Dielectric relaxation studies of binary mixture of pyridine and N,N-dimethylformamide in benzene solution using microwaves absorption data

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Dielectric relaxation studies of binary mixture of pyridine and N,N- dimethylformamide in benzene solution using microwaves absorption data

Sandeep Kumar, D R Sharma, N Thakur, N S Negi & V S Rangra Department of Physics, Himachal Pradesh University, Shimla 171 005 Received 6 July 2005; revised 11 November 2005; accepted 3 January 2006

The dielectric relaxation time (τ) of binary mixtures of different molar concentrations of pyridine (C5H5N) and N,N- dimethylformamide (DMF) in benzene solution has been calculated by using standard microwave techniques and Gopala Krishna’s single frequency (9.875 GHz) concentration variational method at different temperatures (25, 30, 35 and 40°C).

The solute-solute molecular associations have been proposed. The dielectric relaxation has been found to be an activated process. The energy parameters for this activated process have been calculated and compared with that of viscous flow process.

Keywords: Dielectric relaxation, Binary mixture, Pyridine, N,N-dimethylformamide IPC Code: G01R 27/26

1 Introduction

Pyridine (C₅H₅N) is recognized as the non-aqueous aprotic solvent¹ having dielectric constant ε′=12.40 at 25°C and dipole moment² μ=2.22D at 25°C. Its boiling point³ is 115°C. Pyridine is negligibly basic⁴ and it has property to form complexes with many salts, it is used in wide variety of reactions, including electrophilic substitution, nucleophilic substitution, oxidation and reduction. The second constituent of the binary mixture is N, N-dimethylformamide (DMF) is an important non-aqueous solvent having dielectric constant⁵ ε′=37.70 at 25°C and dipole moment⁵ μ=3.86D at 25°C. Its boiling and melting points are 153°C and −61°C respectively⁵. This molecular aspect of pyridine and DMF motivated the authors to study the dielectric relaxation behaviour of the present binary mixture. Dielectric relaxation studies of polar molecules in non-polar solvents from microwave absorption studies have been frequently attempted^6-10. The microwave techniques have been used to measure the dielectric constant (ε′) and dielectric loss (ε′′) for dilute solutions of (C₅H₅N+DMF) binary mixture in benzene solution. Goplala Krishna’s single frequency concentration variational method has been used¹¹. The dielectric relaxation studies in the microwave region provide meaningful information about the self- association, solute-solute and solute-solvent type of the molecular associations among the polar molecules. This is because of the capacity of

microwaves to detect the weaker molecular interactions. The dielectric measurements have been made for binary mixtures of different mole fractions of pyridine (0, 0.3, 0.5, 0.70, 1.0) and DMF at different temperatures (25, 30, 35 and 40°C). The energy parameters have also been calculated for the binary mixture of 0.50-mole fraction of pyridine (At 0.50-mole fraction of pyridine maximum solute-solute molecular association is observed). From the experimental observations, it is found that the dielectric relaxation process like the viscous flow process is a rate process. Solute-solute association has been found, whereas solute-solvent type of the molecular associations for pyridine and DMF has been proposed. The energy parameters for activated process (ΔHε, ΔFε and ΔSε have been calculated by using Eyring rate equations¹² and compared with the corresponding energy parameters (ΔHη, ΔFη and ΔSη)

of viscous flow process at different temperatures 25, 30, 35 and 40°C.

2 Experimental Details

Pure samples of pyridine, dimethylformamide and benzene supplied by standard companies were used after fractional distillation. Pyridine (GR, Merck Limited, Worli, Mumbai 400 018) was kept over 4Å molecular sieves for about 12-14 hr and then was distilled through vertical fractional column.

Dimethylformamide (extra pure, Sisco Research Laboratories PVT. Ltd., Bombay 400 060) was dried with 4Åmolecular sieves for 6-8 hr and occasional

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E-Mail Address: sndp_bhardwaj@yahoo.com

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shaking was done. Then DMF was fractionally distilled. Benzene (AR, central drug house P Ltd, New Delhi) was dried by refluxing over freshly cut sodium metal for 6-8 hr and distilled through a long vertical fractional column. The middle fraction of each distilled solution were collected for use. The dielectric constant (ε′) and dielectric loss (ε′′) of dilute solution of pyridine in benzene solution were calculated by using Hesten et al¹³. technique. The temperature of the solution was controlled by circulating thermosted water around the dielectric cell. The relaxation time (τ) of binary mixture for different mole fractions of pyridine in (pyridine+DMF) binary mixture and dipole moment (μ) for pyridine and DMF were calculated following the single frequency concentration variational method of Gopala Krishna¹¹.

