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

Solubility and Thermodynamics of Solute−Solvent Interactions of Some Amino Acids in Aqueous Sodium Bromide and Potassium Bromide Solutions Partha Sarathi Guin,† Kalachand Mahali,‡ Bijoy Krishna Dolui,§ and Sanjay Roy*,† †

Department of Chemistry, Shibpur Dinobundhoo Institution (College), 412/1, G. T. Road (South), Howrah, West Bengal 711102, India ‡ Department of Chemistry, University of Kalyani, Nadia, West Bengal Pin 741235, India § Department of Chemistry, Visva-Bharati, Santiniketan, Birbhum, West Bengal Pin 731235, India S Supporting Information *

ABSTRACT: The study presents the saturated solubilities of four amino acids such as glycine, DL-alanine, DL-valine, and DL-serine in water media containing sodium bromide and potassium bromide electrolytes at standard temperature (298.15 K) which were evaluated by applying the “gravimetric method”. Various physicochemical and thermodynamical parameters such as enthalpy of solvation, free energy change, density, molar volume, solvent diameter, cosolvent diameter, apparent dipole moment of aqueous electrolyte systems, mole fraction of salt and solvent, mean molecular weight of electrolyte solvent and isothermal expansibility constant of aqueous electrolytes were estimated at standard temperature (298.15 K). The characteristics and degree of solubilities of the experimental molecules in water media in the presence of NaBr and KBr were examined by means of the salting-in effect which was further justified by relative solubility and salting-in constants. Various reasonings influencing the solubility were introduced and interrelated with appropriate thermodynamical issues. Ultimately the chemical transfer Gibbs free energies were measured and used to justify the stability of the experimental molecules in water media in the presence of electrolytes.

1. INTRODUCTION In designing the mode of separation of amino acids, the precise determination of the solubility behavior in experimental mixtures of this class of molecules is indispensable. As amino acids are the basic building blocks of proteins, advancement of knowledge on thermodynamic behaviors of these molecules in aqueous media have been introduced as an active area of research for quite long time.1−5 The solubility of this type of molecules in water and different water-electrolyte solutions and the solvation of these molecules in terms of thermodynamic parameters are imperative in the purification and solution chemistry of proteins.6−8 These studies are also important in chemical, food, and pharmaceutical industries.9−12 Because of the presence of −NH2 and −COOH groups in amino acids, these molecules exist in aqueous media as zwitterions at pH 7. The zwitterions are polar having a significant dipole moment, which results in several weak interactions involving this dipole. Despite the presence of the two above-mentioned functional groups in all amino acids they differ widely because of their difference in chemical structures. In the present study glycine is the most simple molecule containing no hydrophobic side chain, whereas other experimental molecules such as DL-alanine, DL-valine and DL-serine consist of different hydrophobic side chains such as CH3−, CH3−CH(CH3)− and −CH2OH, © XXXX American Chemical Society

respectively [Table S1in the Supporting Information]. For quite a long time various studies on amino acids have been done by numerous researchers, but studies on the stability and thermodynamical parameters of these acids in water media and biological systems remain stagnant. Nowadays the increasing global demand for this class of molecules in different fields has led to the introduction of many techniques for the synthesis of such molecules in which chemical pathways and fermentation method are most crucial. Separation of these molecules from overindulgent reagents and other types of impurities in water media is a difficult job that is normally carried out through crystallization or precipitation methods. It is interesting to note that the separation price of these molecules has been found as about 50% of the entire production charge.12 This aspect is obviously very serious and needs enormous study to overcome the problem. The effects of different ions arising out by the dissociation of electrolytes are of prospective significance for the separation of these molecules. Thus, designing the most efficient extraction and Received: July 14, 2017 Accepted: January 22, 2018

A

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

experimental results was found as ±0.005 mol·kg−1. This means that, in spite of the presence of different amounts of amino acid in the solution, no considerable quantity of electrolyte was precipitated or adsorbed on the solid phase of amino acid. This definitely proves that solid recovered was only due to amino acids.

separation method requires knowledge of saturated solubility of such molecules in the presence of different salts.12 The current article depicts the determination of saturated solubility of the experimental amino acids such as glycine, DLalanine, DL-valine, and DL-serine in aqueous sodium bromide and potassium bromide mixtures at 298.15 K by introducing a simple gravimetric method.3,12−14 In the present study we attempted to evaluate various physicochemical parameters influencing the solubility and various transfer free energies, and the results of the solubility and solvation thermodynamics were also correlated.

