Interactions of some α-amino acids with tetra-n-alkylammonium bromides in aqueous medium at different temperatures

J. Chem. Thermodynamics 39 (2007) 613–620 www.elsevier.com/locate/jct

Interactions of some a-amino acids with tetra-n-alkylammonium bromides in aqueous medium at different temperatures Anwar Ali *, Shahla Khan, Soghra Hyder, Mohd Tariq Department of Chemistry, Jamia Millia Islamia (Central University) New Delhi 110 025, India Received 11 April 2006; received in revised form 31 August 2006; accepted 31 August 2006 Available online 5 September 2006

Abstract The densities and viscosities were measured for binary mixtures of 0.01 M aqueous tetramethylammonium bromide (TMAB) and tetraethylammonium bromide (TEAB) and ternary mixtures of glycine, DL-alanine and DL-valine (0.01 to 0.05 M) in 0.01 M-aqueous tetran-alkylammonium bromides at T = (298.15, 303.15, 308.15, and 313.15) K and at atmospheric pressure. The apparent molar volume, /v, was computed using the density data. The limiting apparent molar volume, /v of amino acids and the slope, S v were obtained using the Masson’s equation. The transfer volume, /v ðtrÞ of the amino acids and their side-chain contribution, /v ðRÞ have also been determined. The viscosity data have been analyzed on the basis of the Jones–Dole equation, the viscosity A and B-coefficients and free energy of # #  activation of viscous flow, per mole of solvent, Dl# 1 and solute, Dl2 have been evaluated. Both /v and Dl2 vary linearly with increase in number of C atoms in the alkyl chain of the amino acids. This linearity is used to find the contribution of the zwitterionic end groups, #   ðNHþ 4 ; COO Þ and those of CH2 groups of side-chain to the /v and Dl2 . The variations of these parameters with concentration and temperature clearly suggests the role of alkyl cation on the solute–solvent interactions in aqueous medium. The structural influence of the large cations of quaternary ammonium salts upon solvent is also taken into consideration.  2006 Elsevier Ltd. All rights reserved. Keywords: Amino acids; Tetra-n-alkylammonium bromides; Partial molar volume; Transfer volume; Free energy of activation

1. Introduction The present work is continuation of our program to study the molecular interactions of amino acids in mixed aqueous solvents from the measurements of various transport and thermodynamic properties [1–3]. As a part of this series of investigations, we report here the density and viscosity measurements of amino acids glycine (Gly), DL-alanine (Ala) and DL-valine (Val) in aqueous tetramethylammonium bromide ((CH3)4NBr) and in aqueous tetraethylammonium bromide ((C2H5)4NBr) at T = (298.15, 303.15, 308.15, and 313.15) K. Amino acids (AAs) are the fundamental structural units of proteins. Study of these model compounds (amino acids) *

Corresponding author. Tel.: +91 11 26981717x3250, 3252; fax: +91 11 26981232. E-mail address: [email protected] (A. Ali). 0021-9614/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2006.08.010

in aqueous electrolytic solutions provide information, which can be of great help in understanding the effects of electrolytes on biomolecules [4–7]. It will be of interest to study a separate class of electrolytes like tetra-n-alkylammonium salts (R4NBr) [R = (CH3)4, (C2H5)4] due to some unusual properties of these salt solutions, apparently caused by their large size and hydrophobic alkyl chain– water interaction. Thus, keeping these considerations, we present various parameters like apparent molar volume, /v, limiting apparent molar volume, /v and its experimental slope S v , transfer volume, /v ðtrÞ and side-chain contribution of the amino acids, /v ðRÞ, computed from the experimental density, q, and viscosity, g, data. The viscosity A and B-coefficients were calculated by using Jones–Dole equation [8], and free energy of activation of viscous flow, per mole of solvent, # #  Dl# 1 and solute, Dl2 were also calculated. /v and Dl2 were split into contributions from the zwitterionic end

614

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620

 group ðNHþ 3 ; COO Þ and the methylene groups (CH2) of the amino acids using their linear variation with the number of carbon atoms in the alkyl chain of the amino acids in aqueous (CH3)4NBr and (C2H5)4NBr at T = (298.15, 303.15, 308.15, and 313.15) K. All these parameters offer a convenient method to study the intermolecular interactions occurring between the various components of the ternary mixtures.

