Volumetric properties of amino acids in aqueous solutions of ammonium based protic ionic liquids

Fluid Phase Equilibria 385 (2015) 258–274

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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d

Volumetric properties of amino acids in aqueous solutions of ammonium based protic ionic liquids Vickramjeet Singh a , Pratap K. Chhotaray a , Parampaul K. Banipal b , Tarlok S. Banipal b, **, Ramesh L. Gardas a,1, * a b

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 August 2014 Received in revised form 8 November 2014 Accepted 12 November 2014 Available online 18 November 2014

Apparent molar volumes, V2,f of amino acids (glycine and DL-a-alanine) in water and in aqueous solutions of ammonium based protic ionic liquids (PILs), namely, propylammonium formate (PAF) and 3-hydroxypropylammonium formate (3-HPAF) determined from precise density measurements, increase with concentration of amino acids and increase in temperature (288.15–318.15) K. Partial

molar volume of transfer, DtV2 are both positive and negative in case of PAF, whereas only negative DtV2 values have been observed in case of 3-HPAF over the studied concentration range. Thermal expansion
coefficients, (@V2 /@T)P of amino acids in presence of PILs are higher as compared to that observed in water
2 and further, (@ V2 /@T2)P values are found to be negative, which suggest that studied amino acids behave as structure breakers in these solutions. Hydration number, nH for studied amino acids in aqueous PILs solutions follows the order: 3-HPAF > PAF, which suggest a strong dehydration effect of PAF. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Protic ionic liquid Amino acids Density Propylammonium formate Partial molar volume

1. Introduction The physicochemical properties of amino acids in aqueous and mixed aqueous solutions have generated a great deal of interest, as studies on these basic building blocks (amino acids) of biomolecules (proteins) provide an understanding of effects of additives on biomolecules [1–7]. Due to the complex structures of proteins, the direct investigations on these biopolymers are very challenging, so various low molecular weight model compounds such as amino acids, peptides and their derivatives have been studied extensively, in order to gain a better understanding of solute–solvent interactions and their role in the conformational stability of proteins [8–12]. Thermodynamic studies on amino acids in presence of various additives (electrolyte, non electrolytes, surfactants, drugs, etc.) as a function of concentration, temperature and pressure have been performed to understand solute–solvent interactions [13–32]. Banipal et al. [6,7,23,28] have reported volumetric properties of various amino acids in aqueous solutions of metal chlorides and organic salts. Yan et al. [26,27,30,31] have also reported various physico-chemical studies on amino acids in a variety of aqueous media. These studies can provide some insights into unfolding

* Corresponding author. Tel.: +91 44 2257 4248; fax: +91 44 2257 4202. ** Corresponding author. Tel.: +91 183 2451357; fax: +91 183 2258819/20. E-mail addresses: [email protected] (T.S. Banipal), [email protected] (R.L. Gardas). 1 http://www.iitm.ac.in/info/fac/gardas. 0378-3812/$ – see front matter ã 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2014.11.016

behavior of proteins. However, thermodynamic properties of ternary solutions containing amino acids, water and ionic liquids (ILs) are limited [33–40]. Furthermore, ILs are also employed in aqueous biphasic systems containing amino acids (consist of two immiscible aqueous-rich phases), due to the ability of tailoring the polarities and affinities by manipulating the cation or anion part of IL [41,42]. Ionic liquids are molten salts having melting point below the boiling point of water. ILs have numerous advantages over other solvents, including high conversion rates, high selectivity, better protein stability, as well as better recoverability and recyclability [43–46]. Shekaari and Jebali [34,35,38] have reported densities, viscosities, electrical conductances and refractive indices of amino acids (glycine, L-alanine and L-valine) in aqueous solutions of imidazolium based ILs at 298.15 K. They observed [34,35,38] positive DtV2
values for glycine in studied ILs and negative DtV2
values for L-alanine and L-valine in cases of 1-hexyl-3-methylimidazolium bromide [38] and 1-propyl-3-methylimidazolium bromide [35]. Fang and Ren [39] have also reported negative DtV2
values for glycine, L-alanine, and L-phenylalanine in presence of 1-ethyl-3-methylimidazolium bromide. Most of the work has been carried out in presence of imidazolium based ILs [34,35,38,39], as bulky cation of imidazolium based ILs are known to orient water molecules around their alkyl chain. Roy et al. [36] have also reported the solvation behaviour of a-amino acids in aqueous solutions of tetrabutylphosphonium tetrafluoroborate at 298.15 K. Furthermore, studies on solubility and stability of amino acids in presence of ammonium based ILs are also available [10,47], but

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

the data on thermodynamic properties of amino acids in aqueous solutions of ammonium based ILs as a function of concentration and temperature are limited. So, in order to understand the solvation behaviour of amino acids, we report herein volumetric properties of glycine and DL-a-alanine in water and in mB [molality of protic ionic liquids (PILs)] = (0.05, 0.10, 0.15, and 0.20) mol kg1 aqueous solutions of propylammonium formate (PAF) and 3-hydroxypropylammonium formate (3-HPAF) at temperatures, T = (288.15–318.15) K and at atmospheric pressure. Various derived thermodynamic parameters, such as volume of transfer, expansion coefficients, interaction parameters and hydration numbers have also been evaluated to study the effect of ILs on the solvation behaviour of amino acids. 2. Experimental 2.1. Materials The provenance, CAS number and mass fraction purity of the chemicals used are given in Table 1. The amino acids were used after drying in a vacuum oven for 24 h. For the synthesis of PILs, the 3-amino-1-propanol, propylamine and formic acid were used as received from the supplier without further purification.

2.3. Apparatus and procedure The densities, r of solutions were measured at desired temperature by using vibrating-tube digital densimeter (Model: DMA 60/602 Anton Paar, Austria), which can measure density from temperature, 263.15 K to 343.15 K with a precision of 1 103 kg m3. The densimeter cell was thermostated by using an efficient constant temperature bath (Julabo F25/Germany) with temperature stability within 0.01 K. The densimeter was calibrated with pure water and dry air. All density measurements were made relative to pure water. Densities of pure water have been taken from the literature [50]. The uncertainty in the measured densities on an average is 3.7  103 kg m3. The solutions were made fresh on mass basis in air tight glass vials by using Mettler-Toledo balance (AB 265S) having a precision of 0.01 mg. The solutions were made in deionized, doubly distilled and freshly degassed water. The water content present in PILs determined by Karl Fischer Titrator has been taken into account for molality correction of stock solutions (PILs + water). The uncertainties in molality (m), density, (r), temperature (T) and pressure (P) are u(m) = 7.2  106 mol kg1, u(r) = 3.7  103 kg m3, u(T) = 0.01 K, and u(P) = 0.5 kPa. 3. Results and discussion

