Rangra, Vir Singh

Sharma, D R

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Indian]. Phys. 78B(1), 111-114 (2004) q A ou .5 U P ®Dielectric relaxation studies of binary mixtures of acetone and N-methylformamide in the benzene solutions using microwaveabsorptio|i dataVir Singh Rangra an(| D R Sharma*Department of Physics. H P University, ShiniD'171 005, Himadial Pradesh, IndiaTReceived 24 July 2003, acrepied 7 November 2003Abstract : Dielectric relaxation times (r) and the dipole moments (//>of the binary mixtures of different molar concentrations of acetone (CH^COCHO in the binary mixtures of acetone and N-methylformamide (NMF) in benzene solutions have been calculated by using standard standing wave microwave techniques and Gopala Krishna’s single frequency (9 88 GHz) concentration vanational method at dififeient tempera> tures (25°C, 30“C, 35°C, 40°C). The dielectric relaxation process have been found to be an activated process. The energy parameters ( ziSJ for the dielectric relaxation process of binary mixtures containing 50% mole-fraction of acetone have been calculated at the respective given temperatures and the comparison has been made with the corresponding energy parameters AS^) for the viscous flow. On the basis of theobservations, it is found tliat the dielectric relaxation process can be treated as the rate process like the viscous flow process. Solute-solute and solute-solvent molecular associations have been predicted.Keywords : Dielectric relaxation binary mixtures (acetone + NMF), microwave absorption.PACS Nos. : 77.22,Gm, 31 70.DkDielectric relaxation studies of polar molecules in non­polar solvents using microwave absorption methods have been attempted by many research workers [1-6]. Acetone (CH3COCH3) is a dipolar, aprotic solvent having dielectric constant [7] = 20.7 and dipole-moment [7] fj = 2.84Din the vapour phase. N-methylformamide (NMF) is an important non-aqueous amide with a large value of dielectric constant [8] {s' = 182.4 at 25®C) and possessing a dipole-moment [8] // = 4.0D approximately. It is possible to prepare binary mixtures of given dielectric constant value lying between that of acetone and NMF by mixing respective solvents in proper proportions. This motivated the authors to undertake the experimental work concerned with the dielectric relaxation process in the binary mixture of acetone and NMF in order to understand the molecular association in the whole concentration range of acetone in the binary mixtures. Dielectric relaxation studies in the microwave region provide meaningful information aboutself-association, solute-solute and solute-solvent type of the molecular associations among the polar molecules [9-11]. This is because of the capability of the microwaves to detect the weak molecular interactions. Dielectric relaxation of the binary mixtures of acetone and NMF have been studied in the whole concentration range of acetone in the binary mixtures using standard microwave standing wave techniques and single frequency (9.88 GHz) concentration variation method of Gopala Krishna [12]. The measurements have been made for the binary mixtures of different mole-fractions of acetone (0.0, 0.3, 0,5, 0.7, 1.0) at four different temperatures (25®C, 30®C, 35®C, 40°C). The dielectric relaxation process has been found to be an activated process. The energy parameters have been calculated for the binary mixtures having 0.5 mole- fraction of acetone. From the experimental observations, it is found that the dielectric relaxation process is a rate process like that of the viscous flow process. Solute-soluteCorresponding Author© 2004 lACS112 W  Singh Rangra and D R Sharmaand solute-solvent types of the molecular associations for NMF have been proposed.Pure samples of acetone (AR Grade) from SISCO Research Laboratories Pvt. Ltd., Mumbai (India) and N- methylformamide (GC Grade) from Fluka, Germany were dried with 4A molecular sieves for about 10 hours with occasional shakings. The pure sample of acetone was distilled through a long vertical fractionating column and the middle fraction was collected for use. The pure sample of NMF was distilled through a long vertical fractionating column under the reduced pressure and the middle fraction of distilled NMF was used. The X-band microwave bench (freq 9.88 