Amino acids glycine (Merck, 99.7%), DL-alanine (Merck, 99%), DL-valine (Loba Chemie, 99%) were used after recrystallization from {ethanol + water} mixtures and drying in vacuum over P2O5 at room temperature for at least 72 h. Tetramethylammonium bromide and tetraethylammonium bromide were procured from, Aldrich Chem. Co., 98% purity, and, s.d.fine, 98% purity, respectively. These salts were purified by recrystallization to ensure maximum purity and dried in vacuum. The solutions were prepared on molarity basis. The concentrations of amino acids were in the range from 0.01 M to 0.05 M while those of tetra-n-alkyl salts were 0.01 M. The weighings were done using Precisa XB 220A (Swiss make) electronic balance precisely up to 1 Æ 104 g. The solutions were prepared with utmost care and stored in special airtight bottles to avoid moisture contamination and evaporation. Density measurements were carried out using a single capillary pycnometer made of Borosil glass having bulb volume of 8 Æ 106 m3. The viscosities were measured using an Ubbelohde-type viscometer. The measured density and viscosity were accurate to 0.01 kg Æ m3 and 3 · 106 N Æ m2 Æ s, respectively. For all the measurements the temperature of the solutions was maintained in an electronically controlled water bath (Julabo, Model: MD GMBH Germany) (±0.02 K).

The densities, q, and viscosities, g, were measured as a function of molar concentrations of amino acids at T = (298.15, 303.15, 308.15, and 313.15) K and are given in Table 1. The apparent molar volumes of the solutes were calculated by using the following relation: /v ¼ ðq0  qÞ=cq0 þ M=q0 ;

ð1Þ

where q and q0 are the density of solution and solvent, respectively, and c is the concentration of the solute (amino acid) of molar mass M. /v values of the amino acids at the four studied temperatures are plotted against c1/2 (figures 1 and 2). A valuable empirical generalization on the change of /v with the square root of molar concentration is given as [9]: þ

/v

S v c1=2 ;

ð2Þ

where is the limiting apparent molar volume at infinite dilution, calculated by linear regression of equation (2),

ð3Þ

/v

where (in water) values at T = (298.15, 308.15, and 313.15) K were taken from the literature [10–13]. The /v data have also been used to estimate the group contributions for the amino acid side-chains [14,15]. These can be derived from the difference between the /v value of the AA and that of glycine, which has the C–H side-chain: /v ðRÞ ¼ /v ðAAÞ  /v ðGlyÞ;

ð4Þ

The quantity /v ðRÞ thus gives the contribution of the AA side-chain, R to the partial molar volume, /v . Jones and Dole [8] have experimentally shown that the viscosities of many solutions can be accurately represented by the equation g=g0 ¼ gr ¼ 1 þ Ac1=2 þ Bc;

ð5Þ

where gr is the relative viscosity, A is the Falkenhagen coefficient and B is the Jones–Dole coefficient. A and B are determined by plotting [(g/g0)  1]/c1/2 against c1/2, the intercepts and slopes of the linear plots are A and B, respectively. A gives a measure of solute–solute interactions and B provides information concerning the solvation of ions and their effects on the strength of the solvent in the near environment of the solute particles. Recently, Feakins et al. [16] found that the B-coefficient of viscosity depends on the molar volume of the solvent according to the equation: # B ¼ ðV 1  V 2 Þ þ V 1 ½ðDl# 2  Dl1 Þ=RT ;

3. Results

/v ¼

/v ðtrÞ ¼ /v ðin aq: tetraalkylammonium bromideÞ /v ðin waterÞ;

2. Experimental

/v

has the same meaning as the partial molar volume,V 2 , of amino acids and S v is the experimental slope. The V 2 values provide the information about solute–solvent interactions and S v gives an insight into the solute–solute interactions. The standard volumes of transfer, /v ðtrÞ for the amino acids from water to aqueous tetra-n-alkylammonium bromide solutions were calculated from the relation:

