Journal of Molecular Liquids 162 (2011) 89–94

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Journal of Molecular Liquids

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Apparent molar volumes and transport behavior of glycine and L-valine in aqueoussolutions of tripotassium citrate at T=(308.15 and 318.15)K

Harsh Kumar a,⁎, Kirtanjot Kaur a, Suresh Kumar b

a Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, 144 011 Punjab, Indiab Department of Chemistry, D A V College, Abohar, 152 116, Punjab, India

⁎ Corresponding author. Tel.: +91 181 2690301x220E-mail addresses: h.786.man@gmail.com, manchand

0167-7322/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.molliq.2011.06.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 December 2010Received in revised form 7 June 2011Accepted 15 June 2011Available online 29 June 2011

Keywords:Apparent molar volumeTripotassium citrateAmino acidsViscosity

Densities and viscosities of glycine and L-valine have been measured at 308.15 and 318.15 K in aqueoustripotassium citrate solutions ranging from 0.2 to 0.8 mol kg−1 of tripotassium citrate. The viscosity data havebeen analyzed by Jones–Dole equation. The activation parameters of viscous flow have been obtained tothrow light on the mechanism of viscous flow. The values of apparent molar volume, partial molar volume atinfinite dilution and relative viscosities of each amino acid in various aqueous tripotassium citrate solutionshave been evaluated from the density and viscosity data. The partial molar volumes of transfer from water toaqueous tripotassium citrate solution at infinite dilution have also been calculated. Transfer volume data havebeen used to calculate the pair and triplet interactions. The results have been discussed in terms of solute–solute and solute–solvent interactions and the structural changes of the solutes in solutions.

7.ah@nitj.ac.in (H. Kumar).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Aminoacids are the fundamental substances for proteins, dipeptides,antibiotics and other compounds of biological relevance. These aresuitable for better understanding of interactions occurring betweenamino acids and entities present in the living cell. It is evident thatdifferent substances result in the conformational changes in proteinswhen added to aqueous protein solutions. A small change in waterstructure can greatly influence the reaction in cells, made up ofbiological macromolecules. Moreover, the stability of protein may beincreased in solutions by the addition of low molecular weightsubstances like carbohydrates, salts etc. So, the study of the volumetricand viscometric properties of amino acids in aqueous salt solutions isvery useful to the nature of interactions [1–3]. There are extreme studieson volumetric, viscometric and other thermochemical properties ofamino acids in aqueous electrolyte solutions [4–12] but very few hasbeendirected toorganic salts of biological and industrial importance likecitrates [13–15], which are used in food, cosmetic, chemical andpharmaceutical industries and they are of significant importance inmany biochemical processes [16–18]. In order to have better under-standing of nature of interactions, in the present study, experimentaldata on density and viscosity of Glycine and L-valine in aqueoussolutions of tripotassium citrate (T.P.C.) (0.2, 0.4, 0.6 and 0.8) mol kg−1

have been reported at (308.15 and 318.15)K. The experimental data ondensity have been used to evaluate apparent molar volume, limitingapparent molar volumes, transfer volume at infinite dilution of amino

acids in aqueous tripotassium citrate solutions. The transfer parameterhas been interpreted in terms of solute–cosolute interactions, valuesobtained for transfer volumes have been used to evaluate the pair andtriplet interactions. The experimental data on viscosity have been usedto evaluate viscosity B coefficients. Some data on volumetric andviscometric properties of L-alanine in aqueous tripotassium citrate andvice versa have been reported earlier [19–21].

2. Experimental

Glycine (AR grade) (SD Fine Chemicals, India) and L-valine (Merck,Germany) with mass fraction purities 0.995 and 0.990, respectivelywere used as such without purification. However, before use theywere dried at 100 °C for 6 h and then under vacuum over silica gel atroom temperature for a minimum of 24 h. Tripotassium citrate (ARgrade) (SD Fine Chemicals, India) with mass fraction purity greaterthan 0.990 was dried under vacuum and kept over P2O5in a vacuumdesiccator for a minimum of 48 h at room temperature before use. Allsolutions were prepared using deionized glass-distilled water (havingspecific conductance less than 10−6 S) that had been freshly degassedby vacuum pump on a Sartorius BP 210S balance precise to within±0.0001 g. Solutions of tripotassium citrate were prepared bymass inthe range 0.2–0.8 mol kg−1 and used on the same day when theywere prepared. Solutions of amino acids were made by mass on themolality concentration scale. Uncertainties in solution concentrationwere estimated at ±2×10−5 mol kg−1 in calculations.