GHz) was used. By mixing acetone with NMF in the calculated proportions, the binary mixture of required mole-fraction was made. Set of dilute solutions of the binary mixtures in the benzene solutions were made. Using microwave techniques of Heston et at [13], the dielectric constant (e ') and the dielectric loss (f") of dilute solutions of binary mixtures of acetone and NMF in benzene solution at different temperatures were calculated. The temperature of the solution was controlled by circulating thermostated water around the dielectric cell. Dielectric relaxation time (r )  and dipole moment of molecular entities in the benzene solutions were calculated following the single frequency concentration variational method of Gopala Krishna [12].Table 1 shows the dielectric relaxation time (r )  and dipole-moment (//) for different molc-fractions of (acetone + NMF) binary mixtures at different temperatures in the benzene solution. The values of the dipole-moments for pure acetone and NMF calculated in this table are very close to the literature values of the dipole-moment. This shows that pure acetone and pure NMF exist in the monomer form in the benzene solution. The values of the relaxation time (z) vary non-linearly with the increase in the mole-fraction of acetone in the binary mixtures (acetone + NMF) at all temperatures (25°C, 30°C, 35°C, 40°C). The relaxation time depends upon the size and shape of 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 the relaxation time from its value corresponding to one constituent to the value corresponding to the other constituent with the mole-fraction variation in the whole concentration range may be taken as the absence of any solute-solute association in the mixture. On the other hand, non-linear variation o f the relaxation time with the mole-ffaction is interpreted as the possible solute- solute molecular association in the binary mixture. In theIkble 1. Dielectric relaxation time (r) and dipole-moment (//) for different mole-fractions of (acetone + NMF) mixture in benzene solu­tion at different temperatures.Temperature( ° C )Mole Fraction of acetone in binary mixturer1 0fx (Debye)0 . 0 0 4 . 2 9 4 . 0 4  (//n m f )0 . 3 0 4 . 8 22 5 0 . 5 0 5 . 3 60 . 7 0 3 . 2 41 .0 0 2 . 0 4 2 .7 8  (/Atceume)0 . 0 0 4 . 2 0 3 ,9 8  OUnmf)0 . 3 0 4 . 7 63 0 0 . 5 0 5 .2 50 . 7 0 3 .1 51 .0 0 1 .8 5 2 .7 6  (/V eto n e )0 . 0 0 4 .1 1 3 ,9 5  (/Jnmf)0 . 3 0 4 . 6 93 5 0 . 5 0 5 . 1 40 . 7 0 3 . 0 6L O O 1 .6 9 2 . 7 6  (//aceionr)0 . 0 0 4 . 0 2 3 .9 10 . 3 0 4 . 6 34 0 0 . 5 0 5 ,0 30 . 7 0 2 . 9 61 .0 0 1 .5 2 2 .7 6 (/4 c c to n e )present case, the non-linear variation of the relaxationtime with the change in mole-fraction of acetone in the binary mixture at 25°C (Figure 1) shows the presence of solute-solute molecular associations. The solute-soluteOT“X0.0 02 04  X 0,8 t.OFigure 1. Variation of relaxation time with mole fraction of acetone in (NMF + acetone] mixture in benzene solution at 25®C,Dielectric relaxation studies o f binary mixtures o f acetone and N-methylformamide etc 113molecular association arising from the interaction of the fractional negative charge on the oxygen atom of acetone and fractional positive charge on the hydrogen atoms of NMF molecule may be proposed (Figure 2). It is foundC H 3,C H 3/= § - - 1 -oII-c-C H ,■NHFigure 2. Solute-solute association of NMF molecule in benzene.that there is a small variation in the dipole-moment values of binary mixtures of acetone and NMF in benzene solution with the rise in temperature (Table 1). This could be explained on the basis of the solvent-effects[14]. The value of the dipole-moment of acetone (binary mixture with l.(X) mole-fraction of acetone in the mixture at all temperatures) does not depend on the temperature. This indicates the absence of solute-solvent association for pure acetone in benzene solution. The value of the dipole-moment of NMF (binary mixture with 0.00 mole- fraction of acetone in the binary mixture) is found to change slightly with the ri.se of temperature. This predicts the solute-solvent type of molecular association for NMF in the benzene solution. NMF molecule shows the resonance hybrid structure. The molecular association arising because of the interaction of -i-ve fractional chaige at the sight of N-atom in NMF and ;r-delocalized electron cloud in the benzene ring of benzene molecule is shown in the Figure 3.