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CHAPTER 6
THERMODYNAMIC AND TRANSPORT PROPERTIES OF
GLYCINE IN AQUEOUS SOLUTIONS OF SODIUM
CARBONATE AT DIFFERENT TEMPERATURES
6.1 INTRODUCTION
The characterization of thermodynamical properties of protein
hydration can assist in understanding the conformational stability and the
unfolding behaviour of globular proteins (Kikuchi et al 1995). Because of the
structural complexities of proteins and the non-feasibility of direct
thermodynamic studies, amino acids and oligopeptides are often used as
model compounds since they are the fundamental components of proteins
(Duke et al 1994, Hakin et al 1994b, Millero 1971). It is recognized that
amino acids in aqueous solution have two oppositely charged carboxyl and
amino groups that may interfere with the hydration of the adjacent amino acid
side chains. In physiological media such as blood, membranes, cellular fluids,
etc., where water happens to be involved in an important manner, the
zwitterionic (dipolar) character of amino acids has an important bearing on
their biological functions (Ali et al 2007b). Also, the interactions of amino
acids with water molecules in aqueous solutions of salts and the temperature
dependence of these interactions play a vital role in understanding the nature
of action of bioactive molecules and /or the thermodynamic behaviour of
biochemical process in the body system.
199
Nagy and Jencks (1965) have discussed that electrolytes induce
dissociation in the protein without causing any conformational change or
denaturation. They have suggested that salts interact directly with the peptide
group of the protein and bring about its dissociation. Ultrasonic studies of
glycine, L-proline and DNA in aqueous solutions are undertaken by
Nambinarayanan and Rao (1989) and they observed that addition of small
quantities of strong structure beakers to water increase the cohesion among
the molecules by breaking the open structure. It is useful to extend the study
of amino acids to a mixed solvent system not only because mixed aqueous
solvents are used in Chemistry and other fields to control factors as solubility,
reactivity and stability of systems but also because biological fluids are
ultimately not pure water (Wadi and Ramasami 1997).
Volumetric properties of solute, such as the partial molar volume
and compressibility are known to be sensitive to the nature of hydration.
Further, the hydration effects are known to be very sensitive to temperature
(Kikuchi et al 1995). Viscosity is another important property that can yield
information on solute-solvent interactions (Badarayani et al 2005). The first
systematic study on the thermodynamic properties of protein solutions, in
particular of the partial volumes, is presented by Cohn and Edsall (1943).
Some workers have studied the compressibility of amino acids in aqueous
solutions (Cabani et al 1981, Chalikian et al 1993, Kharakoz 1991), but the
amount of available compressibility data for amino acids is much less
compared with volume data, although the compressibility seems to sense the
solute hydration structure at a greater distance from the solute than does the
volume (Chalikian et al 1993). Also compressibility is a powerful
thermodynamic parameter for elucidating the behaviour of a solute in a
solvent (Chalikian et al 1994).
200
Volumetric and viscometric studies of glycine in binary aqueous
solutions of sucrose have been carried out by Pal and Kumar (2005a) at
different temperatures. Li et al (2002) have obtained the partial molal volumes
of glycine, L-alanine, and L-serine in aqueous glucose solutions at 298.15 K
and interpreted the transfer volume by the cosphere overlap model. One of the
main uses of amino acids is, they are used as an additive in the food industry,
e.g. glycine is used for sweet jams and salted vegetables, sauce, vinegar and
fruit juice. The reason is that the taste of the naturally occurring amino acids
is categorised as either bitter or sweet (Barrett 1985). Sodium carbonate is
also a food additive used as an acidity regulator, anti-caking agent and
stabilizer.
Literature survey shows that no one has reported the work on AA in
aqueous sodium carbonate solutions. Hence this chapter deals with volumetric
parameters like apparent molal volumes (V ), partial molal volumes (V0),
transfer volumes ( V0), hydration number (nH), pair (VAB) and triplet (VABB)
interaction parameters of glycine in aqueous sodium carbonate solution.