ð6Þ

where V 1 and V 2 are the partial molar volumes of the solvent and solute, respectively. Eyring and co-workers [17] proposed that the free energy of activation of viscous flow of pure solvent Dl# 1 could be calculated by the equation: g0 ¼ ðhN A =V 1 Þ expðDl# 1 =RT Þ;

ð7Þ

which on rearranging gives  Dl# 1 ¼ RT lnðgo V 1 =hN A Þ;

ð8Þ

where R, h and N are gas constant, Planck’s constant and Avogadro’s number, respectively. Equation (6) can also be rearranged to give Dl# as 2 follows: #    Dl# 2 ¼ Dl1 þ ðRT =V 1 Þ½B  ðV 1  V 2 Þ:

ð9Þ

# The values of /v ; S v ; /v ðtrÞ; /v ðRÞ; A; B; Dl# 1 , and Dl2 are compiled in Table 2. Due to the linear relation between /v  and nC [18,19] the contributions of (NHþ 3 , COO ) and

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620 TABLE 1 Values of density, q and viscosity, g and of amino acids, glycine, 303.15, 308.15, and 313.15) K c/(mol Æ L1)

0.00 0.01 0.02 0.03 0.04 0.05

T = 298.15 K

997.3 997.6 997.9 998.3 998.8 999.4

DL-alanine,

and

DL-valine

in aqueous tetra-n-alkylammonium bromide at T = (298.15,

T = 303.15 K

996.0 996.2 996.5 996.9 997.3 997.9

615

T = 308.15 K

{Gly + aq. (CH3)4NBr} q/(kg Æ m3) 994.8 995.0 995.2 995.5 995.9 996.4

T = 313.15 K

993.5 993.6 993.8 994.1 994.5 994.9

103 Æ g/(N Æ m2 Æ s) 0.00 0.01 0.02 0.03 0.04 0.05

0.00 0.01 0.02 0.03 0.04 0.05

0.886 0.890 0.892 0.893 0.895 0.896

997.3 997.6 998.0 998.4 998.9 999.4

0.794 0.797 0.798 0.800 0.801 0.802

996.0 996.1 996.4 996.8 997.2 997.7

0.717 0.719 0.721 0.722 0.723 0.724 {Ala + aq. (CH3)4NBr} q/(kg Æ m3) 994.8 994.8 995.0 995.3 995.6 996.0

0.652 0.653 0.654 0.655 0.655 0.656

993.5 993.3 993.4 993.7 994.0 994.4

103 Æ g/(N Æ m2 Æ s) 0.00 0.01 0.02 0.03 0.04 0.05

0.00 0.01 0.02 0.03 0.04 0.05

0.886 0.891 0.896 0.901 0.906 0.912

997.3 997.6 998.0 998.4 998.8 999.2

0.794 0.797 0.800 0.804 0.809 0.815

996.0 996.2 996.5 996.8 997.1 997.4

0.717 0.719 0.721 0.725 0.729 0.734 {Val + aq. (CH3)4NBr} q/(kg Æ m3) 994.8 994.7 994.9 995.2 995.5 995.8

0.652 0.652 0.654 0.657 0.661 0.665

993.5 993.3 993.4 993.7 994.0 994.3

103 Æ g/(N Æ m2 Æ s) 0.00 0.01 0.02 0.03 0.04 0.05

0.00 0.01 0.02 0.03 0.04 0.05

0.886 0.903 0.914 0.928 0.943 0.955

996.7 997.1 997.5 997.9 998.4 998.9

0.794 0.807 0.816 0.828 0.842 0.853

995.3 995.7 996.0 996.4 996.8 997.3

0.717 0.727 0.735 0.745 0.758 0.767 {Gly + aq. (C2H5)4NBr} q/(kg Æ m3) 994.0 994.3 994.6 995.0 995.4 995.8

0.652 0.658 0.666 0.675 0.685 0.694

992.6 992.8 993.1 993.4 993.8 994.2

103 Æ g/(N Æ m2 Æ s) 0.00 0.01

0.887 0.875

0.794 0.784

0.717 0.709

0.650 0.644 (continued on next page)

