ids 130 (2007) 70–73www.elsevier.com/locate/molliq

Journal of Molecular Liqu

Molecular associations in binary mixtures of pyridine and chlorobenzene inbenzene solution using microwave absorption data

Sandeep Kumar ⁎, D.R. Sharma, N. Thakur, N.S. Negi, V.S. Rangra

Department of Physics, Himachal Pradesh University, Shimla, PIN 171005, India

Received 10 August 2005; accepted 15 April 2006Available online 11 July 2006

Abstract

The dielectric relaxation time (τ) of binary mixtures of different molar concentrations of pyridine (C5H5N) and chlorobenzene (C6H5Cl) in benzenesolution at different temperatures (25, 30, 35 and 40 °C) has been calculated by using standard microwave techniques and Gopala Krishna's singlefrequency (9.875 GHz) concentration variation method. The energy parameters (ΔHε,ΔFε, andΔSε) for the dielectric relaxation process of the binarymixture containing 0.5 mol fraction of pyridine have been calculated at the respective temperatures. Comparisons have been made with thecorresponding energy parameters for the viscous flow (ΔHη,ΔFη, andΔSη). From the observations it is found that the dielectric relaxation process canbe treated as the rate process. Based upon above studies, solute–solvent type of molecular associations arising from the interaction of chlorobenzene andbenzene and pyridine and benzene molecules has been proposed. No solute–solute type of molecular association has been observed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Dielectric relaxation; Binary mixture; Pyridine; Chlorobenzene

1. Introduction

Pyridine (C5H5N) is recognized as the non-aqueous aproticsolvent having dielectric constant ε′=12.40 [1] and dipolemoment μ=2.20 D [2]. Its boiling point is 115 °C [3]. Pyridine isnegligibly basic [3] and forms complexes with many salts. It isused in a wide variety of reactions, including electrophilicsubstitution, nucleophilic substitution, oxidation and reduction.Chlorobenzene (CLB) on the other hand has low dielectricconstant ε′=5.649 [4] and dipole moment μ=1.69 D [4]. Itsboiling and melting points are 132 °C and −45 °C, respectively[3]. Chlorobenzene is a neither acidic nor basic colourless liquidwith a pleasant smell. Chlorobenzene is insoluble in water butsoluble in alcohol, benzene and ether. The above characteristicphysical nature of pyridine and chlorobenzene motivated theauthors to study the molecular associations in binary mixtures ofpyridine and chlorobenzene over the whole concentration range.

Abbreviations: D, Debye; ε, Epsilon; τ, Tau; μ, Mau; CLB, Chlorobenzene.⁎ Corresponding author. Present address: C/0 Prof. D R Sharma, Department

of Physics, H P University, Shimla, PIN-171005, India. Tel.: +91 177 2830950.E-mail address: sndp_bhardwaj@yahoo.com (S. Kumar).

0167-7322/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2006.04.012

Dielectric relaxation studies in the microwave region providemeaningful information about the self-association, solute–soluteand solute–solvent type of the molecular associations among thepolar molecules. This is because of the capacity of microwaves todetect the weaker molecular interactions. Dielectric relaxationstudies of polar molecules in non-polar solvents from microwaveabsorption studies have been frequently carried out [5–7].Microwave standard standing wave techniques have been usedto measure the dielectric constant (ε′) and dielectric loss (ε″) fordilute solutions of (C5H5N+CLB) binary mixture in benzenesolution. Gopala Krishna's single frequency concentrationvariational method is used in the calculation of relaxation time(τ) [8]. Measurements have been made for binary mixtures ofdifferent mole fractions of pyridine (0, 0.3, 0.5, 0.7, 1) in thebinary mixture at different temperatures (25, 30, 35 and 40 °C).The energy parameters have also been calculated for the binarymixture having 0.5-mol fraction of pyridine. From the experi-mental observations it is found that the dielectric relaxationprocess, like the viscous flow process, is a rate process. Nosolute–solute association has been found, whereas solute–solventtypes of the molecular associations for pyridine and CLB havebeen proposed.

