American Journal of Applied Chemistry
Volume 3, Issue 3, June 2015, Pages: 101-104

Thermodynamics of glycine in mixed solvents at 30O – 40O C

Gobardhan Nayak3, Guru Charan Pradhan2, Manas Ranjan Senapati1, *

1Department of Chemistry, Trident Academy of Technology, Bhubaneswar-751024, Odisha, India

2Department of Chemistry, Utkal University, Bhubaneswar-751004, Odisha, India

3Department of Chemistry, Einstein Academy of Technology & Management, Bhubaneswar, Odisha, India

Email address:

(M. R. Senapati)

To cite this article:

Gobardhan Nayak, Guru Charan Pradhan, Manas Ranjan Senapati. Thermodynamics of Glycine in Mixed Solvents at 30O – 40OC. American Journal of Applied Chemistry. Vol. 3, No. 3, 2014, pp. 103-104. doi: 10.11648/j.ajac.20150303.13


Abstract: The ion solvent interaction of glycine in ethanol + water, methanol + water, isopropanol + water, glycol + water, glycerol + water obtained from conductance measurement have been compared at 10, 20 and 30% (w/w) solvent composition with temperature range of 30-40oC have been studied. The physical parameters like K, DGo, DGo (el), and DG0t (Ch) have been calculated and ion-solvent interactions are inferred.

Keywords: Dielectric Constant, Walden Product, Ion-Solvent Interaction & Dipole Moment


1. Introduction

Amines in pure state are self associated through intermolecular hydrogen bonds. They are all electron donors which allow them to have specific interactions with other electron deficient molecules. The physical properties of the mixed solvents like ethanol + water, methanol + water, isopropanol + water, dioxane + water, glycol + water and glycerol + water viz. dielectric constant, dipole moments are very much different from that of water. These organic solvents are more or less aprotic. Water, the most commonly used solvent, is both polar and sustains hydrogen bonds. Water is both an electron donor and electron acceptor. These  and several other properties make a study of their aqueous mixture an interesting thing to explore particularly of the ionic processes accompanying the solution of strong electrolytes.

Fig.1 represents a 2-dimensional section of an idealized electrolyte solution. The ions are shown as spheres with unit electrical charge. The solvent (pale blue) is shown as a uniform medium, without structure. On average, each ion is surrounded more closely by ions of opposite charge than by ions of like charge. Conductivity is traditionally determined by measuring the AC resistance of the solution between two electrodes. Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity of ionic contributions. Lars Onsager gave a theoretical explanation of Kohlrausch's law by extending Debye–Hückel theory.

Fig. 1. An idealized representation of a solution of a 1:1 electrolyte

In the present communication conductivities of glycine in ethanol + water, methanol + water, isopropanol + water, dioxane + water, glycol + water, and glycerol + water mixture at 30o to 40oC have been measured to investigate the ion solvent interaction.

2. Experimental

The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance [8]. An alternating voltage is used in order to avoid electrolysis. The resistance is measured by a conductivity meter. Typical frequencies used are in the range 1–3 kHz. The dependence on the frequency is usually small, but may become appreciable at very high frequencies, an effect known as the Debye–Falkenhagen effect.

The salt used was of E-Merck’ extra pure variety. The purification of solvent, preparation of solvent and solution and measurement of the conductance has been described previously [3]. The conductance measurements were within accuracy of 0.002 and within the concentration range from 0.01 to 0.001 equivalent litre-1. The temperature of investigation was from 303 to 313 + 0.01 K.

3. Discussion

The Onsager equation [1] for a completely dissociated electrolyte is

Ù = Ùo – (A+BÙo) ÖC                                (1)

where A and B are independent of concentration of the electrolytes. It satisfactorily accounts from the change in the equivalent conductivities with concentration. Correct evaluation of Lo can be made by extrapolating to zero concentration of the line obtained by plotting Ù vs. C1/2. However the above method of extrapolation has been reported to the unreliable in case of a number of electrolytes involving incomplete dissociation or ion association. Davies has extended the Onsager’s equation and has tabulated the dissociation constants of a variety of salts, specially of higher valency type. Electrolytic conductivities have been used to study ion solvent interaction and salvation of various cations and anions in aqueous and non-aqueous solution.

The equivalent conductivity of glycine investigated weight % of ethanol, methanol, isopropanol, dioxane, glycol, glycerol (10, 20 and 30%) water mixture at 30-40oC and found to be almost linear with C1/2. The theoretical Slope (ST) calculated from the determined Ùo values for different electrolytes at different solvent composition have been obtained and compared with the experiment slope (S). Dielectric constants were calculated from the data of Akenlof and short, viscosities were determined experimentally. The ST and S values are almost in fair agreement and differ by 2 to 3%; Ùo values are given in table-1.