2.2. Synthesis and characterization of PILs The PILs were synthesized [48,49] by exothermic neutralization of bases with formic acid. The formic acid was added drop wise with vigorous stirring to base in a two necked round bottom flask (fitted with dropping funnel) at a temperature below 283.15 K. After the complete addition, the reaction mixture was then continuously stirred for 24 h at room temperature. The resulting viscous liquids obtained were then dried at room temperature under high vacuum for two days to remove moisture and unreacted starting materials, if any. Synthesized protic ionic liquids were further stored under nitrogen atmosphere. The proton NMR of PILs was recorded on Brukar Avance 500 MHz spectrometer using deuterated DMSO as solvent. For 3HPAF, 1H NMR {(DMSO, dppm): d = 8.43 ppm (s, 1H, HCOO), d = 6.15 ppm (broad, 4H, OH and NH3 +), d = 3.45 ppm (t, 2H, CH2–N), d = 2.77 ppm (t, 2H, CH2–O), d = 1.65 ppm (qn, 2H, CH2–C)}. For PAF, d = 8.43 ppm (s, 1H, HCOO), d = 2.68 ppm (t, 2H, CH2–N), d = 1.53 (sx, 2H, CH2–C), d = 0.88 ppm (t, 3H, CH3–C). IR spectra were recorded on JASCO FT/IR-4100 spectrometer, having maximum resolution of 0.9 cm1 and the signal to noise ratio of 22,000:1. The broad band appeared in the range 3600–2600 cm1 exhibits the characteristic ammonium peak, n(N—H) and n(O—H) stretching vibration. The broad band centered around 1600 cm1 corresponds to the characteristic carbonyl, n(C¼O) stretching and d(N—H) plane bending vibrations. The Analab Karl Fischer Titrator (Micro AquaCal100) was used to determine the water content of synthesized PILs. The water content in 3-HPAF was found to be 7124 ppm and in PAF 6753 ppm. The amount of water present in PILs has been taken into account for the molality correction of the stock solutions (PILs + water). Table 1 Provenance, CAS number and mass fraction purity of the chemicals used. Chemical name

Source

CAS number

Mass fraction puritya

Glycine DL-a-Alanine Formic acid 3-Amino-1-propanol Propylamine

Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich

56-40-6 302-72-7 64-18-6 156-87-6 107-10-8

0.99 0.99 0.95 0.99 0.99

a

259

The amino acids were used after drying in a vacuum oven for 24 h. All the other chemicals were used as received without further purification.

3.1. Apparent molar volume and partial molar volume at infinite dilution The densities, r of glycine and DL-a-alanine have been measured in water and in mB = (0.05, 0.10, 0.15 and 0.20) mol kg1 aqueous solutions of two newly synthesized ammonium based PILs (PAF and 3-HPAF) at T = (288.15, 298.15, 308.15 and 318.15) K and at atmospheric pressure. The densities of amino acids in water and in aqueous PIL solutions increase with solute (amino acid) as well as with cosolute (PILs) concentration but decrease with increase in temperature. The densities of glycine and DL-a-alanine in water at different temperatures agree well with literature values [7,51–53]. Apparent molar volumes, V2,f of glycine and DLa-alanine have been determined at different temperatures using equation: V 2;f ¼ ½M=r  ½ðr  ro Þ=ðmrro Þ

(1)

where M and m, are respectively, the molar mass and molality of the solute; r and ro are the densities of solution and solvent [water or (PILs + water)]. The densities and V2,f results for amino acids in PILs solutions are given in Table 2. The combined uncertainty in the V2,f values resulting from various quantities [u(r) = 3.7  103 kg m3, u(m) = 7.2  106 mol kg1, u(T) = 0.01 K, u(P) = 0.5 kPa] ranges from 0.15  106 to 0.04  106 m3 mol1 at low (0.05 mol kg1) and high concentration ranges of the amino acids, respectively (level of confidence, k = 0.95). A representative plot of V2,f versus molality m as a function of temperature for glycine in water is shown in Fig. 1. The V2,f values for glycine increase with temperature as well as with the molality of the amino acid as the color of the plot changes from blue to orange. The dark blue color represents the lowest V2,f values at low temperature and orange color represents the highest V2,f values at 318.15 K. At infinite dilution, apparent molar volume and partial molar volume, V2
becomes same and V2
have been evaluated by leastsquares fitting of the following relation to V2,f data as:


V 2;f ¼ V 2 þ Sv m

(2)

The V2
and the experimental slope, Sv values are given in Table 3. The V2
values of amino acids in water agree well with

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literature values [7,51,54–55]. Experimental slope, Sv is also called as virial coefficient, provides information regarding solute–solute interactions [56]. The sign of Sv can be used to interpret the nature of interactions occurring between solute molecules. Positive Sv values suggest the overlap of co-spheres when two hydrophilic

Table 2 The densities, r and apparent molar volumes of glycine and T/K

mB/mol kg1a

Glycine in water 288.15

0.0

groups (—NH3+ and —COO) of amino acids interacts in solution, whereas overlap of two hydrophobic co-spheres results a negative Sv [57]. In the present study Sv values are positive for amino acids in water and in aqueous solutions of PILs. The variation of Sv with temperature and cosolute (PILs) concentration is irregular. Further,

a-alanine in water and in aqueous solutions of PILs at temperatures, T = (288.15–318.15) K.

DL-

m/mol kg1b

0.02012 0.05495 0.07199 0.09442 0.11775 0.13344 0.16565 0.18108 0.21473 298.15 0.02012 0.05495 0.07199 0.09442 0.11775 0.13344 0.16565 0.18108 0.21473 308.15 0.02012 0.05495 0.07199 0.09442 0.11775 0.13344 0.16565 0.18108 0.21473 318.15

0.05

0.02012 0.05495 0.07199 0.09442 0.11775 0.13344 0.16565 0.18108 0.21473 Glycine in aqueous PAF solutions

288.15 0.02549 0.05465 0.08165 0.08711 0.10591 0.13692 0.14142 0.15315 0.18293 298.15 0.02549 0.05465 0.08165 0.08711 0.10591 0.13692 0.14142 0.15315 0.18293 308.15

0.02549 0.05465 0.08165

r/kg m3 (ro = 999.129)c 999.786 1000.918 1001.469 1002.190 1002.939 1003.438 1004.465 1004.953 1006.016 (ro = 997.047)c 997.686 998.784 999.318 1000.017 1000.740 1001.223 1002.212 1002.682 1003.703 (ro = 994.063)c 994.693 995.780 996.308 997.001 997.721 998.201 999.187 999.655 1000.676 (ro = 990.244)c 990.867 991.940 992.463 993.148 993.859 994.334 995.309 995.774 996.781 (ro = 999.508)c 1000.360 1001.330 1002.222 1002.400 1003.020 1004.034 1004.180 1004.561 1005.525 (ro = 997.432)c 998.257 999.196 1000.058 1000.229 1000.826 1001.804 1001.940 1002.305 1003.232 (ro = 994.409)c 995.218 996.139 996.988

V2,f106/m3 mol1

42.40 42.45 42.47 42.53 42.56 42.60 42.64 42.67 42.71 43.32 43.42 43.46 43.52 43.58 43.63 43.70 43.74 43.82 43.80 43.82 43.86 43.90 43.92 43.95 43.99 44.02 44.06 44.20 44.26 44.28 44.31 44.34 44.37 44.40 44.42 44.47

41.60 41.66 41.72 41.75 41.77 41.83 41.84 41.87 41.93 42.67 42.75 42.82 42.87 42.91 42.98 43.03 43.07 43.14 43.36 43.40 43.44

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

261

Table 2 (Continued) T/K

mB/mol kg1a

318.15

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.08711 0.10591 0.13692 0.14142 0.15315 0.18293