Densities were measured with single stem graduated pycnometerhaving a total volumeof 8 cm3 and an internal diameter of the capillaryof about 0.1 cm. The pycnometer was calibrated at temperatures of(308.15 and318.15)Kwithwater [22]whichwasfilled to themarks on

Table 1Densities (ρ), and apparent molar volumes (Vϕ) of glycine and L-valine in aqueoussolutions of tripotassium citrate at T=(308.15 and 318.15)K.

m(mol·kg−1)

ρ×103

(kg·m−3)Vϕ×106

(m3·mol−1)ρ×103

(kg·m−3)Vϕ×106

(m3·mol−1)

T=308.15 K T=318.15 K

Glycine+water0.00000 0.99322 0.990620.10190 0.99647 43.11 0.99374 44.440.20142 0.99960 43.19 0.99674 44.540.30079 1.00265 43.39 0.99967 44.710.50528 1.00881 43.62 1.00559 44.910.60426 1.01174 43.69 1.00836 45.050.81047 1.01762 43.98 1.01401 45.300.94755 1.02139 44.19 1.01768 45.44

Glycine+0.2 mol·kg−1 tripotassium citrate0.00000 1.03390 1.028910.13127 1.03794 43.64 1.03282 44.650.20060 1.04004 43.71 1.03484 44.780.39292 1.04571 43.99 1.04033 45.000.50372 1.04888 44.15 1.04339 45.170.59907 1.05149 44.38 1.04593 45.370.74405 1.05542 44.62 1.04980 45.510.79796 1.05684 44.72 1.05120 45.580.90858 1.05981 44.81 1.05403 45.73

Glycine+0.4 mol·kg−1 tripotassium citrate0.00000 1.06825 1.063920.09957 1.07124 43.84 1.06681 44.810.19829 1.07415 43.96 1.06959 45.060.29983 1.07709 44.07 1.07240 45.220.41270 1.08025 44.29 1.07544 45.410.65970 1.08696 44.64 1.08175 45.910.82837 1.09133 44.89 1.08586 46.210.88903 1.09289 44.95 1.08729 46.32

Glycine+0.6 mol·kg−1 tripotassium citrate0.00000 1.09956 1.094420.10068 1.10250 43.99 1.09725 45.010.21099 1.10565 44.15 1.10026 45.240.35720 1.10970 44.38 1.10413 45.490.40514 1.11099 44.47 1.10535 45.610.50729 1.11371 44.63 1.10789 45.860.63993 1.11719 44.77 1.11111 46.110.75422 1.12002 44.99 1.11381 46.310.81669 1.12155 45.09 1.11522 46.440.90651 1.12371 45.24 1.11714 46.691.09814 1.12805 45.63 1.12129 47.01

Glycine+0.8 mol·kg−1 tripotassium citrate0.00000 1.13034 1.126320.11850 1.13368 44.22 1.12950 45.370.19223 1.13570 44.38 1.13141 45.570.39417 1.14108 44.66 1.13645 45.980.59840 1.14629 44.92 1.14125 46.370.79582 1.15099 45.28 1.14559 46.760.96677 1.15492 45.52 1.14918 47.05

L-valine+water0.00000 0.99434 0.990910.05774 0.99590 90.35 0.99238 92.160.15046 0.99835 90.49 0.99469 92.280.25664 1.00109 90.59 0.99725 92.470.34351 1.00329 90.65 0.99930 92.560.45697 1.00608 90.76 1.00192 92.66

L-valine+0.2 mol·kg−1 tripotassium citrate0.00000 1.02819 1.024790.08098 1.03013 91.10 1.02658 93.100.18616 1.03257 91.29 1.02884 93.230.23539 1.03368 91.39 1.02987 93.300.31697 1.03549 91.50 1.03152 93.480.37482 1.03675 91.57 1.03265 93.630.50525 1.03952 91.71 1.03520 93.74