3. RESULT Assuming the initial concentration of the salt (NaBr or KBr) in aqueous media as x, the weight of the salt in 5 mL solution would be W1 = (Mx × 5/1000) g; where, M is the molar mass of the salt. If the weight of the empty dry vessel and the vessel containing dry amino acid are W2 and W3 g, respectively, then the amount of the dissolved amino acid will be W = (W3− W2− W1) g. The maximum solubilities of the present amino acids in mol·kg−1 (number of moles of amino acid dissolved in 1 kg of solvent water) were estimated in 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol·kg−1 aqueous NaBr and KBr systems (i.e., number of moles of the electrolyte dissolved in 1 kg of pure aqueous media) and presented in Table 1. The previous studies15−18 have

2. EXPERIMENTAL METHODS 2.1. Chemicals and Their Purifications. Glycine, DLalanine, DL-valine, and DL-serine (>99.8%) were purchased from Sigma-Aldrich and used as received after drying them in desiccator at 350 K for 5 days. NaBr and KBr (>99%) were purchased from E. Merck, India. These salts were dried in an oven at 400 K for 4 days and then cooled in a vacuum desiccator for 7 days before their use. All aqueous solutions were made in triple distilled water. 2.2. Preparations of Saturated Solutions and Determination of Solubility. By dissolving an exact quantity of NaBr and KBr in a desired volume of water, aqueous electrolyte solutions of concentrations 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 in mol· kg−1 were made. An appropriate thermostat having an accuracy of ±0.10 K at atmospheric pressure was employed in controlling temperatures. The solubilities of the experimental amino acids were estimated by using the gravimetric method3,12−14 in aqueous electrolyte systems at their normal pH range. Initially a saturated solution of the amino acid was prepared at the desired temperature. The amino acid in water−electrolyte mixture was charged in the jacketed glass cell in such a way that a slight excess of amino acid remained to be dissolved in order to get a saturated solution. Three sets of samples were prepared for each composition. The temperature of the solution was controlled by a circulating thermostat in the jacket, and the prepared solution was continuously stirred for 24 h to achieve the equilibrium. The undissolved amino acid was then allowed to settle for 7 h prior to sampling. The solutions were (5 mL) collected from the supernatant phase by using dried pipettes. They were then filtered by using a 0.22 μm HPLC disposable filter inserted into glass vessels and very quickly weighed. The solutions were evaporated to dryness to form crystals of amino acids, and finally dried completely in a drying stove at a temperature of 400.15 K. In the presence of silica gel the samples were cooled in a dehydrator for 48 h and weighed. This was repeated until a constant mass was obtained. In the present study to ensure the chance of adsorption or assimilation of the salts such as NaBr and KBr or degradation of the sample on the solid-phase of the amino acids, atomic absorption spectroscopy was employed to investigate the ions in the mixture as it was done in previous works.15,16 Concentrations of cations in the aqueous electrolyte and in the amino acid−water−electrolyte systems were also compared to validate the fact that the electrolytes were not absorbed or incorporated on the solid phase of the amino acids, that is, the precipitate was formed only by the amino acid.17 Electrolyte solutions containing 5%, 10%, 15%, 30%, 40%, and 50% amino acids in excess to saturation were prepared, and cation concentrations were analyzed for comparison of cation concentration in each solution. The highest difference in the

Table 1. Solubilities of Glycine, DL-Alanine, DL-Valine, and −1 DL-Serine in mol·kg of Water in the Presence of NaBr and KBr Electrolytes at 298.15 K under Atmospheric Pressure, p = 0.1 MPaa salt(s) molality mol·kg

−1

in water

NaBr 0.00 0.50 1.00 1.50 2.00 2.50 KBr 0.00 0.50 1.00 1.50 2.00 2.50

solubility (s) at 298.15 K glycine

DL-alanine

DL-valine

DL-serine

3.389 3.516 3.651 3.710 3.920 4.060

1.938 2.073 2.208 2.364 2.466 2.702

0.690 0.783 0.918 1.020 1.122 1.186

0.479 0.657 0.792 0.975 1.184 1.322

3.389 3.537 3.667 3.765 3.854 3.983

1.938 2.078 2.224 2.389 2.547 2.723

0.690 0.804 0.934 1.112 1.246 1.328

0.479 0.678 0.807 0.998 1.215 1.439

a

Standard uncertainties u are u(T) = 0.10 K and u(m) = 0.01 mol· kg−1; relative uncertainties, and ur(S) = 0.02 and ur(p) = 0.02.