616

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620

TABLE 1 (continued) c/(mol Æ L1) 0.02 0.03 0.04 0.05

0.00 0.01 0.02 0.03 0.04 0.05

T = 298.15 K 0.876 0.879 0.884 0.889

996.7 997.0 997.2 997.4 997.6 997.7

T = 303.15 K

T = 308.15 K

0.786 0.790 0.795 0.799

995.3 995.6 995.8 996.1 996.3 996.4

0.711 0.716 0.720 0.726 {Ala + aq. (C2H5)4NBr} q/(kg Æ m3) 994.0 994.3 994.6 994.9 995.1 995.3

T = 313.15 K 0.647 0.651 0.656 0.662

992.6 992.9 993.3 993.5 993.8 994.0

103 Æ g/(N Æ m2 Æ s) 0.00 0.01 0.02 0.03 0.04 0.05

0.00 0.01 0.02 0.03 0.04 0.05

0.887 0.889 0.894 0.899 0.904 0.908

996.7 997.0 997.3 997.5 997.7 997.8

0.794 0.796 0.800 0.805 0.810 0.814

995.3 995.5 995.7 995.8 996.0 995.9

0.717 0.719 0.724 0.728 0.732 0.736 {Val + aq. (C2H5)4NBr} q/(kg Æ m3) 994.0 994.1 994.2 994.3 994.4 994.5

0.650 0.652 0.656 0.661 0.665 0.669

992.6 992.7 992.7 992.8 992.8 992.8

103 Æ g/(N Æ m2 Æ s) 0.00 0.01 0.02 0.03 0.04 0.05

0.887 0.898 0.905 0.911 0.918 0.927

0.794 0.801 0.806 0.812 0.818 0.825

0.717 0.720 0.725 0.731 0.736 0.743

0.650 0.651 0.655 0.660 0.664 0.670

(CH2) groups to /v were calculated using the following equation:

3.1. Discussion

  /v ¼ /v ðNHþ 3 ; COO Þ þ nC /v ðCH2 Þ:

Volumetric property has been regarded as a sensitive structural tool for understanding interactions in solutions [20]. Figures 1 and 2 show that the apparent molar volumes are large positive for all six systems, suggesting the presence of strong solute–solvent interactions in both aqueous tetra-n-alkylammonium bromides. /v is found to decrease with increase in concentration of amino acids in aqueous (CH3)4NBr, an opposite trend in /v is observed for Ala and Val in aqueous (C2H5)4NBr mixtures. As expected, /v increases with increase in temperature for all the amino acids in both the solutions of tetra-alkylammonium bromides under study. It is known that solute–solute interactions at infinite dilution are absent; therefore, the values of /v provide information regarding solute-solvent interaction. The /v values given in table 2 are positive and increase with rise in temperature. They are larger for (CH3)4NBr than those for (C2H5)4NBr, suggesting that the extent of solute–solvent interactions in the former is greater than in the latter

ð10Þ

The values of /v (CH2), thus obtained, were used to obtain the contributions /v (CH3) and /v (CH) with the help of following equations: /v ðCH3 Þ ¼ 1:5  /v ðCH2 Þ; /v ðCHÞ ¼ 0:5  /v ðCH2 Þ;

ð11Þ ð12Þ

Similarly, Dl# 2 of the AAs also vary linearly with nC as: # #  þ Dl# 2 ¼ Dl2 ðNH3 ; COO Þ þ nC Dl2 ðCH2 Þ;

ð13Þ

which gives contribution of zwitterionic end group #  ðNHþ 3 ; COO Þ and (CH2) group to Dl2 . Since the alkyl chains of the homologous series of the a-amino acids investigated in this work are CH2-(Gly), CH3–CH-(Ala) and CH3–CH3–CH–CH-(Val), the /v (CH2) and Dl# 2 (CH2) obtained characterize the mean contribution of CH and CH3 groups to /v and Dl# 2 of the AAs, respectively. Their results are given in table 3.

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620 6.0

7.0

a

/ (m3.mol-1)

v

4.5

v

10-5.

4.0

4.0

/ (m3. mol-1)

5.5

10-5.

