T Madhu Mohan*, S Sreehari Sastry* & V R K Murthy**

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 48, Septeber 010, pp Microwave dielectric relaxation, therodynaic and conforational studies of hydrogen bonded binary ixtures of propan-1-ol with ethyl benzoate and ethyl benzoate T Madhu Mohan*, S Sreehari Sastry* & V R K Murthy** *Departent of Physics, Acharya Nagarjuna University, Nagarjunanagar **Departent of Physics, Indian Institute of Technology, Chennai *E-ail: ssreeharisastryanu@gail.co Received 19 April 010; accepted July 010 The dielectric relaxation studies of propan-1-ol with alkyl benzoates (ethyl benzoate and ethyl benzoate) have been carried out, for various ole fractions at different teperatures, using LF ipedance analyzer, Plunger ethod and Abbe s refractoeter in radio, icrowave and optic frequency regions, respectively. Kirkwood effective correlation factor, corrective Kirkwood correlation factor, Bruggean paraeter, relaxation tie and thero dynaical paraeters are calculated using the experiental data. Conforational analysis of the foration of hydrogen bond in the equiolar binary ixture systes of propan-1-ol with alkyl benzoates is supported by experiental and theoretical FT-IR values. Keywords: Corrective Kirkwood correlation factor, Relaxation tie, Hydrogen bond, FT-IR 1 Introduction The dielectric relaxation study of solute solvent ixture at icrowave frequency gives inforation about olecular interactions in the syste, foration of onoers and ultiers. Dielectric relaxation spectroscopy was proved to be a powerful tool for the investigation of H-bond rearrangeent dynaics and has been widely applied to study the pure solvents, solvent-solvent ixtures 1,. Dielectric investigation of binary polar liquid ixtures consisting of one associative and other nonassociative liquids, provides valuable inforation regarding olecular coplex foration in solution. Alcohols are industrially and scientifically iportant organic copounds and their physical and cheical properties are largely deterined by the OH group. Alcohols are strongly associated in solution because of dipole-dipole interaction and hydrogen bonding. Alkyl benzoates are non-associated in solution, good hydrogen bonding acceptors and iportant industrial cheicals that are widely used in perfuery and pesticides. The strength of the olecular association depends on several factors including the olecular structure, teperature, solvent and other factors 3. The presence of hydrogen bond brings a considerable change in the dielectric properties of liquid ixtures. Therefore, the solution cheistry of these copounds can be strongly influenced by the aggregation phenoena, which can play a significant role in physical properties like refractive index, dielectric perittivity, density and olar volue. Further, the therodynaic properties of liquids and liquid ixtures have been used to understand the olecular interactions between the coponents of the ixture and also for engineering applications concerning heat transfer, ass transfer, fluid flow, activation energy, enthalpy, entropy, etc of the polar olecules 4-9. In the course of present study, to get better understanding of the nature of olecular orientation processes, we studied the teperature dependent dielectric relaxation in pure and binary ixtures of propan-1-ol with ethyl benzoate and ethyl benzoate at different frequencies. The dielectric data is used to calculate Kirkwood effective correlation factor, corrective Kirkwood correlation factor, Bruggean paraeter, relaxation tie and the therodynaic paraeters-gibb s energy of activation, olar enthalpy and olar entropy Conforational analysis of the foration of hydrogen bond between equiolar ixtures of propan-1-ol with benzoates is studied fro FT-IR spectra. The theoretical vibrational frequencies of the pure and equiolar hydrogen bonded systes are obtained fro Hailtonian quantu echanical calculations using Spartan odeling software.