°  + . C H 3  H - C - NFigure 3. Solute>solvem association of NMF molecule in benzene.For binaiy mixtures (50% mole-fraction of acetone in the binary mixture of acetone + NMF), it is found that the variation of log tT  versus l(PIT is straight line. This indicates that the dielectric relaxation process can betreated as a rate process. The energy parameters AFf, 4lSf) for the dielectric relaxation process have been calculated using Eyring rate equations [15], The energy parameters for the viscous flow AFr,  ^ AS,j) havealso been calculated treating the viscous flow as the rate |M‘occss. These two sets of the energy parameters have been compared as shown in Table 2. It is found that the dielectric relaxation process can be treated as the rate process just like the viscous flow.Im ble 2. Relaxation time (r), free energies of activation (AF,, k|al mole ’), enthalpies of activation AHt, in kcal mole ‘) andcitropics of activation (ASV AV,, in cal mole ’ deg ’) for (acetone 4- iIMF) mixture containing .50 mole% of acetone in benzene solution.■y........' f t m p .( * C )TOF, AHr AS, AH^102 5 5 .3 6 2 .0 8 1 .7 9 - ^ . 0 9 7 2 .9 2 2 .6 3 - 0 . 9 6 3 13 0 5 .2 5 2 .1 ! 1 .7 9 - 0 . 1 0 6 2 .9 2 2 .6 3 - 0 . 9 7 0 33 5 5 .1 4 2 .1 4 1 .7 9 - 0 . 1 1 4 2 .9 3 2 .6 3 - 0 . 9 7 7 34 0 5 .0 3 2 .1 7 1 .7 9 - 0 . 1 2 1 2 .9 4 2 .6 3 - 0 . 9 8 7 2The observations show that the free energy of activa­tion (AFt) for the dielectric relaxation process is less than the free energy of activation (AFt,) 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 viscous flow process, the rotational as well as the translational motion of the molecules is involved. It is found that the enthalpy of activation for the dielectric relaxation process (AH^) is less than the enthalpy of activation (AHrj) for the viscous flow process. The enthalpy of activation depends upon the local environment of the molecules. Different values for the enthalpy of activation indicate that the dielectric relaxation process and viscous flow process involves the breaking of bonds with the neighbouring molecules in a different way and to a different extent. The entropy of a system is the measure of the orderly nature of the system. If the environment of the system is co-operative for the activated process, then the change in entropy (AV )^ be­comes -ve. Whereas the +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 like that of the activated viscous flow state.114 Wr Singh Rangra and D R ShartnaReferences[1] A K Sharma, D R Sharma. K C Shanna and D S Gill leitsch. Fiir Physik. Ckmie Neue Folge, Hd 141 S 15 (1984)[2] Ashok Sharma and D R Sharma Indian J. Pure Appl. Phys 30 185 (1992)[3] Azima L G Saad, Shaiflk Adel H and F F Hanna Indian J. Phys 72B 495(1998)|4J Nagcsh Thakui and D R Sharma Indian J. Pure Appl. Phys 38 328 ( 2000)|5 | S K Sit, N Ghosh, U Saha and S Acharya Indian J. Phys. 71B 553(1997)(6) A D Vyas and V A Rana Indian J. Pure Appl. Phys. 40 69 (2002)[7] Nora E Hill, Worth E Vaughan, A H Price and Mansel Davies Di­electric Properties and Molecular Behaviour (London : Van Nostrand-Reinhold) p270 (1969)[81 J J Lagowski The Chemistry o f Non-Aqueous Solvents (New York • Academic) Vol II  p215 (1967)[9) S S Krishnaraurthy and S Sundrarajan J. Phys. Ckem. 73 4036 (1969)[10| K Higasi Bull. Chem. Soc. (Japan) 39 2157 (1966)[11] J S Dhull, D R Sharma, D S Gill and K N Lakshminarayana IndianJ. Phys. 56B 334 (1982)[12] K V Gopala Krishna Trans. Farad. Soc. 33 767 (1957)[13] W M Heston (Jr.), A D Franklin, E L Hennely and C P Smyth J. Am. Chem. Soc. 72 3443 (1950)[14] Nora E Hill, Worth E Vaughan, A H Price and Mansel Davies Di­electric Properties and Molecular Behaviour (London : Van Nostrand-Reinhold) p253 (1969)[15] H Eyring, S Glasstone and K J Laidler Theory o f Rate Process (New York: McGraw-Hill) p541 (1941)

Dielectric Relaxation Studies of Binary Mixtures of Acetone and N-Methylformamide in the Benzene Solutions Using Microwave Absorption Data

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Dielectric Relaxation Studies of Binary Mixtures of Acetone and N-Methylformamide in the Benzene Solutions Using Microwave Absorption Data

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