6.5 6.0 5.5 5.0

3.5 0.09

0.14

c

1/2

0.19

L )

/ (mol

0.19

0.24

b 6.5 6.0

v

7.5

/ (m3.mol-1)

8.5

10-5.

/ (m3.mol-1) v

10-5.

0.14

7.0

b

6.5 5.5

5.5 5.0

4.5 0.09

0.14

c

1/2

0.19

0.24

0.09

/ (mol

0.14

0.19

0.24

c 1/2 / (mol1/2.L1/2)

1/2. 1/2

L ) 12.0

6.0 5.5 5.0

3.5

c

11.5 11.0 10.5 10.0

v

6.5

9.5

10-5.

c

/ (m3.mol-1)

7.0

/ (m3.mol-1)

3.5

c 1/2 / (mol1/2.L1/2)

9.5

v

4.5

1/2. 1/2

10.5

10 - 5 .

5.0

0.09

0.24

11.5

4.0

a

3.0

3.0

4.5

617

9.0 8.5 8.0

3.0 0.09

0.14

0.19

0.24

0.09

c 1/2 / (mol1/2.L1/2) FIGURE 1. Plot of apparent molar volumes, /v, of: (a) glycine; (b) 1/2 DL-alanine; (c) DL-valine + (CH3)4NBr versus c at temperature T {= 298.15 (r), 303.15 (j), 308.15 (m), and 313.15 (d)} K.

solutions. Smaller values of /v in (C2H5)4NBr solutions might also be due to larger voids created by (C2H5)4NBr, thereby, allowing better accommodation of small AAs’ molecules in the voids. This, in turn, is supported by the values of S v which are larger for (C2H5)4NBr, suggesting stronger ion–ion interactions in the ternary system of amino acids + aqueous (C2H5)4NBr as compared to the ternary systems of amino acids + aqueous (CH3)4NBr. In general, the interactions between the amino acids and tetra-n-alkylammonium bromides can be classified into: (i) ion–ion interactions between zwitterions (NHþ 3 and COO) of AAs and cations (R4N+) and anion (Br) of R4NBr; (ii) ion–nonpolar group interactions between (R4N+, Br) and the hydrophobic side chain of AAs; (iii) ion–nonpolar group interactions between  ðNHþ ; COO Þ and the hydrophobic side chain (R4) of 3 the electrolytes; and (iv) nonpolar–nonpolar group inter-

0.14

0.19

0.24

c 1/2 / (mol1/2.L1/2) FIGURE 2. Plot of apparent molar volumes, /v, of: (a) glycine; (b) 1/2 DL-alanine; (c) DL-valine + (C2H5)4NBr versus c at temperature T {= 298.15 (r), 303.15 (j), 308.15 (m), and 313.15 (d)} K.

actions between the hydrophobic side chains of AAs and those of the salts. According to the cosphere overlap model [21] the effect of overlap of hydration cosphere is destructive. Mishra et al. [22] using this model observed that the overlap of cosphere of two ionic species shows an increase in volume, whereas, the overlap of hydrophobic–hydrophobic groups and ion–hydrophobic groups results in net decrease in volume. Thus, in the light of the above fact, the observed positive /v values explain that the interaction contribution of type (i) is stronger than that of type (ii), (iii), and (iv). Further, the /v values of the three amino acids studied are found to increase in the order: Gly < Ala < Val thus, indicating the trend of the strength of solute–solvent interactions in the ternary mixtures. Strong solute–solvent

618

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620

TABLE 2 Values of limiting apparent molar volume, /v , experimental slope, S v , A, and B coefficients of Jones–Dole equation, and free energy of activation for the solvent, # Dl# 1 and solute Dl2 for amino acids, glycine, DL-alanine, and DL-valine in aqueous tetra-n-alkylammonium bromide at T = (298.15, 303.15, 308.15, and 313.15) K T = 298.15 K

T = 303.15 K

T = 308.15 K

T = 313.15 K

105  /v =ðm3  mol1 Þ 104  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ=ðm3  mol1 Þ 102 Æ A/(dm3/2 Æ mol1/2) 102ÆB/(dm3 Æ mol1) 1 Dl# 1 =ðkJ  mol Þ 1 Dl# 2 =ðkJ  mol Þ