997.156 997.742 998.707 998.843 999.204 1000.116 (ro = 990.560)c 991.354 992.256 993.086 993.252 993.827 994.771 994.903 995.257 996.150 (ro = 999.895)c 1001.077 1001.635 1002.425 1002.837 1003.581 1004.339 1004.800 1005.124 1005.845 (ro = 997.801)c 998.942 999.481 1000.247 1000.644 1001.366 1002.102 1002.547 1002.864 1003.563 (ro = 994.813)c 995.934 996.464 997.215 997.605 998.313 999.031 999.468 999.781 1000.468 (ro = 990.922)c 992.021 992.540 993.276 993.657 994.353 995.060 995.487 995.793 996.466 (ro = 1000.280)c 1001.216 1001.813 1001.965 1002.338 1003.342 1003.548 1004.938 1005.438 1006.115 (ro = 998.180)c 999.085 999.662 999.809 1000.170 1001.141 1001.341 1002.686 1003.168 1003.824 (ro = 995.216)c

43.48 43.52 43.56 43.59 43.62 43.69

0.02549 0.05465 0.08165 0.08711 0.10591 0.13692 0.14142 0.15315 0.18293 0.10

288.15

0.03659 0.05395 0.07870 0.09166 0.11521 0.13923 0.15389 0.16439 0.18755

298.15 0.03659 0.05395 0.07870 0.09166 0.11521 0.13923 0.15389 0.16439 0.18755 308.15

0.03659 0.05395 0.07870 0.09166 0.11521 0.13923 0.15389 0.16439 0.18755

318.15

0.03659 0.05395 0.07870 0.09166 0.11521 0.13923 0.15389 0.16439 0.18755 0.15

288.15

0.02903 0.04761 0.05236 0.06406 0.09558 0.10214 0.14613 0.16213 0.18387

298.15

0.02903 0.04761 0.05236 0.06406 0.09558 0.10214 0.14613 0.16213 0.18387

308.15

44.01 44.08 44.14 44.17 44.20 44.25 44.29 44.32 44.39 42.72 42.74 42.81 42.85 42.92 42.96 42.99 43.04 43.09 43.86 43.88 43.91 43.96 44.00 44.02 44.05 44.08 44.12 44.45 44.47 44.51 44.56 44.61 44.66 44.69 44.70 44.74 45.12 45.14 45.19 45.24 45.27 45.30 45.34 45.36 45.40 42.77 42.80 42.82 42.85 42.90 42.93 42.99 43.03 43.08 43.87 43.90 43.92 43.94 43.99 44.01 44.06 44.11 44.15

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Table 2 (Continued) T/K

mB/mol kg1a

318.15

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.02903 0.04761 0.05236 0.06406 0.09558 0.10214 0.14613 0.16213 0.18387

44.51 44.54 44.57 44.60 44.65 44.69 44.76 44.80 44.83

0.03436 0.04875 0.07008 0.08831 0.11297 0.12174 0.13983 0.15074 0.17669

996.104 996.669 996.813 997.167 998.118 998.312 999.626 1000.099 1000.744 (ro = 991.285)c 992.155 992.710 992.851 993.197 994.128 994.319 995.607 996.069 996.697 (ro = 1000.663)c 1001.777 1002.242 1002.929 1003.514 1004.301 1004.583 1005.159 1005.504 1006.328 (ro = 998.553)c 999.634 1000.084 1000.750 1001.316 1002.081 1002.350 1002.910 1003.244 1004.041 (ro = 995.619)c 996.679 997.121 997.774 998.330 999.079 999.345 999.892 1000.222 1001.003 (ro = 991.647)c 992.684 993.116 993.755 994.298 995.027 995.287 995.822 996.141 996.904

0.02482 0.04387 0.06246 0.06721 0.07269 0.11285 0.12881 0.15951 0.17856 0.02482 0.04387 0.06246 0.06721 0.07269 0.11285 0.12881 0.15951

999.857 1000.410 1000.948 1001.083 1001.240 1002.386 1002.839 1003.703 1004.234 997.759 998.303 998.830 998.963 999.116 1000.236 1000.678 1001.525

59.76 59.83 59.89 59.92 59.95 60.06 60.09 60.17 60.22 60.45 60.48 60.53 60.56 60.60 60.73 60.78 60.84

0.02903 0.04761 0.05236 0.06406 0.09558 0.10214 0.14613 0.16213 0.18387 0.20

288.15

0.03436 0.04875 0.07008 0.08831 0.11297 0.12174 0.13983 0.15074 0.17669

298.15

0.03436 0.04875 0.07008 0.08831 0.11297 0.12174 0.13983 0.15074 0.17669

308.15 0.03436 0.04875 0.07008 0.08831 0.11297 0.12174 0.13983 0.15074 0.17669 318.15

DL-a-alanine in water 288.15

298.15

0.0

45.18 45.20 45.22 45.27 45.33 45.36 45.43 45.48 45.53 42.58 42.61 42.63 42.66 42.70 42.70 42.72 42.74 42.76 43.59 43.62 43.64 43.68 43.70 43.73 43.74 43.76 43.79 44.24 44.26 44.28 44.31 44.35 44.36 44.38 44.39 44.42 44.97 45.00 45.02 45.06 45.12 45.13 45.15 45.18 45.21

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

263

Table 2 (Continued) T/K

mB/mol kg1a

308.15

318.15

288.15

0.05

298.15

308.15

318.15

288.15

298.15

308.15

0.10

m/mol kg1b

r/kg m3

0.17856 1002.040 0.02482 994.763 0.04387 995.297 0.06246 995.816 0.06721 995.947 0.07269 996.099 0.11285 997.211 0.12881 997.650 998.487 0.15951 0.17856 999.005 0.02482 990.936 0.04387 991.464 0.06246 991.976 0.06721 992.106 0.07269 992.256 993.354 0.11285 0.12881 993.784 0.15951 994.610 0.17856 995.117 DL-a-Alanine in aqueous PAF solutions 0.03679 1000.618 0.04092 1000.742 0.07720 1001.824 0.08841 1002.156 0.10084 1002.522 0.12681 1003.286 0.15516 1004.118 0.18546 1004.996 0.23205 1006.340 0.03679 998.513 0.04092 998.633 0.07720 999.691 0.08841 1000.013 0.10084 1000.373 0.12681 1001.117 0.15516 1001.929 0.18546 1002.790 0.23205 1004.096 0.03679 995.464 0.04092 995.582 0.07720 996.611 0.08841 996.927 0.10084 997.276 0.12681 998.004 0.15516 998.796 0.18546 999.638 0.23205 1000.922 0.03679 991.598 0.04092 991.713 0.07720 992.728 0.08841 993.039 0.10084 993.382 0.12681 994.101 0.15516 994.881 0.18546 995.710 0.23205 996.972 0.02398 1000.595 0.03972 1001.053 0.06853 1001.887 0.09230 1002.571 0.12607 1003.539 0.14061 1003.953 0.16533 1004.658 0.19400 1005.470 0.21295 1006.004 0.02398 998.485 0.03972 998.932 0.06853 999.746 0.09230 1000.415 0.12607 1001.361 0.14061 1001.766 0.16533 1002.453 0.19400 1003.247 0.21295 1003.766 0.02398 995.481 0.03972 995.918 0.06853 996.713