Further, the data of density and ultrasonic speed values are used to evaluate
apparent molal compressibilities (K ), partial molal compressibilities (K0),
transfer compressibilities ( K0), hydration number (nH), pair (KAB) and
triplet (KABB) interaction parameters. Viscosity B-coefficients of Jones-Dole
equation, transfer B-coefficient ( B), pair ( AB) and triplet ( ABB) interaction
coefficients, free energy of activation per mole of solvent ( µ10*
) and solute
µ20*
) are estimated from viscosity data. All these parameters are obtained at
T= (303.15, 308.15 and 313.15) K are used to discuss the solute – solute and
solute – solvent interactions occurring in the ternary (glycine + sodium
carbonate + water) system. These properties are very sensitive to the nature of
hydration or interactive changes in solutions (Pal and Kumar 2005a).
201
6.2 EXPERIMENTAL
The densities of the solutions are measured using a single stem
pycnometer. The ultrasonic speed was determined using a multifrequency
ultrasonic interferometer (M-84, Mittal make, India) at a frequency of 2 MHz.
Viscosity was measured by means of a suspended level Ubbelohde
viscometer. Densities, ultrasonic speeds and viscosities of the solutions are
measured at temperatures T = (303.15, 308.15 and 313.15) K. The
procedures of measuring these parameters are discussed in detail in Chapter 2.
6.3 RESULTS
The densities of glycine in aqueous sodium carbonate solutions at
T= (303.15, 308.15 and 313.15) K are summarised in Table 6.1. The
uncertainty values for density are calculated and also given in Table 6.1.
Throughout this chapter m denotes molality of glycine and mS molality of
sodium carbonate.
The values of density are used to calculate the apparent molal
volumes (V ) of the solutes using the equation (1.1) and are presented in
Table 6.1. The apparent molal volumes (V ) calculated from equation (1.1)
are then fitted to equation (1.3) to obtain the limiting values of apparent molal
volumes V0 (partial molal volumes) as intercepts at zero concentrations.
However in those cases where molality dependence of V is found to be either
negligible or having no definite trend, as in the present case, the apparent
molal volumes at infinite dilution, V0
are evaluated by taking an average of
all the data points (Wang et al 1999, Bhat and Ahluwalia 1985, Yan et al
2004). The results are given in Table 6.2.
202
Table 6.1 Density ( ) and apparent molal volume, V , of glycine in aqueous sodium carbonate solutions at different
temperatures
mS = 0 mol·kg-1
mS = 0.1 mol·kg-1
mS = 0.3 mol·kg-1
mS = 0.5 mol·kg-1
m
(mol kg-1
) *103
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
T = 303.15 K
0.00 0.9956 1.0058 1.0238 1.0425
0.20 1.0017 44.36(0.36) 1.0117 45.21(0.32) 1.0295 45.88(0.27) 1.0476 48.31(0.24)
0.40 1.0084 42.57(0.35) 1.0166 47.44(0.31) 1.0334 49.96(0.26) 1.0519 49.94(0.23)
0.60 1.0145 42.81(0.34) 1.0221 47.02(0.30) 1.0386 49.08(0.26) 1.0558 50.96(0.23)
0.80 1.0196 44.07(0.33) 1.0274 46.94(0.30) 1.0431 49.38(0.25) 1.0599 51.14(0.22)
1.00 1.0255 43.94(0.33) 1.0328 46.69(0.29) 1.0480 49.08(0.25) 1.0646 50.60(0.22)
= 4.6×10-3
= 4.1×10-3
= 3.6×10-3
= 3.3×10-3
203
Table 6.1 (Continued)
mS = 0 mol·kg-1
mS = 0.1 mol·kg-1
mS = 0.3 mol·kg-1
mS = 0.5 mol·kg-1
m
(mol kg-1
)*10
3
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
*103
(kg m-3