established that that no significant quantity of adsorption or precipitation of salt (NaBr or KBr) on solid amino acid occurs even when the concentrations of amino acid are different in the solution. All estimations were done at 298.15 K by incubating the mixture at above and below (±0.10 K) 298.15 K. The very simple gravimetry method was then applied to measure the solubilities which were observed to agree within 2.8%. In the present study we also calculated the relative solubility and salting-in constants of the experimental amino acids in aqueous NaBr and KBr media in variable concentrations of salts as presented in Tables 2 and 3, respectively. In the present report the solubilities (mol·kg−1) at 298.15 K in pure water and in H2O−NaBr and H2O−KBr systems were used to estimate the apparent standard Gibbs free energies of the experimental solutions applying eq 1.12 ΔGt0(S) = RT ln(SR /SS) B

(1) DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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presence of salts. ΔG0t,d−d(i) represents the free energy due to dipole−dipole interaction as a result of interaction between zwitter-ionic amino acid molecule and solvent water molecule. Chemical interactions such as short-range dispersion, acid−base interaction, hydrophobic and hydrophilic effects, etc., result in the free energy term ΔG0t,ch(i). Considering solvent and solute molecules as hard sphere and by using scaled particle theory the ΔG0t,cav(i) value was measured. The necessary parameters were taken from Tables 4 and 5. The essential equations applied for cavity measurement are1−5

Table 2. Relative Solubility (SS/SR) and log (SS/SR) of Glycine, DL-Alanine, DL-Valine and DL-Serine in Aqueous NaBr and KBr Solutions in Different Compositions of Electrolytes at 298.15 K salt(s) molality

relative solubility

mol·kg−1 in water

Ss/SR

NaBr 0.50 1.00 1.50 2.00 2.50

1.037 1.077 1.095 1.157 1.198

relative solubility log(SS/ SR)298.15K

Glycine

DL-Alanine

0.016 0.032 0.039 0.063 0.078

1.069 1.139 1.219 1.272 1.394

DL-Valine

0.50 1.00 1.50 2.00 2.50 KBr 0.50 1.00 1.50 2.00 2.50

1.135 1.330 1.478 1.626 1.719 1.044 1.082 1.111 1.137 1.175

0.50 1.00 1.50 2.00 2.50

1.165 1.354 1.612 1.806 1.925

log(SS/ SR)298.15K

Ss/SR

0.029 0.057 0.086 0.104 0.144

0 ΔGcav (i) = GC + RT ln(RT /VS)

where

DL-Serine

0.055 0.124 0.169 0.211 0.235 Glycine

1.372 1.653 2.035 2.472 2.759

GC = RT[−ln(1 − Z) + {3X /(1 − Z)}σx + {3Y /(1 − Z)}σx 2 + {9X2 /2(1 − Z)2 }σx 2]

0.137 0.218 0.309 0.393 0.441

Ζ = πNA /6VS(z R σR 3 + zSσS3) X = πNA /6VS(z R σR 2 + zSσS2)

DL-Alanine

0.019 0.034 0.046 0.056 0.070

1.072 1.148 1.233 1.314 1.405

0.066 0.132 0.207 0.257 0.284

1.415 1.685 2.084 2.537 3.004

DL-Valine

Y = πNA /6VS(z R σR + zSσS)

0.030 0.059 0.091 0.119 0.148

VS = ΜS/dS

where NA is Avogadro’s number, zR and zs are the mole fractions of H2O and electrolytes, respectively. σx, σR, and σs represent hard sphere diameters of amino acid, H2O, and cosolvents, respectively. Ms and ds are the molar mass and molar density of the electrolyte solution, respectively. Thus, the Gibbs free energy for transfer owing to formation of cavity, ΔG0t,cav(i) results eq 51,2,12

DL-Serine

0.151 0.227 0.319 0.404 0.478

SΔGt (cav)

Table 3. Salting-in Constants of Glycine, DL-Alanine, DLValine and DL-Serine in Aqueous NaBr and KBr solutions at 298.15 Ka

a

amino acids

Ksi at 298.15 K in NaBr

Ksi at 298.15 K in KBr

glycine DL-alanine DL-valine DL-serine

0.0310 0.0554 0.0894 0.1566

0.0248 0.0592 0.1122 0.1660

(5)