V2,f106/m3 mol1 60.92 61.05 61.07 61.12 61.14 61.15 61.20 61.22 61.28 61.31 61.50 61.54 61.58 61.60 61.61 61.67 61.72 61.78 61.83 58.86 58.88 58.97 59.00 59.04 59.09 59.12 59.19 59.26 59.73 59.74 59.77 59.82 59.83 59.89 59.92 59.96 60.05 60.53 60.54 60.61 60.64 60.66 60.70 60.73 60.76 60.81 61.13 61.14 61.18 61.21 61.24 61.26 61.29 61.32 61.38 59.86 59.88 59.91 59.94 59.97 59.99 60.00 60.02 60.04 60.60 60.62 60.66 60.68 60.71 60.72 60.74 60.76 60.79 61.36 61.38 61.42

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Table 2 (Continued) T/K

mB/mol kg1a

318.15

288.15

0.15

298.15

308.15

318.15

288.15

298.15

308.15

0.20

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.09230 0.12607 0.14061 0.16533 0.19400 0.21295 0.02398 0.03972 0.06853 0.09230 0.12607 0.14061 0.16533 0.19400 0.21295 0.02875 0.04747 0.06546 0.08858 0.10286 0.12031 0.13452 0.15481 0.18604 0.02875 0.04747 0.06546 0.08858 0.10286 0.12031 0.13452 0.15481 0.18604 0.02875 0.04747 0.06546 0.08858 0.10286 0.12031 0.13452 0.15481 0.18604 0.02875 0.04747 0.06546 0.08858 0.10286 0.12031 0.13452 0.15481 0.18604 0.03681 0.07640 0.08316 0.08979 0.11065 0.13470 0.14133 0.16347 0.18865 0.03681 0.07640 0.08316 0.08979 0.11065 0.13470 0.14133 0.16347 0.18865 0.03681 0.07640 0.08316 0.08979 0.11065 0.13470 0.14133 0.16347

997.366 998.289 998.685 999.352 1000.127 1000.631 991.578 992.007 992.787 993.428 994.330 994.717 995.373 996.132 996.624 1001.113 1001.649 1002.163 1002.819 1003.220 1003.711 1004.106 1004.670 1005.529 998.992 999.517 1000.017 1000.657 1001.046 1001.524 1001.909 1002.462 1003.296 996.006 996.516 997.002 997.623 998.002 998.464 998.836 999.371 1000.174 992.064 992.567 993.048 993.665 994.041 994.501 994.872 995.405 996.214 1001.727 1002.857 1003.049 1003.238 1003.828 1004.504 1004.686 1005.305 1006.001 999.588 1000.690 1000.876 1001.058 1001.632 1002.291 1002.469 1003.073 1003.748 996.624 997.696 997.876 998.053 998.611 999.252 999.425 1000.012

61.45 61.48 61.49 61.53 61.55 61.59 62.00 62.03 62.08 62.10 62.16 62.18 62.21 62.23 62.28 60.07 60.15 60.20 60.27 60.32 60.36 60.41 60.46 60.55 60.85 60.91 60.97 61.04 61.11 61.15 61.20 61.23 61.34 61.73 61.79 61.86 61.93 62.00 62.06 62.12 62.16 62.30 62.26 62.30 62.35 62.38 62.43 62.47 62.51 62.53 62.60 60.11 60.22 60.23 60.24 60.27 60.32 60.36 60.39 60.45 60.97 61.03 61.06 61.09 61.12 61.16 61.19 61.21 61.28 61.87 61.93 61.96 61.98 62.02 62.05 62.08 62.10

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

265

Table 2 (Continued) T/K

mB/mol kg1a

318.15

288.15

0.05

298.15

308.15

318.15

0.10 288.15

298.15

308.15

m/mol kg1b

r/kg m3

0.18865 1000.669 0.03681 992.639 0.07640 993.697 0.08316 993.875 0.08979 994.049 0.11065 994.601 0.13470 995.233 0.14133 995.404 995.982 0.16347 0.18865 996.636 Glycine in aqueous 3-HPAF solutions (ro = 1000.067)c 0.02549 1000.920 0.05465 1001.891 0.08165 1002.787 1002.966 0.08711 0.10591 1003.587 0.13692 1004.607 0.14142 1004.750 0.15315 1005.131 0.18293 1006.103 (ro = 997.857)c 0.02549 998.684 0.05465 999.624 0.08165 1000.488 0.08711 1000.659 0.10591 1001.256 0.13692 1002.236 0.14142 1002.372 0.15315 1002.739 0.18293 1003.667 (ro = 994.830)c 0.02549 995.641 0.05465 996.563 0.08165 997.414 0.08711 997.583 0.10591 998.170 0.13692 999.137 0.14142 999.273 0.15315 999.632 0.18293 1000.551 (ro = 991.083)b 0.02549 991.880 0.05465 992.787 0.08165 993.624 0.08711 993.791 0.10591 994.369 0.13692 995.319 0.14142 995.452 0.15315 995.807 0.18293 996.706 (ro = 1000.950)c 0.03360 1002.065 0.04959 1002.592 0.07086 1003.291 0.07863 1003.545 0.10117 1004.283 0.10552 1004.422 0.11830 1004.838 0.15492 1006.020 0.18715 1007.054 (ro = 998.637)c 0.03360 999.717 0.04959 1000.228 0.07086 1000.904 0.07863 1001.148 0.10117 1001.860 0.10552 1001.996 0.11830 1002.396 0.15492 1003.541 0.18715 1004.536 (ro = 995.589)c 0.03360 996.650 0.04959 997.149 0.07086 997.814 0.07863 998.055 0.10117 998.755

V2,f106/m3 mol1 62.16 62.37 62.43 62.46 62.48 62.51 62.54 62.57 62.60 62.63

41.55 41.61 41.64 41.67 41.69 41.72 41.76 41.79 41.82 42.61 42.69 42.76 42.81 42.85 42.92 42.97 43.01 43.08 43.29 43.35 43.37 43.41 43.45 43.49 43.52 43.57 43.61 43.86 43.93 43.95 43.98 44.01 44.06 44.10 44.13 44.20 41.83 41.89 41.92 41.95 41.98 42.01 42.03 42.12 42.19 42.89 42.93 43.00 43.04 43.09 43.11 43.15 43.22 43.31 43.51 43.59 43.62 43.65 43.69

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V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

Table 2 (Continued) T/K

mB/mol kg1a

318.15

288.15

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.10552 0.11830 0.15492 0.18715

998.887 999.280 1000.400 1001.378 (ro = 991.751)c 992.795 993.287 993.941 994.177 994.864 994.995 995.379 996.477 997.437 (ro = 1001.777)c 1002.816 1003.305 1003.612 1004.334 1005.298 1005.753 1006.358 1007.115 1008.141 (ro = 999.417)c 1000.425 1000.899 1001.195 1001.894 1002.827 1003.266 1003.849 1004.576 1005.563 (ro = 996.348)c 997.340 997.806 998.099 998.787 999.707 1000.141 1000.718 1001.439 1002.422 (ro = 992.439)c 993.416 993.876 994.165 994.842 995.749 996.174 996.744 997.448 998.418 (ro = 1002.696)c 1003.535 1004.441 1005.014 1005.265 1005.995 1006.133 1006.542 1007.834 1008.732 (ro = 1000.198)c 1001.012 1001.890 1002.444 1002.688 1003.392 1003.524 1003.924 1005.174 1006.039 (ro = 997.107)c 997.908 998.773