The Walden product [2] Ùoh0 (table 2) has been actually employed to study ion-solvent interaction in solution from conductivity data. The plot of Ùoh0 vs. t are found to be linear and is almost independent of temperature. Further the mere constancy of the Walden product at different temperature is most presumably due to compensating contribution of the temperature coefficient of the conductivity by the negative temperature coefficient of the viscosity of the solvent. The lesser the value of Ùoh0 the greater is the ion solvent interaction. Also electrostatic charge density of the ion plays an important role in inducing ion solvent interaction and salvation. It also appears that during these migration ions are covered with a sheath of solvent molecules resulting in a larger size of the solvodynamic unit, and a decrease in Ùoh0 (Table-2), so that the size of the solvated ions and the ion-solvent interaction is of the order ethanol + water > methanol + water: isopropanol + water > dioxane + water > glycol + water > glycerol + water is in agreement with the viscosity and apparent molar volume data. (To be published later).

Because of the use of aquo-organic solvents, the dielectric constant of the medium is lowered and there is more probability of ion-pair formation. Hence the method of Fuoss and Krauss [3] and that of Shedlovsky [4] have been utilized to calculate the dissociation constant and Ùo simultaneously; K values calculated by both the methods are in good agreement and are recorded in table-3. The K values decrease with the decrease in dielectric constant, i.e. with increase in non-aqueous solvent.

Table 1. Ùo/W-1.cm2

  Temp oC 10% 20% 30%
Methanol 30 125 99 96
+ 35 130 106 100
Water 40 142 114 104
Ethanol 30 122 97 92
+ 35 142 105 102
Water 40 160 128 127
Isopropanol 30 120 100 97
+ 35 140 110 109
Water 40 160 130 120
Dioxane 30 110 96 87
+ 35 120 99 90
Water 40 128 102 96
Glycol 30 125 109 100
+ 35 131 114 104
Water 40 137 118 109
Glycerol 30 127 110 98
+ 35 138 120 109
Water 40 145 132 120

The standard thermodynamic parameter DGo and DSo have been calculated in the usual manner. The plots of DGo and DSo vs. solvent composition are found to be linear. The extrapolated values give the thermodynamic parameter for water. The standard thermodynamic quantities (DGto and DSto) for transfer process for water from water to 10, 20 and 30% of organic solvent + water have been calculated by using Feakin’s and Tuner’s[5] method. DGto values are tabulated in table-7 and 8. The DGto values are all negative, which indicates that the ion pairs are in a lower free energy state in aquo-organic solvent than in water and hence the ion pair formation is favoured by decreasing the dielectric constant of the medium. Since single ion values of free energies are not available presently for the solvent mixtures studied, the method adopted by Khoo [6] is followed to study ion solvent content is increased. It is possible to split the DGto values into two parts as suggested by Roy et al. [7] i.e. chemical contribution denoted in their terminology by DGot (Ch) and an electrostatic contribution DGot (el), which has been calculated from the Born equation:

DGot(ed) = (Ne2/2) [(1/es+1/ew)(1/r++1/r-)              (2)

Table 2. Ùoh0/W-1 cm2

  Temp oC 10% 20% 30%
Methanol 30 1.11 1.12 1.11
+ 35 1.12 1.12 1.12
Water 40 1.12 1.11 1.12
Ethanol 30 1.09 1.09 1.10
+ 35 1.09 1.09 1.09
Water 40 1.09 1.09 1.09
Isopropanol 30 1.13 1.12 1.12
+ 35 1.13 1.13 1.11
Water 40 1.12 1.13 1.11
Dioxane 30 1.14 1.14 1.15
+ 35 1.15 1.16 1.15
Water 40 1.15 1.15 1.14
Glycol 30 1.11 1.12 1.11
+ 35 1.08 1.09 1.08
Water 40 1.10 1.10 1.10
Glycerol 30 1.16 1.24 1.17
+ 35 1.17 1.25 1.18
Water 40 1.18 1.25 1.19

Table 3. K x 102

  Temp oC 10% 20% 30%
Methanol 30 19.12 14.71 9.78
+ 35 18.52 15.61 11.71
Water 40 18.12 15.71 10.61
Ethanol 30 16.42 14.21 10.50
+ 35 17.42 14.53 10.44
Water 40 16.51 14.23 10.23
Isopropanol 30 17.52 15.41 11.72
+ 35 17.62 15.5 11.84
Water 40 17.39 15.3 11.95
Dioxane 30 20.6 16.5 12.4
+ 35 20.5 16.4 12.5
Water 40 20.4 16.4 12.5
Glycol 30 18.4 15.2 11.2
+ 35 18.5 15.5 11.4
Water 40 18.6 15.9 11.9
Glycerol 30 19.2 17.2 9.51
+ 35 19.4 17.4 9.2
Water 40 19.3 17.6 9.3

Table 4.DGto/J mol-1

  Temp oC 10% 20% 30%
Methanol 30 950 1620 2470
+ 35 915 1515 2440
Water 40 948 1598 2080
Ethanol 30 1040 1719 2568
+ 35 1000 1615 2505
Water 40 1050 1715 2520
Isopropanol 30 851 1800 2470
+ 35 862 1850 2390
Water 40 865 1780 2450
Dioxane 30 714 1415 2203
+ 35 815 1415 2340
Water 40 815 1414 2345
Glycol 30 664 1114 1825
+ 35 715 1214 2050
Water 40 744 1312 2092
Glycerol 30 694 1280 1845
+ 35 700 1215 1850
Water 40 710 1275 1820