L-valine+0.4 mol·kg−1 tripotassium citrate0.00000 1.06825 1.063920.09866 1.07026 91.65 1.06574 93.670.13996 1.07107 91.77 1.06647 93.800.18140 1.07185 91.97 1.06718 93.950.25269 1.07319 92.11 1.06838 94.130.28713 1.07381 92.22 1.06894 94.220.33551 1.07467 92.34 1.06970 94.38

90 H. Kumar et al. / Journal of Molecular Liquids 162 (2011) 89–94

the stem. The weight of dried pycnometer was taken first along withglass stopper, and then the solution was introduced with the help ofhypodermic syringe with utmost care so that no air bubble wasentrapped. The pycnometer was again weighed and put into thethermostat controlled, well stirredwater bathwhere temperaturewascontrolled to ±0.01 K, with built-in temperature controller system,until no change in the level of the solution in the capillary wasobserved. This level was noted and used in the calculation of density.The pycnometer was then put into a separate bath maintained atrequired temperature. An average of triplicate measurements wastaken into account. Sufficient care was taken to prevent air bubbleentrapment. The reproducibility in the density measurement wasbetter than±3×10−2 kg m−3. Viscosity wasmeasured with the helpof suspended-level Ubbelohde viscometer which was calibrated withdouble distilled, deionized water at temperatures of (308.15 and318.15)K. An average of three or four sets of flow times for each fluidwas taken for the purpose of the calculation of viscosity. The flow timemeasurement was made with an electronic stop watch having aprecision of ±0.01 s. Viscosities were reproducible to ±0.003 mPa s.

3. Results and discussions

The densities ρ and viscosities η of amino acids in binary aqueoussolutions of tripotassium citrate (0.2, 0.4, 0.6 and 0.8) mol kg−1 arereported in Tables 1 and 2, respectively at temperatures (308.15 and318.15)K. The apparent molar volumes Vϕ were calculated from theexperimental densities by following equation

Vϕ = M = ρ−1000 ρ−ρ0ð Þ=m ρ ρ0 ð1Þ

where m is the molality (mol kg−1) of the solution, M is the molarmass of the solute (kg mol−1) and ρ0 and ρ are the densities (kg m−3)of the solvent and solution respectively. The calculated values ofVϕ and the molal concentrations (m) of amino acids in aqueoustripotassium citrate solutions are also given in Table 1. The magnitudeof positive values of Vϕ for both the amino acids indicates greatersolute–solvent interactions. The values of Vϕ increase with increase intemperature in each concentration of tripotassium citrate. The plots ofVϕ against m were linear in all cases. The values of Vϕ for glycineagainst different concentrations of tripotassium citrate are graphicallyrepresented in Fig. 1 at 308.15 K. Further, the Vϕ values increase as wemove from glycine to L-valine at both temperatures.

The variation of apparent molar volumes Vϕ with the molalconcentration can be adequately represented by the equation

Vϕ = V0ϕ + S�Vm ð2Þ

where Vϕ0 the limiting value of apparent molar volume is equal to the

infinite dilution partial molar volume, and SV* is the experimentalslope indicative of solute–solute interactions. The values of Vϕ

0 and SV*together with standard errors derived by least squares fitting of theVϕ values to Eq. (2) are reported in Table 3. There is a fairly goodagreement between Vϕ

0 values in water for both the amino acidsobtained in this work and those reported in the literature [23,24]. Atinfinite dilution, the solute–solute interaction is negligible, therefore,the standard partial molar property and its temperature dependenceprovides valuable information of the solute–solvent interactions [25–27]. Table 3 shows that Vϕ

0 values of glycine and L-valine in aqueoustripotassium citrate solutions are positive and increase with increasein the salt concentration and temperature, thereby showing thepresence of strong solute–solvent interactions. The increase in Vϕ

0 forboth the amino acids in aqueous tripotassium citrate solutions may beattributed to the increase in solvation of amino acids at highertemperature as well as at higher region of T.P.C. i.e. release of somesolvent molecules from loose salvation layers of the solute in solution.In general Vϕ

0 values increase with increase in molar mass of amino

43.5

44

44.5

45

45.5

46

0 1.2

V

106

/ m3 ·

mol

-1

m / mol kg-110.80.60.40.2

Fig. 1. Plot of apparent molar volumes Vϕ for glycine in aqueous solutions oftripotassium citrate at T=308.15 K (ο, 0.2 mol kg−1;Δ, 0.4 mol kg−1;□, 0.6 mol kg−1;•, 0.8 mol kg−1).