6.42 ± 0.15 1.35 4.32a 2.09 3.34 ± 0.16 7.38 ± 0.94 9.15 25.58

{Gly + aq. (CH3)4NBr} 7.03 ± 0.11 7.44 ± 0.16 1.45 1.35 4.38a 3.06 2.37 ± 0.24 1.86 ± 0.15 8.96 ± 1.38 9.42 ± 0.88 9.03 8.92 28.77 30.19

8.11 ± 0.12 1.51 4.40b 3.71 0.46 ± 0.09 11.25 ± 0.54 8.82 33.98

104  /v =ðm3  mol1 Þ 104  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ=ðm3  mol1 Þ 105  /v (CH3)/(m3 Æ mol1) 102 Æ A/(dm3 Æ mol1) 101 Æ B/(dm3 Æ mol1) 1 Dl# 1 =ðkJ  mol Þ # Dl2 =ðkJ  mol1 Þ

0.69 ± 0.01 0.95 6.05a 0.80 0.43 0.93 ± 0.81 6.08 ± 0.47 9.15 99.46

{Ala + aq. (CH3)4NBr} 0.98 ± 0.02 1.08 ± 0.02 1.93 1.93 6.10a 4.67 2.73 3.33 3.48 ± 0.97 5.03 ± 1.09 6.47 ± 0.56 6.55 ± 0.63 9.03 8.92 110.10 114.17

1.40 ± 0.04 3.12 6.12c 7.83 5.84 6.82 ± 1.04 6.78 ± 0.60 8.82 123.49

104  /v =ðm3  mol1 Þ 104  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ=ðm3  mol1 Þ 105  /v CHðCH3 Þ2 =ðm3  mol1 Þ 102 Æ A/(dm3/2 Æ mol1/2) B/(dm3 Æ mol1) 1 Dl# 1 =ðkJ  mol Þ 1 Dl# =ðkJ  mol Þ 2

0.93 ± 0.02 0.63 9.10a 0.15 2.84 4.07 ± 1.93 1.37 ± 0.11 9.15 207.36

{Val + aq. (CH3)4NBr} 1.03 ± 0.02 1.50 ± 0.05 0.63 2.45 9.16a 5.80 3.24 7.52 0.72 ± 2.03 2.48 ± 2.17 1.43 ± 0.12 1.49 ± 0.13 9.03 8.92 220.49 238.53

1.66 ± 0.05 3.00 9.17d 7.44 8.50 8.00 ± 0.73 1.66 ± 0.04 8.82 267.66

105  /v =ðm3  mol1 Þ 105  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ/(m3 Æ mol1) 101 Æ A/(dm3/2 Æ mol1/2) B/(dm3 Æ mol1) 1 Dl# 1 =ðkJ  mol Þ 1 Dl# 2 =ðkJ  mol Þ halfline

3.93 ± 0.21 3.23 4.32a 0.39 2.51 ± 0.05 1.16 ± 0.03 9.15 170.69

{Gly + aq. (C2H5)4NBr} 4.50 ± 0.17 5.84 ± 0.12 3.92 8.54 4.38a 1.46 2.56 ± 0.08 2.43 ± 0.04 1.30 ± 0.04 1.33 ± 0.02 9.03 8.92 193.17 202.93

6.48 ± 0.11 9.58 4.40b 2.08 2.37 ± 0.03 1.43 ± 0.02 8.81 220.18

105  /v =ðm3  mol1 Þ 105  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ=ðm3  mol1 Þ 105  /v (CH3)/(m3 Æ mol1) 102ÆA/(dm3 Æ mol1) 101ÆB /(dm3 Æ mol1) 1 Dl# 1 =ðkJ  mol Þ 1 Dl# 2 =ðkJ  mol Þ halfline

5.65 ± 0.05 5.61 6.05a 0.40 1.72 4.77 ± 0.63 7.07 ± 0.36 9.15 111.21

{Ala + aq. (C2H5)4NBr} 5.47 ± 0.09 5.23 ± 0.08 5.42 4.85 6.10a 0.88 0.97 0.61 4.44 ± 0.56 3.69 ± 0.52 7.12 ± 0.32 7.13 ± 0.30 9.03 8.92 113.10 114.42

