): Its Clouding in Salt Solution, Solvation by Water and Isopropanol, and Interaction with Sodium Dodecyl Sulfate. J. Phys. Chem. B 2008, 112, 3617−3624.

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