Table 1 (continued)

m(mol·kg−1)

ρ×103

(kg·m−3)Vϕ×106

(m3·mol−1)ρ×103

(kg·m−3)Vϕ×106

(m3·mol−1)

T=308.15 K T=318.15 K

L-valine+0.6 mol·kg−1 tripotassium citrate0.00000 1.09582 1.091190.10291 1.09762 92.19 1.09279 94.160.14578 1.09834 92.29 1.09343 94.260.20235 1.09926 92.46 1.09425 94.390.25749 1.10014 92.57 1.09503 94.500.29774 1.10075 92.69 1.09557 94.620.34006 1.10139 92.79 1.09612 94.75

L-valine+0.8 mol·kg−1 tripotassium citrate0.00000 1.13268 1.128800.05434 1.13340 93.04 1.12940 95.070.10484 1.13405 93.13 1.12994 95.150.15472 1.13467 93.24 1.13045 95.270.20592 1.13529 93.33 1.13096 95.370.25598 1.13587 93.45 1.13144 95.46

91H. Kumar et al. / Journal of Molecular Liquids 162 (2011) 89–94

acids and also due to the hydrophobicity of alkyl side chain of aminoacids [28,29]. As seen in Table 3, the values of the slope SV* are positive,suggesting solute–solute interactions in the system. Moreover SV*,the experimental slope which indicates solute–solute interactions isinfluenced by number of effects [30]. The positive but smaller valuesof SV* as compared to Vϕ

0 suggest that solute–solute interactions areweaker than the solute–solvent interactions. The values of the slope

Table 2Viscosities (η) of glycine and L-valine in aqueous solutions of tripotassium citrate at T=(30

m(mol·kg−1)

η (mPa·s)

T=308.15 K T=318.15 K

Glycine+0.2 mol·kg−1 tripotassium citrate0 0.791 0.6810.19478 0.813 0.7010.29092 0.824 0.7100.39345 0.838 0.7190.48656 0.847 0.7310.59351 0.862 0.7510.63308 0.873 0.7450.77865 0.890 0.7690.91175 0.904 0.779

Glycine+0.6 mol·kg−1 tripotassium citrate

0 1.011 0.8740.35421 1.053 0.9240.47179 1.072 0.9390.54055 1.076 0.9470.62093 1.086 0.9630.73935 1.100 0.9690.94309 1.124 0.982

L-valine+0.2 mol·kg−1 tripotassium citrate

0 0.801 0.6940.09978 0.819 0.7030.14814 0.829 0.7170.19004 0.842 0.7250.24341 0.868 0.7410.29555 0.886 0.7600.3694 0.908 0.782

L-valine+0.6 mol·kg−1 tripotassium citrate

0 0.990 0.8590.09592 1.043 0.8880.14649 1.051 0.9090.19667 1.078 0.9280.24295 1.099 0.9500.29609 1.128 0.9720.35386 1.168 0.993

increase up to 0.6 mol kg−1 and 0.4 mol kg−1 for glycine and L-valine,respectively at both temperatures. An increase of salt concentrationmay enhance the amino acid–salt physical interactions, indicating thestructure making effect of amino acids and at higher concentrationsof tripotassium citrate. The larger values of the slope at higherconcentrations for both amino acids indicate the structure makingeffect in tripotassium citrate rich region.