3 Results and Discussion

Relaxation time (τ ) has been calculated using the relation:

1 dY dX

⎛ ⎞

τ = ⎜ω⎝ ⎟⎠

where

( )

2

2 2

1 2

X ε ε + + ε −′ ′ 2 ′′

= ε +′ + ε′′ , and

( )

² ²

3 Y 2ε′′

= ε +′ + ε′′

The slope of the straight-line curve plotted Y versus X gives the value of (dY/dX). The variation of relaxation time (τ) with the increase in mole fraction of pyridine in binary mixture presents an interesting behaviour as shown in Fig. 1. Tables 1-4 show the dielectric relaxation time (τ) and dipole moment (μ) for different mole fractions of (pyridine+DMF) binary mixtures at different temperatures in the benzene solution. The values of dipole moments for pure pyridine and DMF calculated in Tables 1-4 are very close to the literature values. This shows that pure pyridine and DMF exists in monomer form in benzene solution. It is found that there is small variation in the dipole moment of pyridine and DMF in benzene solution with rise in temperature (Tables 1-4). This could be explained on the basis of the solvent effects¹⁴. In the binary mixture of pyridine and DMF, relaxation time (τ) varies non-linearly with mole fraction of pyridine as shown in Fig. 1. The relaxation time depends upon the size and shape of

Fig. 1—Variation of relaxation time (τ) with mole fraction of pyridine in (pyridine +DMF) in the binary mixture in benzene solution

Table 1—Dielectric constant (ε′), dielectric loss (ε′′), dielectric relaxation time (τ) and dipole moment (μ) for different mole fractions of pyridine in [pyridine +DMF] binary mixture in benzene at 25°C temperature

Mole fraction Weight ε′ ε′′ τ/10⁻¹² μ of pyridine in fraction (±1%) (±3%) (s) (D)

binary mixture (w)

0.0032 2.3920 0.0176 0.00 0.0052 2.4255 0.0263 3.50 3.86_DMF

0.0073 2.4850 0.0390 0.0092 2.5264 0.0495 0.0032 2.3840 0.0141 0.30 0.0053 2.4150 0.0221 4.07

0.0074 2.4450 0.0310 0.0092 2.4755 0.0381 0.0033 2.4260 0.0199 0.50 0.0052 2.4425 0.0250 4.46

0.0076 2.4644 0.0306 0.0104 2.4896 0.0379 0.0042 2.3790 0.0126 0.70 0.0061 2.3955 0.0162 3.22

0.0087 2.4215 0.0215 0.0110 2.4485 0.0272 0.0030 2.3520 0.0086 1.00 0.0055 2.3680 0.0099 1.46 2.22Py

0.0070 2.3780 0.0109 0.0090 2.3910 0.0123

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Table 2—Values of ε′, ε′′, τ and μ for different mole fractions of pyridine in [pyridine +DMF] binary mixture in benzene at 30°C temperature

binary mixture (w)

0.0032 2.3650 0.0160 0.00 0.0052 2.4073 0.0244 3.40 3.85DMF

0.0073 2.4471 0.0360 0.0092 2.5012 0.0460 0.0032 2.3535 0.0127 0.30 0.0053 2.3860 0.0212 3.87

0.0074 2.4160 0.0279 0.0092 2.4390 0.0342 0.0033 2.3825 0.0188 0.50 0.0052 2.3990 0.0229 4.23

0.0076 2.4160 0.0279 0.0104 2.4410 0.0347 0.0042 2.3384 0.0113 0.70 0.0061 2.3580 0.0151 3.12

0.0087 2.3815 0.0199 0.0110 2.4050 0.0247 0.0030 2.3380 0.0078 1.00 0.0055 2.3530 0.0090 1.43 2.25Py

0.0070 2.3720 0.0110 0.0090 2.3780 0.0117

binary mixture (w)

0.0032 2.3440 0.0150 0.00 0.0052 2.3800 0.0226 3.35 3.84DMF

0.0073 2.4241 0.0340 0.0092 2.4735 0.0430 0.0032 2.3415 0.0133 0.30 0.0053 2.3710 0.0197 3.74

0.0074 2.3955 0.0264 0.0092 2.4194 0.0320 0.0033 2.3650 0.0171 0.50 0.0052 2.3795 0.0208 3.98

0.0076 2.3975 0.0260 0.0104 2.4195 0.0311 0.0042 2.3355 0.0112 0.70 0.0061 2.3520 0.0139 3.04

0.0087 2.3730 0.0182 0.0110 2.3990 0.0235 0.0030 2.3240 0.0070 1.00 0.0055 2.3460 0.0091 1.36 2.50Py

0.0070 2.3590 0.0102 0.0090 2.3750 0.0114

binary mixture (w)

0.0032 2.3387 0.0137 0.00 0.0052 2.3764 0.0210 3.22 3.78DMF

0.0073 2.4087 0.0310 0.0092 2.4646 0.0400 0.0032 2.3296 0.0113 0.30 0.0053 2.3610 0.0190 3.59

0.0074 2.3885 0.0250 0.0092 2.4090 0.0300 0.0033 2.3570 0.0167 0.50 0.0052 2.3696 0.0191 3.82

0.0076 2.3890 0.0250 0.0104 2.4095 0.0294 0.0042 2.3162 0.0107 0.70 0.0061 2.3320 0.0136 2.96

0.0087 2.3510 0.0173 0.0110 2.3704 0.0211 0.0030 2.3190 0.0072 1.00 0.0055 2.3420 0.0088 1.15 2.52_Py