43.73 43.76 43.86 43.94

0.03360 0.04959 0.07086 0.07863 0.10117 0.10552 0.11830 0.15492 0.18715 0.15

0.03147 0.04633 0.05572 0.07774 0.10733 0.12141 0.14018 0.16359 0.19587

298.15

0.03147 0.04633 0.05572 0.07774 0.10733 0.12141 0.14018 0.16359 0.19587

308.15

0.03147 0.04633 0.05572 0.07774 0.10733 0.12141 0.14018 0.16359 0.19587

318.15

0.03147 0.04633 0.05572 0.07774 0.10733 0.12141 0.14018 0.16359 0.19587 0.20

288.15

0.02553 0.05328 0.07086 0.07863 0.10117 0.10552 0.11830 0.15876 0.18715

298.15

0.02553 0.05328 0.07086 0.07863 0.10117 0.10552 0.11830 0.15876 0.18715

308.15

0.02553 0.05328

44.06 44.14 44.18 44.22 44.27 44.29 44.35 44.47 44.55 41.99 42.02 42.04 42.06 42.10 42.14 42.18 42.20 42.29 43.00 43.02 43.08 43.11 43.16 43.21 43.27 43.32 43.43 43.56 43.57 43.61 43.63 43.67 43.71 43.75 43.77 43.84 44.08 44.08 44.12 44.15 44.18 44.24 44.27 44.33 44.38 42.14 42.21 42.23 42.26 42.30 42.33 42.37 42.46 42.53 43.15 43.23 43.27 43.29 43.36 43.40 43.41 43.51 43.60 43.70 43.76

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

267

Table 2 (Continued) T/K

mB/mol kg1a

318.15

DL-a-alanine in aqueous 3-HPAF solutions 288.15 0.05

298.15

308.15

318.15

288.15

298.15

308.15

0.10

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.07086 0.07863 0.10117 0.10552 0.11830 0.15876 0.18715

43.80 43.81 43.86 43.89 43.93 43.99 44.06

0.02553 0.05328 0.07086 0.07863 0.10117 0.10552 0.11830 0.15876 0.18715

999.319 999.559 1000.254 1000.386 1000.776 1002.013 1002.870 (ro = 993.147)c 993.936 994.787 995.324 995.560 996.241 996.370 996.752 997.961 998.798

0.02733 0.04260 0.06170 0.09757 0.10527 0.12846 0.14051 0.16380 0.21551 0.02733 0.04260 0.06170 0.09757 0.10527 0.12846 0.14051 0.16380 0.21551 0.02733 0.04260 0.06170 0.09757 0.10527 0.12846 0.14051 0.16380 0.21551 0.02733 0.04260 0.06170 0.09757 0.10527 0.12846 0.14051 0.16380 0.21551 0.03588 0.04581 0.07743 0.07697 0.10578 0.11992 0.14683 0.17541 0.18751 0.03588 0.04581 0.07743 0.07697 0.10578 0.11992 0.14683 0.17541 0.18751 0.03588 0.04581 0.07743 0.07697 0.10578

1000.894 1001.353 1001.923 1002.992 1003.216 1003.898 1004.249 1004.927 1006.427 998.664 999.113 999.673 1000.714 1000.936 1001.601 1001.943 1002.607 1004.065 995.617 996.053 996.596 997.610 997.825 998.474 998.811 999.455 1000.882 991.858 992.289 992.826 993.829 994.043 994.687 995.019 995.660 997.074 1002.025 1002.319 1003.256 1003.241 1004.089 1004.502 1005.288 1006.115 1006.464 999.690 999.980 1000.901 1000.886 1001.719 1002.126 1002.898 1003.713 1004.056 996.611 996.892 997.783 997.768 998.573

58.78 58.83 58.90 58.94 58.99 59.04 59.08 59.13 59.20 59.56 59.59 59.62 59.70 59.72 59.79 59.83 59.87 59.98 60.42 60.48 60.53 60.59 60.62 60.67 60.68 60.74 60.81 60.97 60.99 61.02 61.07 61.08 61.11 61.13 61.16 61.22 59.03 59.09 59.14 59.16 59.20 59.23 59.26 59.31 59.33 59.71 59.73 59.76 59.78 59.81 59.83 59.86 59.89 59.91 60.68 60.71 60.77 60.79 60.84

44.24 44.31 44.34 44.36 44.44 44.48 44.53 44.63 44.72

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V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

Table 2 (Continued) T/K

mB/mol kg1a

318.15

288.15

0.15

298.15

308.15

318.15

288.15

298.15

308.15

318.15

0.20

m/mol kg1b

r/kg m3

V2,f106/m3 mol1

0.11992 0.14683 0.17541 0.18751 0.03588 0.04581 0.07743 0.07697 0.10578 0.11992 0.14683 0.17541 0.18751 0.03985 0.05831 0.06215 0.07843 0.10418 0.14333 0.16584 0.18109 0.21066 0.03985 0.05831 0.06215 0.07843 0.10418 0.14333 0.16584 0.18109 0.21066 0.03985 0.05831 0.06215 0.07843 0.10418 0.14333 0.16584 0.18109 0.21066 0.03985 0.05831 0.06215 0.07843 0.10418 0.14333 0.16584 0.18109 0.21066 0.03983 0.06304 0.07205 0.07464 0.10179 0.11078 0.13948 0.16490 0.18451 0.03983 0.06304 0.07205 0.07464 0.10179 0.11078 0.13948 0.16490 0.18451 0.03983 0.06304 0.07205 0.07464 0.10179 0.11078 0.13948 0.16490 0.18451 0.03983

998.966 999.707 1000.493 1000.824 992.761 993.039 993.920 993.905 994.704 995.090 995.828 996.603 996.928 1002.966 1003.513 1003.625 1004.106 1004.862 1006.005 1006.656 1007.096 1007.943 1000.580 1001.117 1001.227 1001.697 1002.438 1003.556 1004.197 1004.624 1005.458 997.478 997.997 998.104 998.559 999.278 1000.362 1000.980 1001.397 1002.203 993.552 994.064 994.169 994.618 995.324 996.388 996.997 997.405 998.199 1003.878 1004.559 1004.822 1004.897 1005.688 1005.945 1006.774 1007.500 1008.048 1001.356 1002.026 1002.286 1002.358 1003.134 1003.389 1004.204 1004.922 1005.469 998.229 998.878 999.129 999.199 999.951 1000.197 1000.986 1001.678 1002.211 994.255

60.87 60.94 60.98 61.00 61.15 61.17 61.22 61.24 61.27 61.32 61.35 61.41 61.44 59.13 59.17 59.19 59.21 59.24 59.29 59.33 59.35 59.40 59.85 59.86 59.87 59.90 59.93 59.98 60.00 60.04 60.07 60.78 60.82 60.85 60.88 60.91 60.96 61.00 61.02 61.06 61.34 61.37 61.40 61.42 61.47 61.55 61.58 61.62 61.65 59.26 59.34 59.37 59.39 59.44 59.49 59.53 59.59 59.68 59.93 59.97 59.98 60.01 60.06 60.09 60.12 60.15 60.20 60.95 60.99 61.00 61.03 61.07 61.10 61.14 61.19 61.21 61.43