8.15 and 318.15)K.

m(mol·kg−1)

η (mPa·s)

T=308.15 K T=308.15 K

Glycine+0.4 mol·kg−1 tripotassium citrate0 0.876 0.7370.15164 0.898 0.7490.21368 0.903 0.7540.35700 0.932 0.7790.47637 0.945 0.7900.58427 0.967 0.8160.67587 0.975 0.8210.74028 0.986 0.8260.88659 1.038 0.863

Glycine+0.8 mol·kg−1 tripotassium citrate

0 1.130 0.9680.20953 1.202 1.0540.39109 1.264 1.0820.57368 1.315 1.1160.72217 1.348 1.1290.95927 1.402 1.155

L-valine+0.4 mol·kg−1 tripotassium citrate

0 0.876 0.7370.09820 0.907 0.7510.13601 0.911 0.7590.16683 0.923 0.7690.29053 0.973 0.8090.33282 0.994 0.822

L-valine+0.8 mol·kg−1 tripotassium citrate

0 1.143 0.9680.09402 1.175 1.0240.14256 1.194 1.0480.19577 1.227 1.0710.26404 1.281 1.088

Table 3Limiting apparentmolar volumes, Vϕ

0, experimental slopes SV⁎ and transfer volumes, ΔVϕ0 of glycine and L-valine in aqueous tripotassium citrate solutions at T=(308.15 and 318.15)K.

m(mol·kg−1)

Vϕ0 ×106 (m3·mol−1) SV*×106 (m3·l1/2 mol−3/2) ΔVϕ

0 ×106 (m3·mol−1)

308.15 K 318.15 K 308.15 K 318.15 K 308.15 K 318.15 K

Glycine0.0 42.97 (±0.03) 43.81 [23] 44.32(±0.01) 1.26(±0.04) 1.19(±0.03)0.2 43.39 (±0.03) 44.48(±0.02) 1.60(±0.05) 1.39(±0.04) 0.42 0.160.4 43.68 (±0.02) 44.65(±0.02) 1.45(±0.03) 1.88(±0.03) 0.51 0.330.6 43.89(±0.02) 44.80(±0.02) 1.60(±0.04) 2.03(±0.03) 0.84 0.480.8 44.06(±0.02) 45.18(±0.02) 1.51(±0.04) 1.97(±0.04) 1.09 0.86

L-valine0.0 90.32(±0.02) 91.51 [24] 92.09(±0.03) 0.99(±0.07) 1.29(±0.10)0.2 91.02(±0.03) 92.95(±0.04) 1.43(±0.09) 1.63(±0.13) 0.70 0.860.4 91.38(±0.04) 93.39(±0.02) 2.89(±0.17) 2.93(±0.08) 1.06 1.300.6 91.93(±0.01) 93.90(±0.02) 2.54(±0.06) 2.43(±0.09) 1.61 1.810.8 92.92(±0.01) 94.96(±0.01) 2.02(±0.06) 1.98(±0.07) 2.60 2.87

92 H. Kumar et al. / Journal of Molecular Liquids 162 (2011) 89–94

Transfer volumes ΔVϕ0, of each amino acid from water to aqueous

tripotassium citrate solutions at infinite dilution were calculated byusing the equation

ΔV0ϕ = V0

ϕ in aq: tripotassium citrate solutionð Þ−V0ϕ in waterð Þ: ð3Þ

The results are reported in Table 3 and illustrated in Fig. 2. Thevalues of ΔVϕ

0are by definition free from solute–solute interactionsand therefore provide information regarding solute–solvent interac-tions. As seen from Table 3, the transfer volumes ΔVϕ

0 of each aminoacid give positive values in aqueous tripotassium citrate solutions. Thevalues increase with the increase in salt concentration at bothtemperatures. The positive ΔVϕ

0 values can be explained on the basisthat salt interact directly with the amino acids through electrostaticinteractions with the charged centers of the amino acids, therebyleading to reduction in the electrostriction of the solvent. The ΔVϕ

0

values can also be explained on the basis of cosphere overlap model[31,32] in terms of solute–cosolute interactions. According to thismodel, the ion–hydrophilic and hydrophilic–hydrophilic interactionslead to positive ΔVϕ

0 values and hydrophilic–hydrophobic interactionslead to negative values of ΔVϕ

0. The positive and increasing partialmolar volumes at infinite dilution in tripotassium citrate mixedsolvents suggest that in ternary solutions, the ion–hydrophilic andhydrophilic–hydrophilic group interactions are predominant overhydrophilic–hydrophobic group interactions. It is observed fromTable 3 that the magnitude of ΔVϕ