2 MOHAN et al.: MICROWAVE DIELECTRIC RELAXATION OF HYDROGEN BONDED BINARY MIXTURES 669 Experiental Details The copounds propan-1-ol (1PN), ethyl benzoate (MB) and ethyl benzoate (EB) of AR grade are procured fro E. Merck, Gerany and are purified by standard ethods. The binary ixtures are prepared for different ole fractions i.e., ole fraction (X ) of benzoates (MB and EB) is varied fro 0 to 1 in IPN (with a step increent of 0.1). The teperature controller syste with a water bath, supplied by M/s Sakti scientific instruents copany-india, has been used to aintain the constant teperature within the accuracy liit of ±1 K. Densities at different teperatures are easured by using a 10 l specific gravity bottle and METTLER TOLEDO balance (Model No: AB135- S/FACT) whose accuracy is 0.01g. The static perittivity values at the spot frequencies 1 khz (ε static ), 10 khz, 100 khz, 1 MHz and 10 MHz for the above systes are easured using HP-LF ipedance analyzer (Model No: 419 A) at different teperatures. The real (ε ) and iaginary (ε ) parts of the coplex dielectric perittivity (ε* ε jε ) are deterined in X-Band (8.60 GHz) using Plunger technique 15 for the above teperatures. The high frequency dielectric perittivity (ε =n ) is obtained fro the refractoeter easureents using M/s ASCO ake Abbe s refractoeter with sodiu D light as source (10 15 Hz) at different teperatures. The error in the estiation of ε static, ε, ε and density is 1% and the error in the estiation of ε, τ is %. Dipole oents of the liquids in gaseous state are taken fro literature 16. The FT-IR-spectra of pure and equi olar binary ixture systes were recorded in the c 1 region on Perkin-Eler (spectru bx) series. 3 Theory The olecular interactions between a polar solute and a non-polar solvent can be described by the odified Kirkwood-Frohlich correlation factor. The Kirkwood-Frohlich theory 15 takes into account of the short-range interactions through the introduction of the diensionless correlation factor g, which gives inforation regarding the orientation of the electric dipoles in polar liquids. The Kirkwood correlation factor (g) for the pure liquids is given 17 by the expression: ( ε ε )( ε + ε ) ε( ε + ) 4π N Aµ ρ g = 9kTM (1) where N A is the Avogadro s nuber, µ the dipole oent in the gaseous phase, ρ the density, k the Boltzann constant, T the teperature in kelvin, M the olecular weight, ε the dielectric perittivity at static frequency and ε is the perittivity at optical frequency which is the square of refractive index. For a ixture of two polar liquids, say 1 and the Eq. (1) could be odified 18 using different assuptions. (1) Assuing that g eff is the effective correlation factor for the ixture, the Kirkwood equation for the ixture can be expressed as: 4π N A µ 1ρ 1 µ ρ eff X 1 + X. g 9kT M 1 M ( ε ε )( ε + ε ) = ε ε + or ( ) () () Assuing that the dipole oents of both the liquids are affected by the sae aount g f the Kirkwood equation for the ixture is odified as: 4π N A µ 1ρ 1g1 µ ρg f X1 + X. g 9kT M1 M ( ε ε )( ε + ε ) = ε ε + ( ) (3) where X 1 and X are the ole fractions of liquids 1 and in the ixture respectively and the suffix 1, and represents liquid 1, liquid and ixture, respectively. Here the value of g eff in Eq. () varies fro g 1 to g as the concentration (ole fraction) of liquid increases fro 0 to 1 and the value of g f in Eq. (3) is unity for pure polar liquids and close to unity if there is no interaction between the liquid ixtures. The deviation of g f value fro unity indicates the interaction between liquids 1 and. The solute-solvent interaction between the coponents of a ixture can also be obtained fro another dielectric paraeter is Bruggean 19 factor ( f B ) and it is given by: f 1/3 ε = ε ε = φ ( 1 ) 1 B ε1 ε ε (4) where φ is the volue fraction of liquid in liquid 1, If there is no interaction between the coponents in the ixture then the Bruggean factor (f B ) should

3 670 INDIAN J PURE & APPL PHYS, VOL 48, SEPTEMBER 010 vary linearly with volue fraction φ, but if there is interaction between the coponents then f B varies non-linearly with φ. The dielectric data obtained i.e., ε static, ε*=ε jε, ε and the data fro ipedance analyzer is used to fit in an Argand diagra to calculate the relaxation tie (τ). The distribution of relaxation tie is witnessed fro Cole-Cole arc plot. Although Cole-Cole equations which signify the distribution of relaxation tie, eventually a acroscopic relaxation tie (predoinant relaxation tie) is easured using the equation: v ωτ = u 1 1 α (5) where α (distribution paraeter), u and v values are deterined fro their respective Cole-Cole plots. Eyring 0,1 treated the dipolar rotation on the basis of a cheical rate process. The olecules are assued to jup fro one equilibriu position to another, depending on the following factors: (a) The nuber of collisions that it akes with the neighbouring olecules; (b) The entropy S*, which describes the disturbance of the local structure of the ediu during orientation; (c) The heat of activation H*, which is equal to the potential barrier separating the successive equilibriu positions. Using this concept a relation between the icroscopic relaxation tie and the free energy of activation is given as: geoetry optiization procedure using Spartan Modeling software 4. 4 Results and Discussion The perittivity values ε static and ε for various ole fractions at different teperatures of the binary systes 1PN + MB (Syste 1) and 1PN + EB (Syste ) are deterined experientally and their variation with ole fraction is shown in Figs 1 and Fig. 1 Static perittivity (ε static ) versus ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures. * h G τ = exp kt RT * * h H T S = exp kt RT (6) where h is the Planck s constant, k the Boltzann constant, T the teperature in kelvin and R is the gas constant. By using Eq. (6), the therodynaic paraeters Gibb s energy of activation G*, olar enthalpy of activation H*, olar entropy of activation S* for various ole fractions and at different teperatures can be deterined. Miniu energy structures of the onoers of propan-1-ol, ethyl benzoate, ethyl benzoate and the equi olar hydrogen bonded coplexes for the vibrational frequencies are obtained fro Seiepirical Hailtonian quantu echanical calculations,3 such as Austin Model 1 (AM1), Paraeterized Model nuber 3 (PM3) and Modified Neglect of Differential Overlap (MNDO) converged Fig. High frequency perittivity (ε ) versus ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures

