CHAPTER 6 THERMODYNAMIC AND TRANSPORT PROPERTIES OF GLYCINE IN AQUEOUS SOLUTIONS OF SODIUM CARBONATE AT DIFFERENT TEMPERATURES

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1 198 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.

2 199 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).

3 200 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 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 (V 0 ), transfer volumes ( V 0 ), hydration number (n H ), pair (V AB ) and triplet (V ABB ) 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 (K 0 ), transfer compressibilities ( K 0 ), hydration number (n H ), pair (K AB ) and triplet (K ABB ) 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 ( µ 0* 1 ) and solute µ 0* 2 ) are estimated from viscosity data. All these parameters are obtained at T= (303.15, and ) 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).

4 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, and ) K. The procedures of measuring these parameters are discussed in detail in Chapter RESULTS The densities of glycine in aqueous sodium carbonate solutions at T= (303.15, and ) 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 m S 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 V 0 (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, V 0 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.

5 Table 6.1 Density ( ) and apparent molal volume, V, of glycine in aqueous sodium carbonate solutions at different temperatures m (mol kg -1 ) m S = 0 mol kg -1 m S = 0.1 mol kg -1 m S = 0.3 mol kg -1 m S = 0.5 mol kg -1 *10 3 (kg m -3 ) V *10 6 (m 3 mol -1 ) *10 3 (kg m -3 ) V *10 6 (m 3 mol -1 ) T = K *10 3 (kg m -3 ) V *10 6 (m 3 mol -1 ) *10 3 (kg m -3 ) V *10 6 (m 3 mol -1 ) (0.36) (0.32) (0.27) (0.24) (0.35) (0.31) (0.26) (0.23) (0.34) (0.30) (0.26) (0.23) (0.33) (0.30) (0.25) (0.22) (0.33) (0.29) (0.25) (0.22) = = = =

6 m (mol kg -1 ) *10 3 (kg m -3 ) Table 6.1 (Continued) m S = 0 mol kg -1 m S = 0.1 mol kg -1 m S = 0.3 mol kg -1 m S = 0.5 mol kg -1 V *10 6 *10 3 V *10 6 *10 3 V *10 6 *10 3 V *10 6 (m 3 mol -1 ) (kg m -3 ) (m 3 mol -1 ) (kg m -3 ) (m 3 mol -1 ) (kg m -3 ) (m 3 mol -1 ) T = K (0.36) (0.33) (0.28) (0.23) (0.35) (0.31) (0.27) (0.22) (0.34) (0.31) (0.26) (0.22) (0.33) (0.30) (0.26) (0.21) (0.33) (0.30) (0.26) (0.21) = = = = T = K (0.36) (0.32) (0.28) (0.23) (0.35) (0.31) (0.27) (0.22) (0.34) (0.30) (0.26) (0.22) (0.33) (0.29) (0.26) (0.22) (0.33) (0.29) (0.25) (0.21) = = = = Values within parenthesis indicates the error in V 203

7 204 0 Table 6.2 Partial molal volume ( V ) glycine in aqueous sodium carbonate solutions at different temperatures T / K 0 V * 10 6 / m 3 mol -1 at various m s / mol kg (Water) Present Work Literature (0.36) a b 46.66(0.38) 48.66(0.72) 50.19(0.51) (0.64) 44.2 c 47.59(0.52) 49.40(0.71) 50.85(0.64) (0.44) d 48.00(0.35) 49.76(0.43) 51.19(0.49) Values within parenthesis indicates the error in 0 V a Lark and Bala (1983), b Zhao et al (2004), c Yan et al (1999), d Hakin et al (1994b) The partial molal volumes of transfer V 0 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 ( V 0 ) of glycine in aqueous sodium carbonate solutions at different temperatures T / K V 0 * 10 6 / m 3 mol -1 at various m s / mol kg The hydration numbers n H 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 (n H ) of glycine in aqueous sodium carbonate solutions are calculated using compressibility data by the method proposed by Millero et al(1978).the values of n H calculated using equations (1.18) to (1.20) are also included in Table 6.4.