Table 1Dielectric relaxation time (τ) and dipole moment (μ) for different mole fractionsof pyridine in [pyridine+CLB] binary mixtures at different temperatures

Temp. °C Mole fractions of pyridine in binary mixture τ/10−12 (s) μ(D)

25 0.0 5.08 1.57CLB0.3 4.020.5 3.180.7 2.771.0 1.46 2.22Py

30 0.0 4.42 1.60CLB0.3 3.700.5 2.970.7 2.551.0 1.43 2.25Py

35 0.0 3.85 1.62CLB0.3 3.430.5 2.690.7 2.341.0 1.36 2.50Py

40 0.0 3.77 1.65CLB0.3 3.110.5 2.450.7 1.971.0 1.15 2.52Py

71S. Kumar et al. / Journal of Molecular Liquids 130 (2007) 70–73

2. Experimental details

Pure samples of pyridine, chlorobenzene and benzene suppliedby standard companies were used after fractional distillation.Pyridine (GR, Merck limited, Worli, Mumbai-400018) was keptover 4 Å molecular sieves for about 12–14 h and then wasdistilled through a vertical fractional column. Chlorobenzene(LR, Central DrugHouse (P) LTDNewDelhi) was dried with 4 Åmolecular sieves for 6–8 h and occasional shaking was done.Then CLB was fractionally distilled. Benzene (AR, central drughouse P Ltd, N Delhi) was dried by refluxing over freshly cutsodium metal for 6–8 h and distilled through a long verticalfractional column. The middle fractions of each distilled solutionwere collected for use. The solutions were prepared by makinguse of the molecular weights of the corresponding solute mol-ecules for the particular mole fractions. The following formulaswere used

w1 ¼ wdw12

w12 þ 1−nn

� �w22

and

w2 ¼ 1−nn

� �wdw22

w12 þ 1−nn

� �w22

where w1, w2 are the weights of pyridine and CLB solutions, n isthe mole fraction of pyridine in the binary mixture, w is therequired quantity of binary mixture, w12 and w22 are molecularweights of pyridine and chlorobenzene respectively. A set ofdilute solutions of binary mixtures in benzene solution wasprepared using the concept of weight fractions. For the set ofdilute solutions, dielectric constant (ε′) and dielectric loss (ε″)were measured using the standard microwave technique (Hestenet al [9]). Circulating thermostatt water around the dielectric cellcontrolled the temperature of the solution. Relaxation time (τ) of

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

the binary mixture for different mole fractions of pyridine in(pyridine+CLB) binary mixture and dipole moment (μ) forpyridine and CLB were calculated following the single frequencyconcentration variational method of Gopala Krishna [8].

3. Results and discussion

The variation of relaxation time (τ) with the increase in molefraction of pyridine in binary mixture presents an interestingbehaviour, Fig. 1. Table 1 shows the dielectric relaxation time (τ)and dipolemoment (μ) for different mole fractions of (pyridine+CLB) binary mixtures at different temperatures in the benzenesolution. The values of dipole moments for pure pyridine andCLB calculated in this table are very close to the literaturevalues. This shows that pure pyridine and CLB exists inmonomer form in benzene solution. It is found that there is smallvariation in the dipole moment of pyridine and CLB in benzenesolution with rise in temperature Table 1. This could beexplained on the base of the solvent effects [4].

The relaxation time depends upon the size and shape of therotating molecular entities in the solution. This method determinesthe average value of the relaxation time for the participatingmolecular entities in the solution. The linear variation of relaxationtime from its value corresponding to the one constituent to the valuecorresponding to the other constituent in its whole range may betaken as the absence of any solute–solute molecular association inthe binarymixture. On the other hand the non-linear variation of therelaxation time with the mole fraction of one of the constituents isinterpreted as possible solute–solute molecular associations in thebinarymixture. The value of relaxation time (τ) is found to decreaselinearly in the binarymixture of pyridine+CLBwith increase in themole fraction of pyridine at all temperatures (25, 30, 35 and 40 °C)as shown in Table 1. In the present case, the linear variation ofrelaxation time (τ) with the change of mole fraction of pyridine inthe binary mixture at temperatures 25, 30, 35 and 40 °C, Fig. 1,

Fig. 4. Variation of log (τT) versus1T� 103 at 0.5 mol fraction of pyridine in the

binary mixture.