)
V *106
(m3 mol
-1)
T = 308.15 K
0.00 0.9940 1.0041 1.0216 1.0403
0.20 0.9998 45.90(0.36) 1.0093 48.72(0.33) 1.0270 47.36(0.28) 1.0454 48.36(0.23)
0.40 1.0070 42.08(0.35) 1.0143 48.97(0.31) 1.0305 51.71(0.27) 1.0492 51.16(0.22)
0.60 1.0122 44.02(0.34) 1.0207 46.55(0.31) 1.0361 49.62(0.26) 1.0535 51.18(0.22)
0.80 1.0173 44.99(0.33) 1.0257 46.97(0.30) 1.0408 49.56(0.26) 1.0575 51.44(0.21)
1.00 1.0239 43.94(0.33) 1.0311 46.73(0.30) 1.0462 48.74(0.26) 1.0609 52.10(0.21)
= 4.5×10-3
= 4.2×10-3
= 3.7×10-3
= 3.2×10-3
T = 313.15 K
0.00 0.9922 1.0021 1.0195 1.0378
0.20 0.9980 45.93(0.36) 1.0072 49.27(0.32) 1.0247 48.37(0.28) 1.0427 49.35(0.23)
0.40 1.0047 43.37(0.35) 1.0126 48.27(0.31) 1.0288 50.80(0.27) 1.0468 51.00(0.22)
0.60 1.0106 43.70(0.34) 1.0182 47.43(0.30) 1.0336 50.33(0.26) 1.0506 51.89(0.22)
0.80 1.0158 44.63(0.33) 1.0232 47.64(0.29) 1.0383 50.10(0.26) 1.0546 52.00(0.22)
1.00 1.0216 44.48(0.33) 1.0285 47.38(0.29) 1.0437 49.18(0.25) 1.0589 51.69(0.21)
= 4.5×10-3
= 4.0×10-3
= 3.6×10-3
= 3.2×10-3
Values within parenthesis indicates the error in V
204
Table 6.2 Partial molal volume (0
V ) glycine in aqueous sodium
carbonate solutions at different temperatures
0V * 10
6 / m
3 mol
-1 at various ms / mol kg
-1
0.00 (Water)T / K
Present Work Literature 0.1 0.3 0.5
303.15 43.55 (0.36)43.59
a
43.89b 46.66(0.38) 48.66(0.72) 50.19(0.51)
308.15 44.19 (0.64) 44.2c 47.59(0.52) 49.40(0.71) 50.85(0.64)
313.15 44.42(0.44) 44.01d 48.00(0.35) 49.76(0.43) 51.19(0.49)
Values within parenthesis indicates the error in0
V
a Lark and Bala (1983),
bZhao et al (2004),
c Yan et al (1999),
d Hakin et al (1994b)
The partial molal volumes of transfer V0 of glycine from pure
water to aqueous sodium carbonate solutions are calculated using
equation (1.8). The evaluated values are presented in Table 6.3.
Table 6.3 Partial molal volume of transfer (0
V ) of glycine in aqueous
sodium carbonate solutions at different temperatures
0V * 10
6 / m
3 mol
-1 at various ms / mol kg
-1
T / K0.1 0.3 0.5
303.15 3.11 5.12 6.64
308.15 3.40 5.21 6.66
313.15 3.57 5.33 6.76
The hydration numbers nH are estimated from the volumetric data
using the standard equations (1.9) to (1.13) and are given in Table 6.4.
Further, the hydration number (nH) of glycine in aqueous sodium
carbonate solutions are calculated using compressibility data by the method
proposed by Millero et al(1978).The values of nH calculated using
equations (1.18) to (1.20) are also included in Table 6.4.
205
Table 6.4 Hydration number (nH) of glycine in aqueous sodium
carbonate solutions at different temperatures
nH at various ms / mol kg-1
0.1 0.3 0.5
T / K
From
volume
data
From
compressibility
data
From
volume
data
From
compressibility
data
From
volume
data
From
compressibility
data
303.15 1.23 2.28 0.73 1.98 0.35 1.80
308.15 1.00 2.11 0.54 1.80 0.18 1.58
313.15 0.90 1.97 0.46 1.46 0.10 1.38
The pair (VAB) and triplet (VABB) volumetric interaction parameters
are obtained by fitting V0
data to equation (1.14). The thermodynamic
transfer compressibilities at infinite dilution can be expressed by equation (1.14).
The KAB and KABB are the pair and triplet interaction parameters obtained by
fitting K0 data to equation (1.14). The viscometric pair ( AB) and triplet
ABB) interaction parameters are obtained using equation (1.14). The values
are listed in Table 6.5.