Table 5.DGot (Ch)/J mole-1

  Temp oC 10% 20% 30%
Methanol 30 820 998 1670
+ 35 815 1065 1750
Water 40 810 1062 1686
Ethanol 30 940 1080 1640
+ 35 915 1100 1790
Water 40 910 1090 1680
Isopropanol 30 720 980 1700
+ 35 705 970 1680
Water 40 715 990 1715
Dioxane 30 680 814 1752
+ 35 640 912 1614
Water 40 500 914 1555
Glycol 30 620 930 1455
+ 35 580 870 1415
Water 40 570 815 1392
Glycerol 30 580 870 1415
+ 35 560 850 1380
Water 40 550 840 1390

Table 6.DSot (Ch)/JK-1. mole-1

  Temp oC 10% 20% 30%
Methanol 30 5.45 11.41 13.21
+ 35 5.22 11.52 13.42
Water 40 5.51 11.52 13.41
Ethanol 30 6.72 12.50 13.91
+ 35 6.22 12.12 14.56
Water 40 6.52 12.05 14.14
Isopropanol 30 4.81 11.34 13.14
+ 35 4.62 11.25 13.25
Water 40 4.67 11.78 13.45
Dioxane 30 4.22 10.34 12.64
+ 35 4.82 10.34 12.64
Water 40 4.92 10.54 11.95
Glycol 30 3.1 8.2 10.1
+ 35 3.2 8.6 10.5
Water 40 3.6 8.9 10.9
Glycerol 30 3.1 8.4 10.0
+ 35 3.2 8.6 10.2
Water 40 3.3 8.7 10.4

where r+ and r- are the crystallographic radii of the

dînew/dT & dînes/dT

can be evaluated from the simple empirical equation :

dîneo/dT = -1/q                                    (3)

in which q is a constant characteristic of the medium, so equation may be written as :

DSot (el)= Ne2/2[(1/esqs – 1/ewqw) (1/r+ + 1/r-)       (4)

From the knowledge of DGot (el) and DGot (el), the chemical contribution of the free energy transfer, DGot (Ch) and entropy transfer, DSot (Ch) could be calculated by subtracting the respective electrostatic contribution values from the molar quantities and are tabulated in table 4 to 6. It is evident that the chemical contribution of the free energy transfer is negative in all cases and hence is thermodynamically favourable as far as the chemical interactions are concerned, and is of the order:

Ethanol + water > methanol + water > isopropanol + water > dioxane + water > glycol + water > glycerol + water.

The DSot (Ch) is also negative in all cases indicating chemical interaction and is of the order:

Ethanol + water > methanol + water > isopropanol + water > dioxane + water > glycol + water > glycerol + water.

The reasons for this behaviour are as follows:

Ethanol, methanol and isopropanol have got one –OH and water is both an electron donor and acceptor. Hence, the former could accept a proton from water and hence the three dimensional water structures are easily broken down.

The addition of a small amount of dioxane to water may give rise to two effects; if the dioxane is accommodated in the solvent structure, it may strengthen the water structure because dioxane is a proton acceptor. If it cannot be accommodated because of its bulky size then it may cause a breakdown in three dimensional water structures. Several authors have observed that dioxane + water is less ordered than pure water. It is observed that DE and DG increase with increase in dioxane content and hence the three dimensional water structure is broken down though the quanta are less than that of ethanol and methanol + water mixtures.

Glycol has got two –OH groups and glycerol has got 3-OH groups. So it should have more tendencies to break hydrogen bond more readily than methyl alcohol and ethyl alcohol, but the reverse is seen to be true. This is probably due to the low ion solvent dipole interaction energy which is unable to break the strong intermolecular hydrogen bond.

Hydrogen bonding has an effect on the activation energy and conduction mechanism. In literature, the high electrical conductivity obtained was due to hydrogen bonding. It is known that the increase in conductivity with temperature is related to the increase in the population of electrons in the conduction band.


References

  1. Das, N.C., Mishra, P.P and Das, P.B., Acta Ciencia Indica. 3, 136 (1979).
  2. Das, P.B., Ion-Solvent Interaction D.Sc. Thesis, Sambalpur University (1994).
  3. Fuoss, R.M. and Kraus, A., J.Am Chem Soc. 45, 476 (1933).
  4. Shedlovsky, T., J. Franklin Inst 225, 439 (1939).
  5. Feakin, D. and Turner, D.J. Chem Soc. 4984 (1965).
  6. Khoo, K. and Chan, C., Aust J Chem, 28, 721 (1973).
  7. Roy, R.N., Verson, W. and Bothwell, A.L.M., Electrochmica Acta 15, 826 (1977).
  8. Bockris, J. O'M.; Reddy, A.K.N; Gamboa-Aldeco, M. (1998) Modern Electrochemistry (2nd.ed.). Springer.

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