0 0 0 ΔGt,d ‐ d (i) = (SΔGd ‐ d (i) −R ΔGd ‐ d (i)) 0 sΔGd‑d(i)

(6)

in aqueous electrolyte solvent “s”, can be obtained as

follows: 0 SΔGd ‐ d (i)

where, SR and SS are solubilities of amino acids in aqueous media in the absence and presence of salt, respectively. In the mole fraction scale the standard transfer free energies (ΔG0t (i)) were determined using eq 2.1,2

where

= −(8Π/9)N 2μS2 μx2 σS−−3x(kT )−1V S−1 = A /TVS

A = −(8Π/9)N 2μs2 μx2 σS−−3x(k)−1

and VS = MS/dS

(2)

(7)

where N is the Avogadro’s number, k stands for Boltzmann constant, μx and μs are apparent dipole moments of amino acid and solvent molecules, respectively, which are presented in Tables 4 and 5; σs−x = 1/2 (σs + σx), which represents the distance at which the repulsive and attractive forces between solute and solvent molecules become equal. By varying the mole fraction of the binary mixtures, the value of σs−x was measured at different mole fractions. This value was then multiplied by Xs1 in accordance to Marcus19 and Kim et al.20 to obtain ΔG0t,d‑d(i) in mole fraction scale (eq 8),8

where MS and MR are the molar masses of electrolytes (NaBr/ KBr) and H2O (reference solvent), respectively. In this case uncertainty of molar mass (MS) was found as ±0.0003 (Tables 4 and 5). The determined values of ΔG0t (i) are shown in Tables 6 and 7. Considering dipole−induced dipole factor to be infinitesimally small1−5 one may ascribe ΔGt0(i) as the summation of various free energy terms;1−5 0 0 0 ΔGt0(i) = ΔGt,cav (i) + ΔGt,d ‐ d (i) + ΔGt,ch (i)

− RΔGt(cav) = ( SGc − RGc) + RT ln(VR /VS)

0 For the calculation of ΔGt,cav (i), the required solvent parameters were considered from Tables 4 and 5. By applying Keesom-orientation eq 6, ΔG0t,d‑d(i) values were estimated.19

u(T) = 0.10 K.

ΔGt0(i) = ΔGt0(S) − RT ln(MS/MR )

(4)

(3)

ΔG0t,cav(i)

where represents transfer free energy due to cavity formation which involves the creation of cavities by experimental molecule in aqueous media in the absence and

XS1 = XS(μS /σS3)/(μR /σR3) C

(8) DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. Values of Solvent Parametersa salt(s) molality

mol fraction

mol % salt

mol fraction

molar mass

density (ds)

molar volume (Vs)

apparent dipole moment (μs)

mol·kg−1 in water

zs

mol fraction (χa) × 100

zR

MS

kg·dm−3

dm3·mol−1

D

α(x 103)

0.0000 0.0089 0.0177 0.0263 0.0348 0.0431

0.00 0.89 1.77 2.63 3.48 4.31

1.0000 0.9911 0.9823 0.9737 0.9652 0.9569

18.0150b 18.7704 19.5173 20.2472 20.9687 21.6731

0.9970b 1.0167 1.0362 1.0552 1.0740 1.0924

18.0692 18.4621 18.8355 19.1880 19.5239 19.8399

1.830b 1.895 1.959 2.022 2.084 2.144

0.257b 0.257 0.257 0.257 0.257 0.257

0.0089 0.0177 0.0263 0.0348 0.0431

0.89 1.77 2.63 3.48 4.31

0.9911 0.9823 0.9737 0.9652 0.9569

18.9138 19.8024 20.6709 21.5293 22.3675

1.0125 1.0279 1.0428 1.0577 1.0721

18.6803 19.2649 19.8225 20.3548 20.8633

1.906 1.982 2.056 2.129 2.199

0.257 0.257 0.257 0.257 0.257

NaBr 0.00 0.50 1.00 1.50 2.00 2.50 KBr 0.50 1.00 1.50 2.00 2.50

a Mole fraction of salt (zs), water (zR), mean molecular weight of electrolyte solvent (MS), density (ds), molar volume (Vs), solvent diameter (σs), σs−x, μs, and isothermal expansibility constant (α) and apparent dipole moment (D) of the H2O + salt systems at 298.15 K. u(MS) = 0.0003, [u for uncertainty]. bFrom ref 32.