Table 6.5 Pair interaction coefficients, VAB / KAB / AB and triplet
interaction coefficients VABB / KABB / ABB of glycine in
aqueous sodium carbonate solutions at different temperatures
T / K VAB * 106
m3mol
-2kg
VABB * 106
m3
mol-3
kg2
KAB * 1014
m3
mol-1
kg Pa-1
KABB * 1014
m3
mol-1
kg Pa-1
AB * 103
m3
mol-2
kg
ABB * 103
m3mol
-3kg
2
303.15 16.925 -14.850 3.508 -3.593 0.060 -0.057
308.15 18.555 -17.256 3.526 -3.570 0.049 -0.050
313.15 19.505 -18.510 3.653 -3.553 0.038 -0.043
206
The experimental data on ultrasonic speed of glycine in aqueous
sodium carbonate solutions at T= (303.15, 308.15 and 313.15) K are given in
Table 6.6. The uncertainty values u for ultrasonic speed are also included in
Table 6.6.
Table 6.6 Ultrasonic speed (u) of glycine in aqueous sodium carbonate
solutions at different temperatures
u / m s-1
at various ms / mol kg-1
m
(mol kg-1
) 0.00 (Water) 0.1 0.3 0.5
T = 303.15 K
0.00 1512.0 1530.5 1554.1 1587.8
0.20 1522.5 1539.3 1563.1 1597.8
0.40 1531.2 1547.9 1573.0 1608.4
0.60 1540.6 1553.7 1578.6 1616.8
0.80 1550.9 1559.8 1587.0 1625.8
1.00 1560.6 1565.5 1593.1 1632.9
uncertainty u = 0.737 u = 0.531 u = 0.596 u = 0.695
T = 308.15 K
0.00 1520.4 1534.1 1558.8 1591.9
0.20 1530.8 1543.9 1567.7 1601.2
0.40 1538.8 1551.8 1578.9 1612.2
0.60 1549.8 1557.8 1584.7 1622.2
0.80 1558.6 1565.8 1592.7 1630.5
1.00 1566.7 1573.1 1599.4 1639.6
uncertainty u = 0.711 u = 0.583 u = 0.622 u = 0.735
T = 313.15 K
0.00 1528.0 1538.6 1564.3 1596.8
0.20 1538.2 1548.0 1572.6 1605.9
0.40 1546.9 1556.5 1581.8 1616.0
0.60 1556.1 1563.7 1588.3 1625.5
0.80 1565.8 1571.1 1596.0 1634.6
1.00 1574.9 1580.5 1602.4 1643.7
uncertainty u = 0.712 u = 0.625 u = 0.584 u = 0.720
207
The apparent molal compressibilities (K ) of the solutes can be
calculated, from density and compressibility data, using the equation (1.16)
and the values are reported in Table 6.7.
Table 6.7 Apparent molal compressibility (K ) of glycine in aqueous
sodium carbonate solutions at different temperatures
K * 1015
/ m3
mol-1
Pa-1
at various ms / mol kg-1
m
(mol kg-1
) 0.00 (Water) 0.1 0.3 0.5
T = 303.15 K
0.20 -24.48 -17.36 -15.25 -13.48
0.40 -23.14 -15.07 -13.00 -12.82
0.60 -22.47 -12.67 -10.41 -10.55
0.80 -21.69 -11.53 -10.23 -9.97
1.00 -21.53 -10.76 -9.33 -9.22
T = 308.15 K
0.20 -22.58 -17.13 -13.81 -11.86
0.40 -22.17 -14.20 -13.11 -11.59
0.60 -21.95 -13.49 -11.13 -11.36
0.80 -20.22 -13.08 -10.70 -10.09
1.00 -20.05 -12.82 -10.36 -9.32
T = 313.15 K
0.20 -21.78 -15.57 -11.48 -10.68
0.40 -21.58 -15.03 -10.58 -10.49
0.60 -20.82 -14.01 -9.02 -9.69
0.80 -20.05 -13.06 -8.93 -9.28
1.00 -19.76 -13.80 -8.81 -9.26
The apparent molal compressibilities (K ) calculated from
equation (1.16) are fitted to equation (1.17) to obtain the partial molal
compressibilities K0. In the present case the values of K
0 are obtained from
the linear plots of K vs m (Figure 6.1). The values of K0
and the
experimental slopes Sk are given in Table 6.8.