8 205 Table 6.4 Hydration number (n H ) of glycine in aqueous sodium carbonate solutions at different temperatures T / K From volume data n H at various m s / mol kg From compressibility data From volume data From compressibility data From volume data From compressibility data The pair (V AB ) and triplet (V ABB ) volumetric interaction parameters are obtained by fitting V 0 data to equation (1.14). The thermodynamic transfer compressibilities at infinite dilution can be expressed by equation (1.14). The K AB and K ABB are the pair and triplet interaction parameters obtained by fitting K 0 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, V AB / K AB / AB and triplet interaction coefficients V ABB / K ABB / ABB of glycine in aqueous sodium carbonate solutions at different temperatures T / K V AB * 10 6 m 3 mol -2 kg 14 m 3 V ABB * K AB * 10 K ABB * 10 AB * 10 3 m 3 mol -3 kg 2 m 3 mol -1 kg Pa -1 mol -1 kg Pa -1 m 3 mol -2 kg ABB * 10 3 m 3 mol -3 kg

9 206 The experimental data on ultrasonic speed of glycine in aqueous sodium carbonate solutions at T= (303.15, and ) 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 m u / m s -1 at various m s / mol kg -1 (mol kg -1 ) 0.00 (Water) T = K uncertainty u = u = u = u = T = K uncertainty u = u = u = u = T = K uncertainty u = u = u = u = 0.720

10 207 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 m K * / m 3 mol -1 Pa -1 at various m s / mol kg -1 (mol kg -1 ) 0.00 (Water) T = K T = K T = K The apparent molal compressibilities (K ) calculated from equation (1.16) are fitted to equation (1.17) to obtain the partial molal compressibilities K 0. 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 K 0 and the experimental slopes S k are given in Table 6.8.

11 K / (10-15 m 3.mol -1.Pa -1 ) m/(mol.kg -1 ) Figure 6.1 Plot of apparent molal compressibility (K ) against molality (m) of glycine at T = ( ) K,( ) K, ( ) K, of 0.1 M sodium carbonate solution

12 Table 6.8 Partial molal compressibility (K 0 ), slopes (S k ) of glycine in aqueous sodium carbonate solutions at different temperatures K 0 * S k * K 0 * S k * K 0 * S k * K 0 * S k * T / K m 3 mol -1 Pa -1 kg m 3 mol -2 Pa -1 m 3 mol -1 Pa -1 kg m 3 mol -2 Pa -1 m 3 mol -1 Pa -1 kg m 3 mol -2 Pa -1 m 3 mol -1 Pa -1 kg m 3 mol -2 Pa -1 various m s / mol kg (Water) (0.04) (0.07) (0.09) (0.05) (0.05) e (0.10) (0.05) (0.04) (0.02) f (0.06) (0.06) (0.02) 2.03 e Wadi and Ramasami (1997), f Kharakoz (1991) 209

13 210 The partial molal compressibilities of transfer K 0 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 ( K 0 ) of glycine in aqueous sodium carbonate solutions at different temperatures T / K K 0 * / m 3 mol -1 Pa -1 at various m s / mol kg 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 Table 6.10 Viscosity ( ) of amino acids in aqueous sodium carbonate solutions at different temperatures m / m Pa s at various m s / mol kg -1 (mol kg -1 ) 0.00 (Water) T = K uncertainty = = =0.020 = 0.022

14 211 Table 6.10 (Continued) m / m Pa s at various m s / mol kg -1 (mol kg -1 ) 0.00 (Water) T = K uncertainty = = = = T = K uncertainty = = = = 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).