Table 5. Values of σs−x = 1/2(σs + σx) of the Amino Acids Present in Water−Electrolytes Systems at 298.15 K σs−x (nm)

salt(s) molality mol·kg

−1

in water

NaBr 0.00 0.50 1.00 1.50 2.00 2.50 KBr 0.00 0.50 1.00 1.50 2.00 2.50

bromide than in aqueous potassium bromide solution. However, the salting in effect is more pronounced in potassium bromide than in the sodium bromide for DL-alanine, DL-valine, and DL-serine which is evident in the relative solubility data (Table 2) and salting in constants. A more positive salting in constant indicates more pronounced salting in effect (Table 3). The relative solubility (SS/SR) is the ratio of the solubility of an amino acid in an aqueous electrolyte mixture (SS) to that in pure aqueous media (SR). In different concentrations of salts, values of (SS/SR) of were measured and are mentioned in Table 2. From Table 2 it is evident that as the molality of electrolytes increases, the relative solubility increases linearly (salting-in). The difference in the solubility and salting-in effect is possibly due to different sort of complex formations in water media by the amino acids of dissimilar size with the anion of electrolytes which is in accordance with the study done by Held and co-workers.23 The salting- in constant (Ksi) is a quantitative estimate of salting- in effect which was evaluated with the help of eq 9.23−25

σs (nm)

glycine

DL-alanine

DL-valine

DL-serine

0.2740 0.2742 0.2744 0.2747 0.2749 0.2751

0.4190 0.4191 0.4192 0.4194 0.4195 0.4196

0.4450 0.4451 0.4452 0.4454 0.4455 0.4456

0.4830 0.4831 0.4832 0.4834 0.4835 0.4836

0.4335 0.4336 0.4337 0.4339 0.4340 0.4341

0.2740 0.2745 0.2751 0.2756 0.2761 0.2766

0.4190 0.4193 0.4196 0.4198 0.4201 0.4203

0.4450 0.4453 0.4456 0.4458 0.4461 0.4463

0.4830 0.4833 0.4836 0.4838 0.4841 0.4843

0.4335 0.4338 0.4341 0.4343 0.4346 0.4348

log(SS/SR ) = K siC

where Xs1 is the actual mole fraction which arises owing to dipole−dipole interaction. In this study the value of ΔG0t,ch(i) of the amino acids was obtained by subtracting the values of ΔG0t,cav(i) and ΔG0t,d‑d(i) from ΔG0t (i) and the values are presented in Tables 6 and 7.

(9)

where SS and SR represent the solubility of amino acid in water media in the absence and presence of salt of concentration. C is the concentration of the salt in a salt−water binary mixture. By using the linear relationship 25,26 of log(S S /S R ) vs C (concentration), values of Ksi were measured (Table 3). The salting in constants (Table 3) of the experimental amino acids in various salt mixtures also supports the solubility values shown in Tables 1 and 2. Solubility results suggest that there is an increase in solubility by a maximum of 2−11% for KBr compared to NaBr [Table 1] in the experimental range of salt concentration for the experimental amino acids. The difference in solubility in aqueous NaBr and KBr solutions probably is due to dissimilarity in their ability of dispersion interaction and hydrogen bonding with different size amino acids. A larger K+ (ionic radius 1.38 Å)27,28 and Br¯ (ionic radius = 1.96 Å)27,28 possibly results in stronger hydrogen bonding and dispersion interactions for comparable size in forming an ion-pair complex with the zwitterionic species of amino acids.2,12,16,29

4. DISCUSSIONS 4.1. Solubility. Earlier studies17,21,22 have shown that experimental temperature and physicochemical characteristics of both the amino acid molecule and salt strongly influence the effect of salts on the solubilities of amino acids. In the current report we estimated the solubilities of glycine, DL-alanine, DLvaline, and DL-serine in water−NaBr and water−KBr solvent systems at standard temperature (298.15 K). From the experimental results (Table 1 and Figure 1) it can be clearly said that with increasing concentrations of salts in the experimental solutions the solubility of amino acids increases. The salting-in effect was found to be operative with sodium bromide and potassium bromide. This indicates that the salts affect the solubility of these amino acids considerably. Salting in effect is more pronounced for glycine in aqueous sodium D