208
K /
(10
-15m
3.m
ol
-1.P
a-1
)
-18.0
-17.0
-16.0
-15.0
-14.0
-13.0
-12.0
-11.0
-10.0
0.00 0.20 0.40 0.60 0.80 1.00 1.20
m/(mol.kg-1
)
Figure 6.1 Plot of apparent molal compressibility (K ) against molality
(m) of glycine at T = ( ) 303.15K,( ) 308.15K, ( ) 313.15K,
of 0.1 M sodium carbonate solution
209
Table 6.8 Partial molal compressibility (K0), slopes (Sk) of glycine in aqueous sodium carbonate solutions at different
temperatures
K0 * 10
15Sk * 10
18K
0 * 10
15Sk * 10
18K
0 * 10
15Sk * 10
18K
0 * 10
15Sk * 10
18
m3mol
-1Pa
-1kg m
3mol
-2Pa
-1m
3mol
-1Pa
-1kg m
3mol
-2Pa
-1m
3mol
-1Pa
-1kg m
3mol
-2Pa
-1m
3mol
-1Pa
-1kg m
3mol
-2Pa
-1
various ms / mol kg-1
T / K
0.00 (Water) 0.1 0.3 0.5
303.15 -24.86(0.04) 3.67 -18.50(0.07) 8.37 -16.03(0.09) 7.31 -14.62(0.05) 5.69
308.15 -23.49(0.05)
-23.5e
3.50 -17.07(0.10) 4.87 -14.61(0.05) 4.65 -12.82(0.04) 3.30
313.15 -22.47(0.02)
-22.4f
2.78 -15.95(0.06) 2.76 -11.83(0.06) 3.49 -11.09(0.02) 2.03
eWadi and Ramasami (1997),
fKharakoz (1991)
210
The partial molal compressibilities of transfer K0
of glycine from
pure water to aqueous sodium carbonate solutions at different temperatures
are calculated using equation (1.8) and the results are given in Tables 6.9.
Table 6.9 Transfer partial molal compressibility (0
K ) of glycine in
aqueous sodium carbonate solutions at different temperatures
0K * 10
15 / m
3 mol
-1 Pa
-1 at various ms / mol kg
-1
T / K
0.1 0.3 0.5
303.15 6.36 8.83 10.24
308.15 6.42 8.88 10.67
313.15 6.52 10.64 11.38
In order to complement the results obtained from volumetric and
compressibility data, the viscosity ( ) values are also obtained for the same
system at the studied temperatures. The viscosity values and the uncertainty
values for viscosity are given in Table 6.10.
Table 6.10 Viscosity ( ) of – amino acids in aqueous sodium
carbonate solutions at different temperatures
/ m Pa s at various ms / mol kg-1
m
(mol kg-1
) 0.00 (Water) 0.1 0.3 0.5
T = 303.15 K
0.00 0.797 0.836 0.923 1.026
0.20 0.814 0.857 0.944 1.041
0.40 0.837 0.879 0.965 1.069
0.60 0.854 0.902 0.994 1.099
0.80 0.877 0.927 1.025 1.131
1.00 0.901 0.954 1.056 1.173
uncertainty = 0.015 = 0.018 =0.020 = 0.022
211
Table 6.10 (Continued)
/ m Pa s at various ms / mol kg-1
m
(mol kg-1
) 0.00 (Water) 0.1 0.3 0.5
T = 308.15 K
0.00 0.719 0.763 0.842 0.931
0.20 0.740 0.782 0.854 0.940
0.40 0.762 0.801 0.868 0.959
0.60 0.781 0.826 0.899 0.983
0.80 0.800 0.849 0.926 1.019
1.00 0.819 0.869 0.953 1.055
uncertainty = 0.015 = 0.016 = 0.017 = 0.019
T = 313.15 K
0.00 0.653 0.694 0.763 0.846
0.20 0.670 0.709 0.772 0.853
0.40 0.688 0.727 0.785 0.870
0.60 0.709 0.750 0.815 0.891
0.80 0.725 0.768 0.837 0.924
1.00 0.742 0.790 0.860 0.955
uncertainty = 0.014 = 0.015 = 0.016 = 0.017
The relative viscosity r of glycine in water and in cosolute
solutions are calculated using the equation (1.21). The viscosity B
coefficients are calculated by fitting the r values to the Jones – Dole equation
by the method of least squares using equation (1.23).