5.04 ± 0.08 4.77 6.12c 1.08 1.44 3.45 ± 0.53 7.52 ± 0.31 8.81 121.19

104  /v =ðm3  mol1 Þ 105  S v =ðm3  mol3=2  L1=2 Þ 105  /vðwaterÞ =ðm3  mol1 Þ 105  /v ðtrÞ=ðm3  mol1 Þ 105  /v CHðCH3 Þ2 =ðm3  mol1 Þ 102ÆA /(dm3/2Æmol1/2) 101ÆB /(dm3Æmol1) 1 Dl# 1 =ðkJ  mol Þ 1 Dl# =ðkJ  mol Þ 2

0.80 ± 0.02 6.65 9.10a 1.14 4.03 5.48 ± 1.08 6.16 ± 0.62 9.15 101.90

{Val + aq. (C2H5)4NBr} 0.91 ± 0.04 1.07 ± 0.01 5.80 0.59 9.16a 1.55 4.57 4.86 1.00 ± 0.62 4.67 ± 0.87 7.16 ± 0.36 9.15 ± 0.50 9.03 8.92 118.67 150.65

1.12 ± 0.01 0.77 9.17d 2.01 4.70 7.56 ± 0.93 9.51 ± 0.54 8.81 158.43

a b c d

Data taken from reference [19]. Data taken from reference [20]. Data taken from reference [18]. Data taken from reference [21].

A. Ali et al. / J. Chem. Thermodynamics 39 (2007) 613–620

619

TABLE 3 #    þ Contribution of ðNHþ 3 ; COO Þ, (CH), (CH2) and (CH3) groups to the limiting apparent molar volume, /v , and of ðNH3 ; COO Þ and (CH2) to Dl2 , of the amino acids glycine, DL-alanine, and DL-valine in aqueous tetra-n-alkylammonium bromide solution at T = (298.15, 303.15, 308.15, and 313.15) K T = 298.15 K

 ðNHþ 3 ; COO Þ (CH) Gly (CH2) (CH3) Ala (CH3CH) Val (CH3CH3CHCH)

 ðNHþ 3 ; COO Þ (CH2)

 ðNHþ 3 ; COO Þ (CH) Gly(CH2) (CH3) Ala (CH3CH) Val (CH3CH3CHCH)

 ðNHþ 3 ; COO Þ (CH2)

T = 303.15 K

T = 308.15 K

T = 313.15 K

{(CH3)4NBr} 105  /v =ðm3  mol1 Þ 5.22 0.49 0.98 1.47 1.96 3.93

6.77 0.48 0.96 1.44 1.93 3.85

28.37 59.64

26.43 62.66

5.35 1.22 2.45 3.67 4.89 9.79

6.78 1.31 2.62 3.93 5.24 1.48

31.99 68.41

38.10 77.06

1 Dl# 2 =ðkJ  mol Þ

(C2H5)4NBr} 105  /v =ðm3  mol1 Þ 2.78 0.66 1.32 1.97 2.63 5.26

2.70 0.78 1.56 2.34 3.12 6.25

175.34 20.32

190.39 20.89

3.10 0.89 1.78 2.67 3.56 7.121

3.41 0.89 1.78 2.67 3.56 7.13

1 Dl# 2 =ðkJ  mol Þ

interactions have also been reported for some tetra-n-alkylammonium perchlorates in 2-methoxyethanol [23]. Increase in /v with rise in temperature is due to loosening of solvation layers of the solutes in solution, thereby, releasing some solvent molecules. The transfer volumes, /v ðtrÞ are mostly positive (Table 2) and decrease with increase in the size of alkyl chain of both the AAs as well as electrolytes. This indicates that increased hydrophobicity of solutes induces more dehydration effect. The interaction of (R4N+, Br) ions with the charged centers  ðNHþ 3 ; COO Þ of zwitterions of AAs reduces the electrostriction effect, thereby, giving rise to a positive volume transfer from water to aqueous R4NBr. However, negative /v ðtrÞ is shown for Ala in aqueous (C2H5)4NBr at all temperatures, and for Gly and Val at T = 298.15 K. This may be due to shielding effect of larger alkyl group of (C2H5)4NBr. The results of /v ðRÞ (table 2) reveal that contribution of side-chain, R group is significant and positive for all the AAs and increase as the size of alkyl chain increases from Ala to Val. It may be stressed that /v ðRÞ is not the absolute partial molar volume of the side-chain, R, but gives the contribution to /v on replacing a C–H group by a C–R group [15,24] and the volume contribution of the H-atom in Gly is assumed to be neglected. An analysis of A and B coefficients derived from viscosity data, included in table 2, shows that B coefficients are larger than A coefficients, supporting the behaviour of /v and S v , respectively; both suggesting stronger solute– solvent interactions as compared to solute–solute interactions.