0 increases from glycine to L-valinewhich means that glycine–tripotassium citrate interactions are lessthan L-valine–tripotassium citrate interactions, i.e. lesser electrostric-tion of solvent water is produced in cases of L-valine leading to highervalues of ΔVϕ

0 and this shows that hydrophilic–ionic group in-teractions are less important in case of glycine. Moreover the additionof tripotassium citrate to aqueous amino acid solutions, as perKirkwood model, will coordinate the hydration spheres of K+ withthose of carboxylate ions and those of Cit3−or H2Cit−with the

0

1

2

3

0m / mol kg-1

V

106

/ m3 ·

mol

-1

0.80.60.40.2

0

Fig. 2. Plot ofΔVϕ0 of glycine (solid line) and L-valine (dash line) in aqueous tripotassium

citrate solutions at different temperatures (ο, 308.15 K; •, 318.15 K).

hydration sphere of ammonium ions so that molecules of watersolvent relax to bulk state due to interactions and results in thepositive value of ΔVϕ

0 of amino acid [13,33]. The most probableinteraction i.e. hydrophilic ionic group interaction between the ionspecies, those from dissociation of electrolyte and water moleculesinduces the dehydration of zwitterionic center of amino acids andtherefore increases the infinite dilution partial molar volume.

McMillar and Mayer [34] proposed a formalism to calculate theinteraction coefficients, which permits the separation of effects due tointeractions between the pairs of solutemolecules and those due to itsinteraction between two or more solute molecules. So, thermody-namic transfer function at infinite dilution can be expressed as

ΔV0ϕ water to aqueous tripotassium citrate solutionð Þ

= 2Vxymy + 3Vxyym2y + 4Vxyyym

3y + … …

ð4Þ

where x denotes amino acid, y denotes tri-potassium citrate andmy isthe molality of tri-potassium citrate. Constants Vxy, Vxyy, and Vxyyy

denote pair, triplet and quartet interaction coefficients. Theseconstant were calculated by fitting the ΔVϕ

0 values to Eq. (4) and arereported in Table 4.

The pair interaction coefficients Vxy and quartet interactioncoefficient Vxyyy are positive for both amino acids, whereas tripletinteraction coefficient Vxyy coefficient parameters are negative at bothtemperatures. The Vxyy has lower negative values for glycine whereaslarger negative values in case of L-valine. The higher Vxy values thanVxyy and Vxyyy show that interactions between tri-potassium citrateand amino acid are mainly pair wise i.e. the domination of pairinteractions for both the amino acids. The higher magnitude for theseinteraction coefficients in case of L-valine than glycine furthersuggests that interaction between L-valine–tri-potassium citrate isstronger than glycine–tri-potassium citrate and thus tri-potassiumcitrate has a stronger dehydration effect on L-valine. The increase inVxy values from glycine to L-valine arises due to difference in alkyl sidechains of amino acids with the T.P.C. This suggests that alkyl sidechains of amino acids play an important role in change of transfer

Table 4Pair, triplet and quartet interaction coefficients for glycine and L-valine in aqueoustripotassium citrate solutions at T=(308.15 and 318.15)K.

T(K)

Vxy×106

(m3·mol−2·kg)Vxyy×106

(m3·mol−3·kg2)Vxyyy×106

(m3·mol−4·kg3)

Glycine308.15 1.2173 −1.1703 0.6846318.15 0.5891 −0.6445 0.5623

L-valine308.15 2.3901 −2.8134 2.0418318.15 3.0752 −3.7627 2.5264

Table 6Values of V

01, V

02, Δμ1

0 *, and Δμ20 * for glycine and L-valine in aqueous tripotassium citratesolutions at T=(308.15 and 318.15) K.

m (mol·kg−1) T=308.15 K T=318.15 K

Glycine0.2 V

01 ×106 (m3 mol−1) 18.51 18.60

V02 ×106 (m3 mol−1) 42.97 44.48

Δμ10 * (kJ mol−1) 26.93 27.41Δμ20 * (kJ mol−1) 53.01 54.61

0.4 V01 ×106 (m3 mol−1) 18.90 18.98

V02 ×106 (m3 mol−1) 43.68 44.65

Δμ10 * (kJ mol−1) 27.24 27.68Δμ20 * (kJ mol−1) 62.44 65.51

0.6 V01 ×106 (m3 mol−1) 19.36 19.45

93H. Kumar et al. / Journal of Molecular Liquids 162 (2011) 89–94

volumes. This further suggests that L-valine having long alkyl sidechain may undergo stronger dehydration effect in the presence ofT.P.C. which results in higher values of transfer volumes and highvalues of Vxy.