212
c/(mol dm-3
)
(r)
1.00
1.05
1.10
1.15
0.00 0.20 0.40 0.60 0.80 1.00
Figure 6.2 Plot of relative viscosity ( r) against molarity (c) of glycine
at T= ( ) 303.15K,( ) 308.15K, ( ) 313.15K of 0.1 M
sodium carbonate solution
The values of B coefficients and error values in B coefficients are
given in Table 6.11 along with the literature values. Good agreement between
experimental and literature values has been observed in the case of glycine in
water.
Table 6.11 Viscosity B - Coefficient of glycine in aqueous sodium
carbonate solutions at different temperatures
B * 103 / m
3 mol
-1 at various ms / mol kg
-1
0.00 (Water)T / K
Present Work Literature0.1 0.3 0.5
303.15 0.141(0.005) 0.137g
0.152(0.004) 0.159(0.007) 0.162(0.009)
308.15 0.144(0.002) 0.1447h 0.142
g 0.153(0.004) 0.157(0.010) 0.159(0.014)
313.15 0.147(0.003) 0.145g
0.154(0.004) 0.155(0.009) 0.156(0.013)
g Bhattacharya and Sengupta (1988),
h Mason et al (1952),
213
Transfer B coefficients B of glycine from water to aqueous
sodium carbonate solutions have been calculated using equation (1.8) and are
reported in Table 6.12.
Table 6.12 Viscosity B - coefficient transfer ( B ) of glycine in aqueous
sodium carbonate solutions at different temperatures
B * 103 / m
3 mol
-1 at various ms / mol kg
-1
T / K0.1 0.3 0.5
303.15 0.011 0.018 0.021
308.15 0.009 0.013 0.015
313.15 0.007 0.008 0.009
The solvation of any solute can be judged from the magnitude of
the ratio of viscosity B coefficient to partial molal volume. The B /V0 values
are calculated and are given in Table 6.13.
Table 6.13 Ratio of B - coefficient to partial molal volume (B / V0) of
glycine in aqueous sodium carbonate solutions at different
temperatures
B / V0 at various ms / mol kg
-1
T / K0.00 (Water) 0.1 0.3 0.5
303.15 3.23 3.26 3.27 3.23
308.15 3.26 3.21 3.18 3.13
313.15 3.31 3.21 3.11 3.05
214
The mean volume of the solvent ( 0
1V ) is calculated using
equation (1.26). The free energy of activation of viscous flow ( µ10*
) per
mole of solvent and free energy of activation of viscous flow ( µ20*
) per mole
of solute have been calculated by using the relations 1.26 and 1.27. The
values of 0
1V , µ1
0* and µ2
0*are given in Table 6.14.
Table 6.14 Mean volume of solvent ( 0
1V ), free energy of activation of
solvent ( 0*
1 ) and free energy of activation of solute ( 0*
2 )
of glycine in aqueous sodium carbonate solution at different
temperatures
ms
mol kg-1
0
1V
m3
mol-1
0*
1
kJ mol-1
0*
2
kJ mol-1
T = 303.15 K
0.0 18.09 9.04 32.23
0.1 18.07 9.16 34.35
0.3 18.06 9.41 35.87
0.5 18.03 9.67 36.81
T = 308.15 K
0.0 18.12 8.93 32.97
0.1 18.10 9.08 34.91
0.3 18.10 9.33 35.99
0.5 18.07 9.59 36.77
T = 313.15 K
0.0 18.16 8.83 33.68
0.1 18.13 8.99 35.38
0.3 18.13 9.23 36.03
0.5 18.11 9.50 36.67
215
6.4 DISCUSSION
Density of the solution (Table 6.1) increases with increase in
concentration of sodium carbonate. The increase in the values of density
attributed to increase in hydrophilic interactions (Malasane and Aswar 2005).
The increase in density may also be interpreted to the structure maker of the
solvent due to the added solute (Thirumaran and Sabu 2009).