0.0070 2.3520 0.0097 0.0090 2.3690 0.0109

the rotating molecular entities in the solution. This method determines the average value of the relaxation time for the participating molecular entities in the solution. The linear variation of relaxation time from its value corresponding to the one constituent to the value corresponding to the other constituent in its whole concentration range may be taken as the absence of any solute-solute molecular association in the binary mixture. On the other hand, the non-linear variation of the relaxation time with the mole fraction of one of the constituent is interpreted as possible solute-solute molecular associations in the binary mixture. In the present case, the non-linear variation of relaxation time with change in mole fraction of pyridine in binary mixture shows solute-solute or solute-solvent molecular associations. The value of relaxation time (τ) is found to increase linearly from the pure value of relaxation time of DMF in benzene solution. In the binary mixture of pyridine+DMF, with increase in the mole fraction of pyridine at XPy=0.3, the relaxation time increases at all temperatures (25, 30, 35 and 40°C) as shown in the Tables 1-4. The value of τ in the binary mixture increases with the increase in the concentration of

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pyridine in the binary mixture (up to X_Py= 0.5). At X_Py = 0.5, there are pyridine and DMF molecules nearly equal in ratio, so solute-solute molecular association is proposed. Further increasing the mole fraction of pyridine in the binary mixture, the concentration of pyridine increases and the relaxation time reaches to the pure value of pyridine in the benzene solution. In the present case, the non-linear variation of τ is shown in Fig. 1. Increasing the mole fraction of pyridine in the binary mixture at temperatures 25, 30, 35 and 40°C, shows the presence of solute-solute molecular associations (Fig. 2) at XPy=0.5. The value of the dipole moment of pyridine (binary mixture with 1.00 mole fraction of pyridine in the mixture) (Tables 1-4) depends on the temperature.

This indicates the presence of solute-solvent association of pure pyridine in the benzene solution.

The value of dipole moment of DMF (binary mixture with 0.00 mole fraction of pyridine in binary mixture) (Tables 1-4) is found to change slightly with temperature. The polar molecule may associate with the non-polar solvent molecules. The association may depend upon the temperature. Thus, the slightly temperature dependent variation of dipole moment of the rotating molecular entity may predict the solute- solvent type of molecular association. DMF molecule shows the resonance hybrid structure. It is proposed that the molecular association arise because of the interaction of +ve fractional charge at the site of carbon atom in DMF and π-delocalized electron cloud in the benzene ring of the benzene molecule (Fig. 3).

For the binary mixtures of 0.5 mole fraction of pyridine in the binary mixture of pyridine+DMF, it is found that the variation of logτT versus 10³/T is a straight line. This indicates that the dielectric relaxation process can be treated as the rate process.

The energy parameters (ΔH_ε, ΔF_ε and ΔS_ε) for the dielectric relaxation process have been calculated using Eyring rate equations¹². The energy parameters for the viscous flow (ΔHη, ΔFη and ΔSη) have also been calculated treating the viscous flow as the rate process. These two sets of the energy parameters have been compared as shown in Table 5. It is found that the dielectric relaxation process can be treated as the rate process just like the viscous flow.

It is found that the enthalpy of activation for the dielectric relaxation process (ΔHε) is less than the enthalpy of the viscous flow process (ΔHη). The enthalpy of activation depends upon the local environment of the molecules. The different values for the enthalpy of activation indicate that the

dielectric and viscous flow process involve the breaking of bonds with the neighbouring molecules in a different way and to a different extent. It is found that the free energy of activation (ΔFε) for the dielectric relaxation process is less than the free energy of activation (ΔFη) for the viscous flow process. This may be explained on the basis that the dielectric relaxation process involves the rotation of molecular entities whereas in the flow process, the rotational as well as the translational motion of the molecules is involved. The entropy of the system is the measure of the orderly nature of the system. If the environment of the system is cooperative for the activated process, then the change in entropy becomes –ve. Where as +ve value of the change in the entropy for activated process indicates the non-cooperative environment of the system and the activated state is unstable. In the present case, it is observed that the change in entropy of the dielectric relaxation process is -ve, indicating that the environment of the system is cooperative and stable, like the activated viscous flow state.

Fig. 2—Solute-solute molecular associations between pyridine and DMF in benzene

Fig. 3—Solute-solvent association between DMF and benzene Table 5—Enthalpies of activation (ΔHε, ΔHη, in kcal mol^-1), free energy of activation (ΔFε, ΔFη, in kcal mol⁻¹), entropies of activation (ΔSε, ΔSη, in cal mol⁻¹ deg K⁻¹) for pyridine+DMF mixture containing 0.50 mole fraction of pyridine in benzene solution at different temperatures

Temp. ΔHε ΔFε ΔSε ΔHη ΔFη ΔSη

°C

25 1.344 1.97 −2.100 2.615 2.917 −1.012 30 1.344 1.98 −2.099 2.615 2.924 −1.018 35 1.344 1.99 −2.097 2.615 2.930 −1.023 40 1.344 2.00 −2.097 2.615 2.945 −1.054

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