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

269

Table 2 (Continued) mB/mol kg1a

T/K

m/mol kg1b 0.06304 0.07205 0.07464 0.10179 0.11078 0.13948 0.16490 0.18451

r/kg m3 994.896 995.144 995.214 995.958 996.202 996.982 997.669 998.196

V2,f106/m3 mol1 61.47 61.48 61.50 61.53 61.56 61.59 61.62 61.65

Standard uncertainties u are u(m) = 7.2  106 mol kg1,u(r) = 3.7  103 kg m3, u(T) = 0.01 K, u(P) = 0.5 kPa and the combined uncertainty Uc is Uc (Vf) = (0.15– 0.04)  106 m3 mol–1 for low and high concentration range of amino acids, respectively (level of confidence, k = 0.95). The amount of water present in PILs has been taken into account for the molality correction of the stock solutions (PILs + water). a mB is the molality of PILs (PAF/3-HPAF) in water. b m is the molality of amino acids in water or water + PILs solutions. c ro is the density of solvent (water or water + PILs).

it has been reported that Sv is influenced by a number of effects [56] and its interpretation becomes a difficult task [56], so in the present study the detailed discussion has been avoided. 3.2. Volume of transfer, interaction coefficients and expansibility The partial molar volumes of transfer, DtV2
of amino acids from water to aqueous PILs solutions have been calculated as:





Dt V 2 ¼ V 2 ðin aqueous PAF or 3  HPAF solutions Þ


 V 2 ðin waterÞ

(3)

The variation of DtV2
values with mB are shown in Figs. 2 and 3. The transfer volumes, DtV2
are both positive and negative in case of PAF, whereas in case of 3-HPAF, the DtV2
values are negative at all temperatures studied for both the amino acids. The DtV2
values after passing through minima in the lower concentration range start increasing with concentration of PILs at all temperatures, except for glycine in case of PAF, where DtV2
values increase upto mB  0.15 mol kg1, afterwards DtV2
values start decreasing. The DtV2
values for glycine in presence of PAF are negative upto 0.087 mol kg1 at 288.15 K, 0.080 mol kg1 at 298.15 K, 0.074 mol kg1 at 308.15 K and 0.062 mol kg1 at 318.15 K, whereas in case of DL-a-alanine the transfer volumes are negative upto slightly higher concentration of PAF i.e. 0.092 mol kg1 at 288.15 K,

Fig. 1. Plot of apparent molar volumes, Vf versus molalities, m of glycine in water at temperatures, T = (288.15, 298.15, 308.15, and 318.15) K.

0.086 mol kg1 at 298.15 K, 0.081 mol kg1 at 308.15 K and 0.073 mol kg1 at 318.15 K. It may be noted that the DtV2
values pass through minimum at about 0.05 mol kg1 of PAF for both the amino acids at all temperatures studied. In case of 3-HPAF, DtV2
values for DL-a-alanine are more negative and DtV2
values also pass through minima for both the amino acids which lie at concentrations of 3-HPAF higher than 0.05 mol kg1 as observed in case of PAF. Therefore, interactions occurring in these cases are dependent upon the concentrations of PILs. Overall, DtV2
values are higher for glycine in comparison to DL-a-alanine. The presence of hydrophobic group in DL-a-alanine may be responsible for the lower DtV2
values as compared to glycine. Shekaari and Jebali [38] have also reported decrease in transfer values with increase in hydrophobic group of amino acid in aqueous solutions of 1-propyl3-methylimidazolium bromide. The type of interactions possible between solute and cosolute in ternary solutions (amino acid + PIL + water) are: (1) hydrophilicionic interactions between ions of PILs (—NH3+, HCOO) and hydrophilic group (NH3+, COO) of amino acid; (2) hydrophobicionic interactions between nonpolar groups of amino acid and ions of PILs; (3) hydrophobic–hydrophobic interactions between the hydrophobic groups of amino acid and alkyl groups of PILs; (4) hydrophilic–hydrophobic interactions between hydrophilic groups of amino acids and the hydrophobic parts of the cation of 3-HPAF and PAF. Considering co-sphere overlap model [58], the overlap of the hydration co-spheres of hydrophilic and ionic part (type 1) results in positive DtV2
values, whereas, other types of interactions (types 2, 3 and 4) result in negative transfer volumes. The positive DtV2
values in case of PAF indicate that type 1 interactions predominate over the type (2, 3 and 4) interactions. Both positive and negative DtV2
values observed in case of PAF suggest the presence of competing interactions between amino acids and ions of PAF, which depends upon the concentration of PAF. This type of behavior shows that with the increase of concentration of PAF, the contributions from hydrophilic-ionic types of interactions increases and dominates over the other types of interactions at higher concentration of PAF. Such type of variation in DtV2
values from negative to positive with increase in concentration of cosolute has also been reported for glycine in aqueous solutions of 1-ethyl-3-methylimidazolium bromide [39], L-valine, L-phenylalanine and L-leucine in aqueous glycerol solutions [59] and also for L-alanine in aqueous solutions of
N-methylformamide [60]. However, the negative DtV2 values in aqueous 3- HPAF and PAF solutions suggest the dominance of
hydrophobic type of interactions. The DtV2 values follow the order: PAF > 3-HPAF, which suggests that introduction of hydroxyl group in cationic part of 3-HPAF, cause the strengthening of interactions between cosolute and water, which may be responsible for reduction of interactions between cosolute and amino acids.

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V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

Table 3
Partial molar volumes, V2 at infinite dilution of amino acids in water and in aqueous PILs solutions at T = (288.15–318.15) K.


mB/mol kg1

V2 106/m3 mol1a

T/K

288.15

298.15

308.15

318.15

42.36  0.01b (1.66)c [42.42d , 42.48e , 42.40f]

43.28  0.01 (2.56) [43.35d , 43.26e, 43.30f , 43.24g ] 42.59  0.02 (3.04) 43.79  0.01 (1.74) 43.82  0.01 (1.75) 43.55  0.01 (1.40)

43.76  0.01 (1.40) [43.79d , 43.80e]

44.18  0.01 (1.35) [44.14d , 44.17g ]

43.29  0.01 (2.12) 44.37  0.01 (2.04) 44.46  0.01 (2.10) 44.20  0.01 (1.30)

43.95  0.01 (2.37) 45.05  0.01 (1.88) 45.11  0.01 (2.29) 44.91  0.01 (1.76)

60.36  0.02 (3.16) [60.37d , 60.47e , 60.49g ] 59.67  0.01 (1.03) 60.59  0.01 (0.91) 60.77  0.01 (3.11) 60.89  0.01 (2.02)

61.01  0.01 (1.67) [61.02d, 60.90f , 61.01g ]

61.45  0.01 (2.08) [61.35d , 61.31g ]

60.50  0.02 (1.44) 61.34  0.01 (1.14) 61.62  0.01 (3.60) 61.80  0.01 (1.90)

61.09  0.01 (1.26) 61.97  0.01 (1.42) 62.20  0.01 (2.19) 62.31  0.01 (1.75)

42.53  0.02 (3.04) 42.81  0.01 (2.73) 42.91  0.02 (2.54) 43.08  0.01 (2.77)

43.23  0.02 (2.08) 43.43  0.01 (2.73) 43.50  0.01 (1.69) 43.64  0.01 (2.25)