It is seen from Table 6.2 that the partial molal volume V0 of
glycine increases with increase in sodium carbonate concentration. The partial
molal volume V0 values are by definition free from solute-solute interactions
and therefore provide information regarding solute-solvent interactions. It is
observed that V0 of glycine are positive in aqueous sodium carbonate at
different temperatures thereby showing the presence of strong solute-solvent
interactions. Similar results are reported by Pal et al (2010) for glycine in
aqueous saccharide solutions at different temperatures. At neutral pH, amino
acids exist as zwitterions and on dissolution in water there is an overall
decrease in the volume of the water. This is due to the contraction of the water
near the end groups, and is termed as electrostriction. According to the
Kirkwood model, addition of sodium carbonate will coordinate the hydration
spheres of the sodium ions with those of the carboxylate ions and those of
carbonate ions with the hydration spheres of the ammonium ions. As a result
of these interactions, the water molecules are allowed to relax to the bulk
state, and this accounts for the positive transfer volumes of the amino acids.
This is a qualitative interpretation of the results.
The magnitudes of the transfer volumes V0
of glycine increase
continuously with sodium carbonate concentration (Table 6.3). The positive
value of V0
indicates that the interaction between the charged centres of
glycine and ions dominates other forms of interactions. A similar conclusion
has been reported for some amino acids in aqueous Na2SO4 (Wadi and
216
Ramasami 1997), NH4Cl (Natarajan et al 1990), NaCl (Yuan et al 2006) and
NaC6 (Wang et al 2004).
The result obtained from V0 can also be viewed on the basis of the
continuum model of a solution (Wadi and Ramasami 1997, Millero et al
1978). This model has been used to obtain the equation
V0
= Vm + DVh = Vm + nH (Vh- Vb),
where Vm is the intrinsic volume of the solute molecule, DVh is the change in
the volume of hydration and Vb and Vh are the partial molar volumes of water
in the bulk state and in the hydration shell of a solution. Thus, addition of
sodium carbonate decreases the electrostriction and this also means that DVh
in the equation decreases as the electrostricted water becomes more like bulk
water. Hence V0 increases on the addition of sodium carbonate and V
0 is
positive assuming that Vm remains almost the same. Increasing the
concentration of sodium carbonate further decreases DVh and hence V0
increases. Increasing the temperature also reduces the electrostriction and V0
increases.
The changes in electrostriction are reflected in hydration numbers.
The decrease in nH values with the increase in the concentration of sodium
carbonate and temperature shows that sodium carbonate has dehydration
effect on amino acids. This also supports the view that electrolytes have a
dehydration effect on the amino acids in solution (Wang et al 1999, Ogawa
et al 1984b, Lin et al 2006). The reduction in the electrostriction with
increasing sodium carbonate and temperature is confirmed by the decrease in
nH, as given in Table 6.4.
In Table 6.5 it is noted that in aqueous sodium carbonate solution
the values of VAB are positive and VABB are negative. The positive values of
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the pair interaction coefficients VAB suggest that in mixture, the primary
interaction mode of glycine is large and the multimolecule interaction is
small. Hence the volume contributions mostly come from the interaction of
two molecules.
The values of pair and triplet interaction parameters (KAB and
KABB) are given in Table 6.5. The KAB values are positive and KABB values are
negative showing that ion/hydrophilic – hydrophilic interactions are
dominating in the solution. Banipal and Singh (2003) have reported similar
trend for glycine in aqueous n-propanol.
The viscometric pair ( AB) and triplet ( ABB) interaction parameters,
presented in Table 6.5, are positive and negative respectively. The positive
values of AB suggest the domination of pair interaction for glycine in aqueous
sodium carbonate solutions. But the small magnitude of AB indicates that pair
interaction parameters are sensitive to both cation and anion of the salt
(Banipal et al 2006a).
The increase in ultrasonic speed (Table 6.6) shows that molecular
association is being taking place in these liquid mixtures (Banipal et al 2007).
It is known that aqueous solution of glycine contain in addition to the
uncharged molecules NH2CH2COOH, an electrically neutral molecule, viz.,
+NH3CH2COO
- dipolar ions (zwitterions). When the amino acid is dissolved
in aqueous sodium carbonate the cations NH3+ and anions COO
- are formed.