The variation of relative viscosity ηr for amino acids in aqueous tri-potassium citrate can be represented by Jones–Dole equation [35]

ηr = η = η0 = 1 + AC1=2 + BC ð5Þ

where η and η0 are the viscosity of the solution and solvent(tripotassium citrate+water) respectively, A and B are the constantcharacteristics of ion–ion and ion–solvent interactions, respectively.Experimental data was fitted in Jones–Dole Eq. (5) to calculate thevalues of A and B. These values are reported in Table 5. It is observedfrom Table 5 that the values of A-coefficients are either negative(small) or positive (small) over the entire composition range oftripotassium citrate except at 0.6 mol kg−1 with L-valine indicatingthe presence of weak ion–ion interactions. Further the values of B-coefficients are positive for both the amino acids in aqueoustripotassium citrate solutions except for L-valine in 0.6 mol kg−1

tripotassium citrate solution. Large and positive B-values as comparedto A-values indicates that the ion–solvent interactions are strongwhich support the result of Vϕ

0 and SV*, both suggesting strongersolute–solvent interactions as compared to solute–solute interactions.It is also observed that at 0.2 and 0.4 mol kg−1 of tripotassium citrate,B-coefficients are high for both the amino acids. A perusal of Table 5shows that the magnitude of B-value for both the amino acidsdecreases with increase in temperature at higher concentration whileit increases at lower concentration of T.P.C. So its derivative oftemperature dB/dT [36] changes sign from positive to negative forboth amino acids which suggests that we can classify glycine and L-valine as a structure-maker in tripotassium citrate water mixtures.The sign of dB/dT values give important information regarding thestructure-making and structure-breaking ability of the solute in thesolvent media [37,38] rather than simply the B-coefficients.

The viscosity data were analyzed on the basis of transition statetreatment for relative viscosities of glycine and L-valine solutions assuggested by Feakins et al. [39]. The B-coefficients in terms of thistheory is given by following relationship

B = V01−V0

2

� �= 1000 + V0

1 = 1000RT Δμ0�2 −Δμ0�

1

� �ð6Þ

where V01 (=∑xiMi/ρ) is the mean volume of the solvent and

V02(=Vϕ

0) is the partial molar volume at infinite dilution of the solute.

Table 5Values of A and B parameters of Jones–Dole equation for glycine and L-valine in aqueoustripotassium citrate solutions at T=(308.15 and 318.15)K.

m(mol·kg−1)

T(K)

A×103/2

(m3/2·mol−1/2)B×103

(m3·mol−1)

Glycine0.2 308.15 −0.0138(±0.0076) 0.1640(±0.0093)

318.15 −0.0112(±0.0141) 0.1654(±0.0173)0.4 308.15 −0.0419(±0.0243) 0.2349(±0.0308)

318.15 −0.0587(±0.0174) 0.2459(±0.0220)0.6 308.15 0.0082(±0.0066) 0.0969(±0.0076)

318.15 0.0589(±0.0184) 0.0645(±0.0213)0.8 308.15 0.0694(±0.0176) 0.1545(±0.0197)

318.15 0.1778(±0.0103) 0.0070(±0.0116)L-valine

0.2 308.15 −0.1086(±0.0226) 0.5299(±0.0435)318.15 −0.1415(±0.0118) 0.5606(±0.0227)

0.4 308.15 −0.0717(±0.0251) 0.5235(±0.0507)318.15 −0.1199(±0.00054) 0.5561(±0.0108)

0.6 308.15 0.6476(±0.0752) −0.1293(±0.0142)318.15 0.6327(±0.0259) −0.1395(±0.0489)