43.80  0.02 (2.11) 43.96  0.01 (3.18) 44.00  0.01 (1.91) 44.14  0.02 (3.08)

59.49  0.01 (2.31) 59.67  0.01 (1.28) 59.79  0.01 (1.33) 59.86  0.02 (1.84)

60.39  0.02 (2.05) 60.61  0.01 (2.12) 60.74  0.01 (1.56) 60.88  0.01 (1.86)

60.94  0.01 (1.28) 61.08  0.01 (1.87) 61.27  0.01 (1.86) 61.38  0.01 (1.47)

In aqueous PAF solutions Glycine 0.00

41.55  0.01 (2.07) 42.62  0.01 (2.51) 42.72  0.01 (1.95) 42.54  0.01 (1.27)

0.05 0.10 0.15 0.20

DL-a-Alanine 0.00

59.71  0.02 (2.93) [59.93d , 59.67e , 59.90f ] 58.81  0.02 (2.03) 59.85  0.01 (0.94) 60.00  0.01 (2.99) 60.04  0.01 (2.16)

0.05 0.10 0.15 0.20

In aqueous 3-HPAF solutions Glycine 0.05 41.51  0.01 (1.72) 0.10 41.76  0.01 (2.29) 0.15 41.93  0.01 (1.72) 0.20 42.07  0.01 (2.44) DL-a-Alanine 0.05

58.74  0.02 (2.28) 59.00  0.02 (1.81) 59.09  0.01 (1.48) 59.17  0.02 (2.66)

0.10 0.15 0.20

a b c d e f g

mB, molality of PILs (PAF/3-HPAF) in water. Standard deviation. Sv/m3 kg mol2. Ref. [7]. Ref. [51]. Ref. [54]. Ref. [55].




The magnitude of V2 values of amino acids in PILs can also be explained by considering the modified equation of Shahidi and Farrell [61]


V 2 ¼ V v:w þ V void  Vshrinkage

(4)

where, Vv.w is the van der Waal’s volume, Vvoid is the associated void volume and Vshrinkage is the shrinkage in volume that arises from electrostriction of solvent caused by hydrophilic groups present in solute. Assuming that Vv.w and Vvoid remain almost unchanged in PIL solutions, the positive DtV2
values observed at higher concentrations of PAF, can therefore be attributed to a decrease in the shrinkage, whereas negative DtV2
values may be due to increase in Vshrinkage.

The volumetric interaction coefficients have been calculated
from transfer parameter, DtV2 based on McMillan–Mayer theory of solutions [62,63] by using the equation:


Dt V 2 ¼ 2V AB mB þ 3V ABB mB2 þ . . .

(5)

where A denotes solute (amino acid) and B denotes PIL (PAF/3-HPAF), constants VAB and VABB are pair and triplet volumetric interaction coefficients, respectively. The values of VAB increase whereas that of VABB decrease with rise of temperature (Table 4). In presence of PAF, pair interaction coefficients, VAB for glycine are positive (except at 288.15 K), whereas their values for DL-a-alanine are negative (except at 318.15 K). The triplet, VABB coefficients are positive for glycine (except at 308.15 K and

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

(a)

271

(b)

0 0

1.2

0.05

0.1

0.15

0.2

-0.2

1 0.6 0.4 0.2 0 -0.2 0

0.05

0.1

0.15

0.2

∆ tV 2 º . 1 0 6 /m 3 . m o l -1

∆tV 2 º.10 6 /m 3.m ol -1

0.8

-0.4

-0.4 -0.6 -0.8 -1

-0.6 -0.8

-1.2

-1

m B/mol ●kg-1

m B/m ol●kg-1

Fig. 2. Plots of standard partial molar volumes of transfer DtV
2 versus molalities, mB of PAF (propylammonium formate) of (a) glycine and (b) DL-a-alanine at temperatures, T = ^, 288.15 K; &, 298.15 K; ~, 308.15 K; , 318.15 K.

318.15 K) and DL-a-alanine. In case of 3-HPAF, VAB values are negative, whereas VABB values are positive for both the amino acids. Overall, the values of pair interactions coefficients are higher in PAF solutions as compared to 3-HPAF solutions, which again suggest that amino acid-PAF interactions are stronger.
The temperature dependence of V2 can be expressed as:


V 2 ¼ v0 þ v1 T þ v2 T 2

(6)




solute should have negative (@2V2 /@T2)P values, where as positive
(@2V2 /@T2)P values suggest that solute behaves as structure maker
(kosmotrope). The negative (@2V2 /@T2)P values for amino acids in water and in aqueous solutions of PAF and 3-HPAF suggest that these amino acids behave as structure breaker in these solutions. It is well documented in literature that glycine and DL-a-alanine behave as structure breaker in water and also in aqueous solutions of some co-solutes [7,66,67].




where n0, n1 and n2 are constants. The derivative of V2 with respect
to temperature at constant pressure, (@V2 /@T)P and its second
2 2 derivative, (@ V2 /@T )P have been evaluated from Eq. (6). The
partial molar expansibility, (@V2 /@T)P values of amino acids in
water agree well with literature values [7,54,64]. The (@V2 /@T)P values in PIL solutions are higher as compared to their values in water and decrease with increase in temperature (Table 5). Hepler

[65] used the thermodynamic relation; (@Cp,2 /@P) T = –T (@2V2 / 2 @T )P by which qualitative information regarding hydration of a solute could be judged from thermal expansion of aqueous solutions. According to this relation [65], the structure breaking

3.3. Hydration numbers The hydration number, nH for the amino acids in water and in
aqueous solutions of PILs (Table 6) have been calculated from V2 data by using the method reported by Millero et al. [68] as:








V 2 ¼ V 2 ðintÞ þ V 2 ðelectÞ


(7)




where V2 (int) and V2 (elect) are the intrinsic and electrostriction
partial molar volumes of the amino acids, respectively. The V2 (int) further consists of two terms; van der Waal’s volume and volume

Fig. 3. Plots of standard partial molar volumes of transfer DtV
2 versus molalities, mB of 3-HPAF (3-hydroxypropylammonium formate) of (a) glycine and (b) DL-a-alanine at temperatures, T = ^, 288.15 K; &, 298.15 K; ~,308.15 K; , 318.15 K.

272

V. Singh et al. / Fluid Phase Equilibria 385 (2015) 258–274

Table 4 The pair, VAB and triplet, VABB interaction coefficients for glycine and VAB 106/ m3 mol2 kg

Saccharide

a-alanine in aqueous PILs solutions at T = (288.15–318.15) K.