The water molecules are attached to the ions strongly by the electrostatic
forces, which introduce a greater cohesion in the solution (Dash et al
2004).The factors apparently responsible for such behaviour may be the
presence of interactions caused by the proton transfer reactions of glycine and
hydrophilic nature of aqueous sodium carbonate.
218
As seen from Table 6.7 that the K values of glycine are negative at
all temperatures investigated. This indicates the presence of strong solute-
solvent interactions.
The partial molal adiabatic compressibility K0 is by definition free
from solute-solute interactions and hence provides information regarding
solute-solvent interactions. Solute-solute interactions can be understood from
the Sk values. It can be seen from Table 6.8 that the partial molal adiabatic
compressibilities (K0) of glycine in aqueous sodium carbonate solutions are
negative and this is due to the large negative contribution of the charged
atomic groups. The positive value of Sk indicate weak solute – solute
interactions.
Using the same continuum model, an equation can be written for
the partial molal adiabatic compressibility K0 of a solute (Wadi and
Ramasami 1997):
K0 = K m + nH (K m
0+ K b
0)
The bulk water has an open structure compared with electrostricted
water and is therefore more compressible. The electrostricted water becomes
like bulk water on addition of sodium carbonate and this accounts for the
apparent molal compressibilities for the amino acids in mixed solvents being
larger than the corresponding ones in water.
The values of transfer partial molal compressibility K0 are
positive and increases with increasing concentration of sodium carbonate
(Table 6.9). These positive values of transfer may be attributed due to the
interactions occurring between the glycine and sodium carbonate molecules.
Due to these interactions, the electrostriction of neighbouring water molecules
around the charged centres of glycine will be reduced in the presence of
219
sodium carbonate. Therefore the electrostricted water goes out of the
hydration spheres of these ions and enters into the bulk which is more
compressible (Hedwig and Hoiland 1994, Cabani et al 1979).
From Table 6.10, it is observed that the values of viscosity increase
with increase in glycine concentration as well as sodium carbonate
concentration. This increasing trend indicates the existence of molecular
interaction occurring in these systems.
Viscosity B coefficient is a measure of order or disorder introduced
by the solute in to the solvent (Kannappan and Palani 2007). It is also a
measure of solute – solvent interaction and the relative size of the solute and
solvent molecules. The behaviour of B coefficient (Table 6.11) of glycine in
aqueous sodium carbonate solutions suggests the existence of strong ion –
solvent interactions. The increase of B values with increasing sodium
carbonate molality reveals that this electrolyte gains a progressively more
structured environment. The sodium carbonate - glycine and sodium
carbonate - water interactions enhance the overall structure of the solvent
resulting in the increased B coefficient with increase in sodium carbonate
molality. Similar results are reported by Lark et al (2007) for glycine in
aqueous magnesium chloride solutions.
The transfer B coefficient B values are positive and increases with
increase in sodium carbonate molality. It is also seen from Table 6.12 that B
decreases with the increase in temperature. The positive values of B of
glycine in aqueous sodium carbonate solutions may be attributed to the more
structured medium in the presence of sodium carbonate solutions.
Also one improvement in the B coefficient concept is to divide the
B coefficient by the limiting apparent molal volume (V0) of the solute (Zhao
2006). A high B /V0 is an indication of the formation of a primary solvation
220
shell. The B /V0 ratio lies between 0 and 2.5 for unsolvated spherical spieces
(Stokes and Mills 1965) and greater than 2.5 for solvated spiecies. The B /V0
values listed in Table 6.13 shows that the values of B /V0
are greater than 2.5
and hence glycine is highly solvated in aqueous sodium fluoride solutions.
Table 6.14 shows that 2* values are positive and much larger
than 1 * suggesting that the interactions between solute and solvent
molecules in the ground state are stronger than in the transition state. Thus,
the solvation of the solute in the transition state is unfavourable in free energy
terms. Mishra and Gautam (2001) have observed similar results for glycine in
aqueous solution of transition metal chlorides. It is well-known that greater
the value of 2*, greater will be the stability of the structural arrangement of
the complexes. The findings are in accordance with the proposition of Feakins
et al (1986). On considering the system as a whole, it has been found that the
interaction generated out of solute-solute and solute-solvent are under active
observation. Here, the changes recorded in the measureable properties are the
consequences of the interactions between water, glycine and sodium
carbonate.