0.8 308.15 −0.1495(±0.0390) 0.6599(±0.0825)318.15 0.1134(±0.0322) 0.2095(±0.0682)

The terms xi and Mi denote the mole-fractions and molar volumes ofwater (1) and tripotassium citrate (2), and ρ is the density of solventmixture (tripotassium citrate+water). The free energy of activationpermole of the solventΔμ10 * and the free energy of activation permoleof solute Δμ20 * were calculated [40] using relationships

Δμ0�1 = R T ln η0V

01 = h N

� �ð7Þ

Δμ0�2 = Δμ0�

1 + R T = V01 1000B– V0

1−V02

� �h ið8Þ

where h is Planck's constant, N is Avogadro number, η0 is the viscosityof the solvent and other symbols have usual significance. Thecalculated values of V0

1, V02, Δμ1

0 *, and Δμ20 * at both the temperaturesare given in Table 6.

It is clear from Table 6 that the values of Δμ20 * are positive andlarger than Δμ10 * for both the amino acids except for L-valine in0.6 mol kg−1 tripotassium citrate. This indicates stronger solute–solvent interactions which suggest that the formation of transitionstate is less favored in the presence of amino acids and the formationof transition state is accompanied by rupture and distortion of theintermolecular bonds in solvent structure i.e. the interaction betweensolute (amino acids) and solvent (T.P.C.+water) are stronger inground state in comparison to the transition state. The magnitude ofΔμ20 * decreases with increase in concentration of tripotassium citrateup to 0.6 mol kg−1 in case of L-valine and again increases for0.8 mol kg−1 whereas the values increase up to 0.4 mol kg−1 forglycine. The higher values of Δμ20 * for L-valine suggest that moreenergy is needed for the transfer from ground state solvent totransition state solvent for the amino acids with longer alkyl side

V02 ×106 (m3 mol−1) 43.89 44.80

Δμ10 * (kJ mol−1) 27.67 28.19Δμ20 * (kJ mol−1) 43.73 40.41

0.8 V01 ×106 (m3 mol−1) 19.83 19.90

V02 ×106 (m3 mol−1) 44.06 45.18

Δμ10 * (kJ mol−1) 28.02 28.53Δμ20 * (kJ mol−1) 51.11 32.28

L-valine0.2 V

01 ×106 (m3 mol−1) 18.64 18.70

V02 ×106 (m3 mol−1) 91.02 92.95

Δμ10 * (kJ mol−1) 26.98 27.48Δμ20 * (kJ mol−1) 109.64 168.92

0.4 V01 ×106 (m3 mol−1) 18.90 18.98

V02 ×106 (m3 mol−1) 91.38 93.39

Δμ10 * (kJ mol−1) 27.24 27.68Δμ20 * (kJ mol−1) 108.02 115.54

0.6 V01 ×106 (m3 mol−1) 19.48 19.57

V02 ×106 (m3 mol−1) 91.93 93.90

Δμ10 * (kJ mol−1) 27.63 28.16Δμ20 * (kJ mol−1) 20.15 19.35

0.8 V01 ×106 (m3 mol−1) 19.81 19.87

V02 ×106 (m3 mol−1) 92.92 94.96

Δμ10 * (kJ mol−1) 28.04 28.52Δμ20 * (kJ mol−1) 94.79 66.40

94 H. Kumar et al. / Journal of Molecular Liquids 162 (2011) 89–94

chains. As discussed in case of volumetric coefficients, the increase inΔμ20 * values from glycine to L-valine results from the difference ininteractions of alkyl side chains with tripotassium citrate whichincreases with increasing alkyl side chain length of amino acids.

According to Feakins model [39], the greater the value of Δμ20 *, thegreater is the structure-making ability of the solute. Table 6 showsthatΔμ20 * is greater at lower and higher concentrations of tripotassiumcitrate and decreases with temperature at 0.6 and 0.8 mol kg−1 oftripotassium citrate for both the amino acids having negative value ofdB/dT showing that the amino acids act as an effective structure-maker in tripotassium citrate–water mixtures.

Acknowledgments

One of the authors (KK) is thankful to The Director and Head,Department of Chemistry, Dr B R Ambedkar National Institute ofTechnology, Jalandhar for providing MHRD fellowship.

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