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 6. Solubility in mol·kg−1 and ΔG0t (i), ΔG0t,cav(i), ΔG0t,d−d(i), ΔG0t,ch(i) and ΔH0t,cav (i) of Glycine, DL-Alanine, DL-Valine and a −1 DL-Serine from Water to Aqueous NaBr Solutions in Different Compositions at 298.15 K (on Mole Fraction Scale in kJ·mol ) molality NaBr mol·kg

a

−1

in water

solubility mol·kg

−1

ΔG0t (i)

ΔG0t,cav(i)

−1

−1

kJ·mol

0.00 0.50 1.00 1.50 2.00 2.50

3.389 3.516 3.651 3.710 3.920 4.060

0.000 −0.193 −0.383 −0.514 −0.737 −0.906

0.00 0.50 1.00 1.50 2.00 2.50

1.938 2.073 2.208 2.364 2.466 2.702

0.000 −0.269 −0.522 −0.782 −0.974 −1.282

0.00 0.50 1.00 1.50 2.00 2.50

0.690 0.783 0.918 1.020 1.122 1.186

0.000 −0.415 −0.906 −1.258 −1.582 −1.801

0.00 0.50 1.00 1.50 2.00 2.50

0.479 0.657 0.792 0.975 1.184 1.322

0.000 −0.885 −1.445 −2.051 −2.619 −2.975

kJ·mol

Glycine 0.000 −0.200 −0.383 −0.550 −0.704 −0.844 DL-Alanine 0.000 −0.215 −0.411 −0.590 −0.754 −0.903 DL-Valine 0.000 −0.238 −0.456 −0.653 −0.834 −0.998 DL-Serine 0.000 −0.208 −0.399 −0.572 −0.731 −0.877

ΔG0t,d−d(i)

ΔG0t,ch(i)

ΔH0t,cav(i)

−1

kJ·mol

kJ·mol−1

0.000 −0.067 −0.279 −0.641 −1.170 −1.860

0.000 0.074 0.279 0.677 1.137 1.798

0.000 −0.218 −0.411 −0.579 −0.729 −0.861

0.000 −0.057 −0.239 −0.549 −1.000 −1.590

0.000 0.003 0.128 0.357 0.780 1.211

0.000 −0.262 −0.487 −0.684 −0.859 −1.010

0.000 −0.046 −0.189 −0.436 −0.795 −1.260

0.000 −0.131 −0.261 −0.169 0.147 0.457

0.000 −0.318 −0.595 −0.838 −1.050 −1.240

0.000 −0.030 −0.126 −0.289 −0.528 −0.840

0.000 −0.647 −0.920 −1.190 −1.360 −1.258

0.000 −0.239 −0.450 −0.634 −0.797 −0.942

−1

kJ·mol

u(T) = 0.10 K.

A comparison table [Table S2] of solubility of the present amino acids was shown here to clarify the results of cation and anions of the electrolytes studied. Comparing the present solubility results of the experimental amino acids with that of the earlier in water media in the presence of various alkali metal salts it is observed that the solubility follows the order: MF < MBr (where M = Na and K) [Table S2]. This shows that amino acids are most soluble in water−KBr media while they are least soluble in water−NaF media. This means that the size and nature of the anion of the salt has a profound role in stabilizing the zwitterionic species of amino acids. Different anions have different potential in monitoring hydrophobic and dispersion interactions involving amino acids, which probably is due to their difference in nature and size. As the ionic radius of K+ is greater than that of Na+, so KF is a much stronger electrolyte than NaF. On the basis of this fact the observed salting-in activities of the present amino acids can be elucidated. The solubility of present amino acids in different water−salt mixtures increases as (Table S2)