DL-

VABB 106/ m3 mol2 kg2

VAB 106/ m3 mol2 kg

VABB 106/ m3 mol2 kg2

In aqueous PAF solutions T/K = 288.15 1.62  4.55 3.21  4.41 T/K = 308.15 1.63  4.03 1.29  3.00

Glycine DL-a-Alanine Glycine a-Alanine

DL-

T/K = 298.15 0.32  4.74 2.19  3.62 T/K = 318.15 3.64  3.55 0.52  2.93

8.49  17.67 14.95  17.11 0.37  15.63 11.68  11.63

2.77  18.38 12.90  14.07 4.95  13.78 6.43  11.39

In aqueous 3-HPAF solutions T/K = 288.15 6.73  2.19 7.52  2.46 T/K = 308.15 4.09  1.34 4.85  1.65

Glycine DL-a-Alanine Glycine a-Alanine

DL-

T/K = 298.15 5.78  1.99 7.00  2.12 T/K = 318.15 3.00  0.99 4.19  1.35

20.90  8.51 21.53  9.56 13.12  5.20 15.72  6.39




due to a packing effect. The values [68] of V2 (int) for amino acids have been obtained from their molar crystal volume by using the relationship [69]:  

0:7 (8) V ðcrystÞ V 2 ðintÞ ¼ 0:634 2 where 0.7 is the packing density for the molecule in an organic crystal and 0.634 is the packing density for a random packing sphere. Millero et al. [68] reported a relationship between the electrostriction volume and the hydration number of the nonelectrolyte as: 

(9) V elect ¼ nH V e  V b

18.32  7.71 20.04  8.21 9.98  3.83 14.00  5.23







where V e is the molar volume of electrostricted water and V b is the molar volume of bulk water. For every water molecule taken

from the bulk phase to the region near amino acids, the (V e – V b) values are: (–2.9, –3.3, and –4.0) cm3 mol1 at (288.15, 298.15, and 308.15) K, respectively. The hydration number, nH of amino acids in water (Table 6) agrees well with literature values [7,67,68]. The nH values of amino acids in water as well as in presence of PILs decrease with increase in temperature. In 0.05 mol kg1 aqueous PAF solutions, nH values for amino acids are slightly higher as compared to their magnitude in water, but as the concentration of PAF increases from (0.10 to 0.20) mol kg1, the nH values decrease (remains almost constant in case of glycine). In case of 3-HPAF solutions, nH values of both amino acids are higher in comparison to that observed in water, however nH values decrease slightly with

Table 5 Partial molar expansion coefficients, (@V21/@T) P and (@2V21/@T2)P of glycine and DL-a-alanine in water and in aqueous PILs solutions at temperatures, T = (288.15–318.15) K. mB /mol kg1

(@V21/@T)P 106/m3 mol1 K1

T/K

288.15

(@2V21/@T2)

298.15

308.15

318.15

SD

0.072 (0.071a , 0.063b, 0.073c) 0.089 0.091 0.089 0.085

0.047 (0.041a , 0.049c ) 0.070 0.066 0.067 0.070

0.022 (0.012a ) 0.051 0.042 0.044 0.055

0.085

0.0025

0.067 0.154 0.105 0.094

0.0019 0.0025 0.0023 0.0015

0.064 (0.052a , 0.062b, 0.072c) 0.083 0.074 0.079 0.086

0.053 (0.046a , 0.050c ) 0.070 0.068 0.070 0.069

0.043 (0.041a ) 0.056 0.063 0.060 0.052

0.047

0.0011

0.047 0.029 0.078 0.121

0.0014 0.0006 0.0010 0.0017

In aqueous 3-HPAF solutions Glycine 0.05 0.110 0.10 0.111 0.15 0.104 0.20 0.106

0.087 0.085 0.080 0.080

0.064 0.059 0.056 0.055

0.041 0.033 0.032 0.029

0.036 0.076 0.067 0.087

0.0023 0.0026 0.0024 0.0026

DL-a-Alanine 0.05 0.10 0.15 0.20

0.080 0.077 0.079 0.081

0.070 0.067 0.071 0.072

0.060 0.057 0.062 0.062

0.112 0.165 0.150 0.190

0.0010 0.0010 0.0009 0.0010

In aqueous PAF solutions Glycine 0.00 0.097 (0.099a ) 0.108 0.05 0.10 0.115 0.15 0.112 0.20 0.100 DL-a-Alanine 0.00

0.05 0.10 0.15 0.20

0.074 (0.057a ) 0.097 0.079 0.089 0.103

0.090 0.087 0.088 0.091

P

106/m3 mol1 K2

SD, standard deviation. The amount of water present in PILs has been taken into account for the molality correction of the stock solutions (PILs + water). a Ref. [7]. b Ref. [54]. c Ref. [64].

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273

Table 6 Hydration numbers, nH of amino acids in water and in aqueous PILs solutions at temperatures, T = (288.15–308.15) K. mB/mol kg1

nH

T/K In aqueous PAF solutions Glycine 0.00 0.05 0.10 0.15 0.20 DL-a-Alanine 0.0

0.05 0.10 0.15 0.20

288.15

298.15

308.15

3.27 (3.26a ) 3.55 3.18 3.15 3.21

2.60 (2.58a , 2.63b , 2.64c ) 2.81 2.44 2.43 2.52

2.02 (2.02a ) 2.14 1.87 1.85 1.91

4.15 (4.08a ) 4.46 4.10 4.05 4.04

3.45 (3.45a , 3.41b , 3.43c) 3.66 3.38 3.33 3.29

2.68 (2.68a ) 2.81 2.60 2.53 2.49

In aqueous 3-HPAF solutions Glycine 0.05 0.10 0.15 0.20

3.57 3.48 3.42 3.37

2.83 2.74 2.71 2.66

2.15 2.11 2.09 2.05

DL-a-Alanine 0.05 0.10 0.15 0.20

4.48 4.40 4.36 4.34

3.71 3.66 3.62 3.60

2.84 2.78 2.75 2.72

The amount of water present in PILs has been taken into account for the molality correction of the stock solutions (PILs + water). a Ref. [7]. b Ref. [67]. c Ref. [68].

concentration of 3-HPAF. These observations suggest that magnitude of hydration numbers depends upon the nature of interactions between amino acids and PILs. The higher nH values at 0.05 mol kg1 aqueous PAF and in (0.05–0.20 mol kg1) aqueous 3-HPAF solutions suggest that PILs-amino acid interactions are
weak, and this has also been observed from negative DtV2 values. The lower nH values for amino acids in (0.10, 0.15 and 0.20) mol kg1 aqueous PAF solutions suggest that the interactions involving the (NH3+, HCOO) ions of PIL and amino acids are strong. The hydration numbers are found to be higher in presence of 3HPAF as compared to PAF, which further suggest that PAF has strong dehydration effect on amino acids in comparison to 3-HPAF. 4. Conclusions





Transfer volumes, DtV2 calculated from V2 data are both
positive and negative in case of PAF, whereas only negative DtV2
values have been observed in case of 3-HPAF. Overall the DtV2 values decrease in the order: glycine > DL-a-alanine. The negative
DtV2 values suggest the dominance of hydrophobic type of interactions over the hydrophilic type of interactions. Further,
DtV2 are higher in case of PAF in comparison to 3-HPAF, which suggest that amino acid-PIL interactions are stronger in case of PAF. The values of pair interaction coefficients (VAB) are also higher in presence of PAF, which further supports the view, that amino acid interacts more strongly with PAF. In aqueous PAF (only at 0.05 mol kg1) and in 3-HPAF solutions, the hydration numbers, nH are higher as compared to their values in water. Overall, nH values are higher in aqueous solutions of 3-HPAF as compared to their values in aqueous PAF solutions, which indicate that PAF has strong dehydration effect on amino acids in comparison to 3-HPAF.

Acknowledgements Authors are thankful to Council of Scientific and Industrial Research (CSIR), India and Department of Science and Technology (DST), India for their financial support. Dr. Vickramjeet Singh acknowledges the financial support from CSIR through his postdoctoral fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

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