15 ( r) c/(mol dm -3 ) Figure 6.2 Plot of relative viscosity ( r ) against molarity (c) of glycine at T= ( ) K,( ) K, ( ) K 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 * 10 3 / m 3 mol -1 at various m s / mol kg -1 T / K 0.00 (Water) Present Work Literature (0.005) g 0.152(0.004) 0.159(0.007) 0.162(0.009) (0.002) h g 0.153(0.004) 0.157(0.010) 0.159(0.014) (0.003) g 0.154(0.004) 0.155(0.009) 0.156(0.013) g Bhattacharya and Sengupta (1988), h Mason et al (1952),

16 213 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 Table 6.12 Viscosity B - coefficient transfer ( B ) of glycine in aqueous sodium carbonate solutions at different temperatures T / K B * 10 3 / m 3 mol -1 at various m s / mol kg The solvation of any solute can be judged from the magnitude of the ratio of viscosity B coefficient to partial molal volume. The B /V 0 values are calculated and are given in Table Table 6.13 Ratio of B - coefficient to partial molal volume (B / V 0 ) of glycine in aqueous sodium carbonate solutions at different temperatures T / K B / V 0 at various m s / mol kg (Water)

17 214 The mean volume of the solvent ( V ) is calculated using equation (1.26). The free energy of activation of viscous flow ( µ 1 0* ) per mole of solvent and free energy of activation of viscous flow ( µ 2 0* ) per mole of solute have been calculated by using the relations 1.26 and The values of 0 V 1, µ 0* 1 and µ 0* 2 are given in Table Table 6.14 Mean volume of solvent ( V ), free energy of activation of 0 1 solvent ( ) and free energy of activation of solute ( ) 0* 1 of glycine in aqueous sodium carbonate solution at different temperatures 0* 2 m s mol kg -1 0 V 1 m 3 mol -1 0* 1 kj mol -1 0* 2 kj mol -1 T = K T = K T = K

18 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 V 0 of glycine increases with increase in sodium carbonate concentration. The partial molal volume V 0 values are by definition free from solute-solute interactions and therefore provide information regarding solute-solvent interactions. It is observed that V 0 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 V 0 of glycine increase continuously with sodium carbonate concentration (Table 6.3). The positive value of V 0 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 Na 2 SO 4 (Wadi and

19 216 Ramasami 1997), NH 4 Cl (Natarajan et al 1990), NaCl (Yuan et al 2006) and NaC 6 (Wang et al 2004). The result obtained from V 0 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 V 0 = V m + DV h = V m + n H (V h - V b ), where V m is the intrinsic volume of the solute molecule, DV h is the change in the volume of hydration and V b and V h 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 DV h in the equation decreases as the electrostricted water becomes more like bulk water. Hence V 0 increases on the addition of sodium carbonate and V 0 is positive assuming that V m remains almost the same. Increasing the 0 concentration of sodium carbonate further decreases DV h and hence V 0 increases. Increasing the temperature also reduces the electrostriction and V increases. The changes in electrostriction are reflected in hydration numbers. The decrease in n H 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 n H, as given in Table 6.4. In Table 6.5 it is noted that in aqueous sodium carbonate solution the values of V AB are positive and V ABB are negative. The positive values of

20 217 the pair interaction coefficients V AB 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 (K AB and K ABB ) are given in Table 6.5. The K AB values are positive and K ABB 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 NH 2 CH 2 COOH, an electrically neutral molecule, viz., + NH 3 CH 2 COO - dipolar ions (zwitterions). When the amino acid is dissolved in aqueous sodium carbonate the cations NH + 3 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.

21 218 As seen from Table 6.7 that the K values of glycine are negative at all temperatures investigated. This indicates the presence of strong solutesolvent interactions. The partial molal adiabatic compressibility K 0 is by definition free from solute-solute interactions and hence provides information regarding solute-solvent interactions. Solute-solute interactions can be understood from the S k values. It can be seen from Table 6.8 that the partial molal adiabatic compressibilities (K 0 ) 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 S k indicate weak solute solute interactions. Using the same continuum model, an equation can be written for 0 the partial molal adiabatic compressibility K of a solute (Wadi and Ramasami 1997): K 0 = K m + n H (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 K 0 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

22 219 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 (V 0 ) of the solute (Zhao 2006). A high B /V 0 is an indication of the formation of a primary solvation

23 220 shell. The B /V 0 ratio lies between 0 and 2.5 for unsolvated spherical spieces 0 (Stokes and Mills 1965) and greater than 2.5 for solvated spiecies. The B /V values listed in Table 6.13 shows that the values of B /V 0 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.