DL-alanine, DL-valine, and DL-serine under similar experimental conditions. There might be measurement errors during the determination of solubility of this type of molecules. 4.2. Transfer Free Energetics Due to Solvent−Solute Interactions. Figure 2 and Tables 6 and 7 show the change of ΔG0t (i) for the present amino acid molecules against the mol % of electrolytes. The variation of ΔG0t (i) was mainly observed with the collective and sequential variation in interactions 0 (i)), dipole−dipole owing to cavity formation (ΔG t,cav 0 (ΔGt,d‑d(i)) and other chemical interactions (ΔG0t,ch(i)) such as hydrogen bonding, dispersion, and acid−base interaction.1,33,34 It was found that for the present experimental salt systems, the studied amino acids showed a negative increment of Gibbs energies of transfer (ΔGt0(i)) with increasing concentration of salts. This hints that the present amino acids are stabilized in the presence of higher concentration of salts. Here it is important to mention that except for glycine, the other three amino acids showed more stability, that is, more negative Gibbs energies of transfer (ΔG0t (i)) in aqueous KBr than the aqueous NaBr system. These types of variations may be due the size factors of the solute and cosolvent molecules. The smaller amino acid glycine match very well in size with comparatively smaller cosolvent molecules, that is, aqueous NaBr and hence becomes stabilized more by stronger solute− solvent interaction. On the other hand the comparatively larger amino acids match properly with the larger cosolvent molecules, that is, aqueous KBr thereby exhibiting pronounced solute−solvent interactions and becomes stabilized more in

NaF < KF < NaBr < KBr

However, for amino acids in water−Na2SO4 mixtures, the present solubility results are conflicting. A salting-in effect was observed in an earlier study by El-Dossoki15 for the present amino acids. In other studies by Islam,30 Pinho et al.,31 and Dolui et al.,32 the salting-in effect has been found only for glycine, while the salting-out effect was found for others such as E

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 7. Solubility in mol·kg−1 and ΔG0t (i), ΔG0t,cav(i), ΔG0t,d−d(i), ΔG0t,ch(i) and ΔH0t,cav (i) of Glycine, DL-Alanine, DL-Valine and a −1 DL-Serine from Water to Aqueous KBr Solutions in Different Compositions at 298.15 K (on Mole Fraction Scale in kJ·mol ) molality KBr mol·kg

−1

in water

Glycine 0.00 0.50 1.00 1.50 2.00 2.50 DL-Alanine 0.00 0.50 1.00 1.50 2.00 2.50 DL-Valine 0.00 0.50 1.00 1.50 2.00 2.50 DL-Serine 0.00 0.50 1.00 1.50 2.00 2.50

solubility mol·kg

−1

ΔG0t (i) −1

kJ·mol

ΔG0t,cav(i) −1

ΔG0t,d−d(i) −1

kJ·mol

kJ·mol

ΔG0t,ch(i)

ΔH0t,cav(i)

−1

kJ·mol−1

kJ·mol

3.389 3.537 3.667 3.765 3.854 3.983

0.000 −0.208 −0.394 −0.550 −0.695 −0.859

0.000 −0.308 −0.586 −0.837 −1.060 −1.270

0.000 −0.065 −0.273 −0.635 −1.160 −1.830

0.000 0.165 0.465 0.922 1.525 2.241

0.000 −0.332 −0.614 −0.854 −1.060 −1.240

1.938 2.078 2.224 2.389 2.547 2.723

0.000 −0.275 −0.539 −0.808 −1.054 −1.301

0.000 −0.331 −0.628 −0.896 −1.140 −1.360

0.000 −0.056 −0.235 −0.545 −0.992 −1.570

0.000 0.112 0.324 0.633 1.078 1.629

0.000 −0.396 −0.725 −1.010 −1.250 −1.460

0.690 0.804 0.934 1.112 1.246 1.328

0.000 −0.481 −0.949 −1.473 −1.841 −2.081

0.000 −0.367 −0.695 −0.990 −1.260 −1.500

0.000 −0.044 −0.187 −0.433 −0.789 −1.250

0.000 −0.070 −0.067 −0.050 0.208 0.669

0.000 −0.483 −0.889 −1.230 −1.530 −1.790

0.479 0.678 0.807 0.998 1.215 1.439

0.000 −0.964 −1.492 −2.109 − 2.684 −3.185

0.000 −0.321 −0.609 −0.869 −1.110 −1.320

0.000 −0.029 −0.124 −0.287 −0.522 −0.828

0.000 −0.614 −0.759 −0.953 −1.052 −1.037

0.000 −0.364 −0.672 −0.934 −1.160 −1.360

a

u(T) = 0.10 K; hard sphere diameters of water, NaBr, and KBr are 2.74,32 2.99, and 3.34 Å,27,28 respectively. The dipole-moment values of NaBr, KBr, and water are 9.12, 10.41, and 1.830 D, respectively, taken from the references.27,28 The required diameter of glycine, DL-alanine, DL-nor-valine, and DL-serine are 5.64, 6.16, 6.92, and 5.93 Å, respectively, which are taken from the references.1,12 The required dipole moment of glycine, DLalanine, DL-valine, and DL-serine are 15.7, 15.9, 16.0, and 11.10 D, respectively.1,12

Figure 1. Variation of saturated solubility (S) in mol·kg−1 of glycine (1,2), DL-alanine (3,4), DL-valine (5,6), and DL-serine (7,8) with mol % of salt(s) at 298.15 K.