Table 2Enthalpies of activation (ΔHε,ΔHη in kcal mol−1), free energy of activation(ΔFε,ΔFη in kcal mol−1) and entropies of activation (ΔSε,ΔSη in cal mol−1 degK−1) for (pyridine+CLB and pyridine+NB) binary mixtures containing 0.5 molfraction of pyridine at different temperatures

T°C

s10−12s

ΔHε

(0.054)aΔFε

(0.004)aΔSε(0.035)a

ΔHη

(0.008)aΔFη

(0.003)aΔSη(0.005)a

Pyridine+CLB in benzene solutionat 0.5 mol fraction of pyridine inbinary mixture

25 3.18 2.66 1.77 2.99 2.615 2.917 −1.01230 2.97 2.66 1.77 2.94 2.615 2.924 −1.01835 2.69 2.66 1.75 2.95 2.615 2.930 −1.02340 2.45 2.66 1.73 2.97 2.615 2.945 −1.054

Pyridine+NB in benzene solution

Fig. 2. Solute–solvent molecular association of pyridine and benzene.

72 S. Kumar et al. / Journal of Molecular Liquids 130 (2007) 70–73

shows the absence of solute–solute molecular associations. Thevalue of dipole moments of pyridine and CLB (Table 1) is found tochange slightly with temperature. This predicts the solute–solventtype of molecular association for both pyridine and CLBmoleculesin benzene solution. Molecular association between pyridine andbenzene arises because of the interactions of positive chargedelocalized at the pyridine molecule and π delocalized electroncloud at the benzene ring of the benzene molecules shown in Fig. 2where in benzene solution CLB molecule shows resonance hybridstructure. The molecular association arising because of theinteraction of +νe fractional charge at the site of chlorine atom inCLB and π-delocalized electron cloud in the benzene ring ofbenzene molecule is shown in Fig. 3.

For the binarymixture (0.5mol fraction of pyridine in the binarymixture) of pyridine+CLB, it is found, that the variation of log τTversus 103/T a straight line and is shown in Fig. 4. This indicatesthat, the dielectric relaxation process can be treated as the rateprocess. The energy parameters (ΔHε, ΔFε, and ΔSε) for thedielectric relaxation process have been calculated using Eyring rateequations [10]. The energy parameters for the viscous flow process(ΔHη,ΔFη, andΔSη) have also been calculated treating the viscousflow as a rate process. These two sets of the energy parameters areshown in Table 2. Energy parameters (for the dielectric relaxationprocess) for the binarymixture of pyridine+nitrobenzene [11] havebeen calculated and are shown in Table 2. For both the binarymixtures it is found that the dielectric relaxation process can betreated as the activated process.

It is found that ΔHε (enthalpy of activation for the dielectricrelaxation for CLB binary mixture) is greater than the ΔHη

(enthalpy of activation for the viscous flow process), whereas forthe binary mixture of pyridine+nitrobenzene, the enthalpy ofactivation for the dielectric relaxation process ΔHε is less thanthe enthalpy of activation for the viscous flow processΔHη. Theenthalpy of activation depends upon the local environment of themolecules. Different values for the enthalpy of activationindicate that the dielectric relaxation process and the viscousflow process involve the breaking of bonds with the neighbour-ing molecules in a different way and to a different extent.

It is found that the (in both the binary mixtures pyridine+CLB and pyridine+nitrobenzene) free energy of activation(ΔFε) of the dielectric relaxation process is less than the freeenergy of activation (ΔFη) of the viscous flow process. This may

Fig. 3. Solute–solvent molecular associations between chlorobenzene andbenzene.

be explained on the basis that the dielectric relaxation processinvolves the rotation of molecular entities whereas in the flowprocess the rotational as well as the translational motion of themolecules are involved.

If the environment of the system is cooperative for the activatedprocess, then the change in entropy becomes −νe. Where as +νevalue of the change in the entropy for activated process indicatesthe non-cooperative environment of the system and the activatedstate is unstable. In the present case, it is observed that, the changein entropy of the dielectric relaxation process is +νe, indicating

at 0.5 mol fraction of pyridine inbinary mixture

25 7.10 0.08 2.25 3.32 2.615 2.917 −1.01230 6.97 0.08 2.28 3.27 2.615 2.917 −1.01835 6.86 0.08 2.32 3.16 2.615 2.917 −1.02340 6.71 0.08 2.36 3.36 2.615 2.917 −1.054a Indicates that the standard deviation is the calculation of the thermodynam-

ical parameters.

73S. Kumar et al. / Journal of Molecular Liquids 130 (2007) 70–73

that the environment of the system is non-cooperative andunstable, unlike the activated viscous flow state.

Acknowledgements

The authors are grateful to theHead of the PhysicsDepartment,H P University, and Shimla-5 for providing necessary experi-mental facilities. Authors are also thankful to Prof. I J S Kour andDr. M S Chauhan of the Chemistry Department, H P University,Shimla-5 for their useful discussions.

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