184.82 12.35

201.56 14.98

The values of B (table 2) are found to increase with increase in temperature. Since dB/dT is negative for structure-makers and positive for structure-breakers [25], we can conclude that Gly, Ala and Val behave as structurebreakers in both the aqueous tetra-n-alkylammonium bromide solutions. This is consistent with our /v results and the cosphere overlap theory [24], according to which the effect of overlap of hydration cosphere is destructive, thus contributing to structure-breaking effect. Further, the values of B of amino acids in (CH3)4NBr electrolyte increase from Gly to Val while a reverse trend is observed in case of (C2H5)4NBr. This may be attributed to the higher polarity of (CH3)4NBr, due to smaller apolar group (CH3)4, than that of (C2H5)4NBr, with bigger apolar group (C2H5)4, resulting in stronger interaction of ((CH3)4N+, Br) ions than those of ((C2H5)4N+, Br) ions  with the charged centers ðNHþ 3 ; COO Þ of amino acids in the mixtures. This, in turn, facilitates stronger interaction  between ðNHþ 3 ; COO Þ polar ends of amino acids with the water molecules in (C2H5)4NBr mixture than in (CH3)4NBr mixture, and becomes increasingly stronger as the polar nature of amino acid increases from Val to Gly in (C2H5)4NBr electrolyte, causing an increase in B value from Val to Gly. But, because of high polarity of (CH3)4NBr and almost insignificant effect of its apolar group (CH3)4, the strength of interaction between amino acid and water molecules shows the opposite trend in B values in this electrolyte. The Dl# 2 values calculated from equation (9), given in table 2, are found to be positive and larger than Dl# 1 . This suggests that the formation of transition state is less

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favoured in the presence of AAs since the breaking and distortion of the intermolecular bonds were accompanied in this process. Dl# is greatly influenced by temperature, 2 increasing positively with increase in temperature. Table 3 shows the contribution of the zwitterionic group  ðNHþ 3 ; COO Þ and methylene group (CH2), of amino  acids to /v and Dl# obtained by using equations (10) 2 and (13), respectively. The contributions of the other alkyl-chains of the AAs to /v were calculated by equations (11) and (12). It can be seen from table 3 that   /v ðNHþ 3 ; COO Þ are larger than those of /v (CH2) and increase with temperature in both the tetra-n-alkylammonium bromides. These results indicate that the interaction between R4NBr and the charged end groups  ðNHþ 3 ; COO Þ is much stronger than that between R4NBr and CH2 group. Similar results were reported for a-amino acids in aqueous guanidine hydrochloride [26].  The contribution of ðNHþ 3 ; COO Þ to the activation free # energy, Dl2 (table 3) is found to be lower than (CH2) contribution in (CH3)4NBr solution, while a reverse trend is observed in (C2H5)4NBr solution. This, further, supports the above view that due to higher polarity of (CH3)4NBr (with smaller apolar group (CH3)4 than that of (C2H5)4NBr, having bigger apolar group (C2H5)4, interaction (type (i)) is more pronounced in (CH3)4NBr than in (C2H5)4NBr mixture. Acknowledgement The authors are grateful to the Head, Department of Chemistry, Jamia Millia Islamia, New Delhi, for providing necessary research facilities. References [1] A. Ali, S. Hyder, S. Sabir, D. Chand, A.K. Nain, J. Chem. Thermodyn. 38 (2006) 136–143.

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JCT 06-87