Figure 2. Variation of ΔG0t (i) of glycine, DL-alanine, DL-valine, and DLserine with mol % of NaBr/KBr at 298.15 K.

aqueous KBr than aqueous NaBr. The stability of the amino acids in the experimental water−salt mixture follows the order: DL-serine > DL-valine > DL-alanine > glycine (Figure 2). Results of Tables 6 and 7 show that ΔG0t,cav(i) values reduce upon increasing concentrations of both the salts. This hints that amino acids attain greater stability with a higher concentration

of salts content in the binary mixtures in comparison to pure aqueous media. This may occur because the amino acid molecules may be more readily accommodated in the salt− water mixture than in pure water by releasing certain energy owing to a comparatively greater size of the salt such as KBr F

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(0.334 nm)27,28 and NaBr (0.299 nm)27,28 in aqueous solvent systems in comparison to that of pure H2O (0.274 nm).19 The ΔG0t,d‑d(i) values for the experimental solutes were observed to reduce systematically with increasing concentrations of salts and thus achieving greater stability. The dipole moments of NaBr (9.12 D)27,28 and KBr (10.41D)27,28 are higher than that of H2O (1.83 D),19 which is supportive in justifying such a tendency.2 The chemical transfer Gibbs energies, that is, ΔG0t,ch(i) for the studied solutes were evaluated through elimination of ΔG0t,cav(i) and ΔG0t,d‑d(i) from ΔG0t (i). Figure 3 exhibits the

more stable in aqueous NaBr solution than in KBr aqueous solution. The study is definitely a valuable addition in the area of amino acid research particularly in pharmaceutical, biochemical, and industrial areas.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00647. Specifications of chemical samples; comparison of the solubility data of the present work and literature results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Sanjay Roy: 0000-0001-6841-4961 Notes

The authors declare no competing financial interest.

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Figure 3. Variation of ΔG0t,ch(i) of glycine, DL-alanine, DL-serine with mol % of NaBr/KBr at 298.15 K.

DL-valine,

REFERENCES

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and

change of ΔG0t,ch(i) with concentration of salts. From Tables 6 and 7 and Figure 3 it is clear that ΔG0t,ch(i) rises systematically in the positive direction with the rise in salts concentrations for glycine, DL-alanine and DL-valine clearly indicating destabilization of such amino acids in the media. However, in the case DLserine in aqueous NaBr and KBr, ΔG0t,ch(i) decreases gradually with the increasing concentration of the electrolyte. Previous studies showed that dipole−dipole and cavity forming interactions direct the mode for stabilizing experimental amino acid molecules. However, their collective effect and other factors associated with total transfer free energy, ΔG0t (i) led to ΔG0t,ch(i), which was found to be positive for glycine, DLalanine, and DL-valine in aqueous mixtures. The deviation of such a trend in the case of DL-serine might be due to the structural effect of DL-serine (Table S1). The presence of an alcoholic group only in DL-serine in the present study makes it different from the others which definitely plays an important role in acid base and hydrogen-bonding interaction. This interaction is more effective in NaBr solution than in KBr which might be due to higher charge density of the former electrolyte cation due to smaller size. The change in enthalpy because of cavity forming interaction also supports the above results of stability of the present amino acids.

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CONCLUSION In the current study the saturated solubilities of glycine, DLalanine, DL-valine, and DL-serine were measured in water−NaBr and water−KBr solvent systems at standard temperature (298.15 K). The nature and degree of solubilities of such amino acids in the experimental solvents were analyzed in terms of salting-in effect. The stability of the experimental molecules in the solution was explained in terms of physicochemical, chemical, and structural aspects. The study concludes that glycine, DL-alanine, DL-valine, and DL-serine are G

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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H

DOI: 10.1021/acs.jced.7b00647 J. Chem. Eng. Data XXXX, XXX, XXX−XXX