4 MOHAN et al.: MICROWAVE DIELECTRIC RELAXATION OF HYDROGEN BONDED BINARY MIXTURES 671 (syste 1), 3 and 4 (syste ). Fro Figs 1-4, it is observed that there is non-linear variation of ε static and ε with ole fractions at all teperatures, which indicates the interaction between the copounds. Siilar results were reported by Kroeger 6 for the ixture of alcohols and polar liquids. The values of g eff, g f for syste 1 and syste at different teperatures are presented in Tables 1 and, respectively. The g eff values for the pure copounds 1PN and MB are greater than unity at all teperatures, which suggest that olecular dipoles have parallel orientation aong theselves. For the pure copound EB, the values of g eff are less than unity at all teperatures indicating anti-parallel orientation of the electric dipoles. But for both the ixture systes, the paraeter g eff exhibits a steadily decreasing tendency as the concentration of MB or EB goes on increasing in 1PN at all teperatures. This tendency leads to the conclusion that the heterogeneous interaction between the copounds in binary systes, possibly between the OH group of alcohol and CH group of benzoates, is in such a way to for ultiers with decreasing parallel orientation of electric dipoles 5. The strength of this heterogeneous interaction is found to depend on the teperature and concentration of the ixture, which shows that teperature and concentration variations ipact the structural properties. The corrective Kirkwood correlation factor (g f ) is a dielectric paraeter, which can give inforation regarding the interaction between the copounds in the ixture. For an ideal non-interacting ixture, g f ust be unity and the agnitude of deviation of g f fro unity indicates the strength of interaction Fig. 3 Static perittivity (ε static ) versus ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures Fig. 4 High frequency perittivity (ε ) versus ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures Table 1 Values of g eff and g f with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures T=303 K T=308 K T=313 K T=318 K T=33 K X g eff g f g eff g f g eff g f g eff g f g eff g f

5 67 INDIAN J PURE & APPL PHYS, VOL 48, SEPTEMBER 010 Table Values of g eff and g f with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures T=303 K T=308 K T=313 K T=318 K T=33 K X g eff g f g eff g f g eff g f g eff g f g eff g f Table 3 Values of Bruggean paraeter (f B ) with volue fraction (φ ) of ethyl benzoate and ethyl benzoate in propan-1-ol at different teperatures φ T=303 K T=308 K T=313 K T=318 K T=33 K 1PN+MB 1PN+EB 1PN+MB 1PN+EB 1PN+MB 1PN+EB 1PN+MB 1PN+EB 1PN+MB 1PN+EB between the copounds i.e., greater the deviation fro unity larger the strength of interaction 6. In the present case, for both the systes at low concentrations of benzoate in 1PN, the values of g f are closer to unity indicating weaker inter olecular interactions between the copounds, but as the concentration of MB and EB increases in 1 PN g f values deviate ore fro unity indicating the increent in the strength of hetero interaction between the copounds. The values of g f are found to depend ore on concentration than teperature as reported by Krishnan S et al 7. Further, it is observed that g f values deviate ore fro unity, in syste than in syste 1, at all teperatures indicating stronger hetero interaction in case of syste. The other dielectric paraeter, which gives inforation about the interaction between the coponents in the ixture, is Bruggean paraeter (f B ). The values of f B, for both the systes, are found to vary non-linearly with volue fraction φ at all teperatures are provided in Table 3. This phenoenon indicates that there is an interaction taking place between the coponents of the binary systes 8. The relaxation ties (τ) of pure 1PN are higher and is due to the foration of intra olecular hydrogen bonding between one alcohol olecule and another (R-O H OH R), which leads to the foration of self associated groups 1. The increase in the nuber of self associated groups causes to absorb ore electroagnetic energy. Due to this, olecules relax very slowly leading to higher relaxation ties 9. But the relaxation ties of pure MB and EB are very low 30 copared to pure1pn. The relaxation ties of pure EB are greater than that of pure MB due to the increase in olecular size and effective radius of the rotating unit 31. The relaxation ties are found to decrease as the concentration of MB and EB increases in 1PN at all teperatures as shown in Figs 5 and 6, respectively. At high concentrations of alcohol in the ixture, there are a large nuber of 1PN olecules surrounding the benzoate olecules and these associative alcohol olecules are foring hydrogen

6 MOHAN et al.: MICROWAVE DIELECTRIC RELAXATION OF HYDROGEN BONDED BINARY MIXTURES 673 Table 4 Values of Gibb s energy of activation ( G* kjol 1 ) with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures X T=303 K T=308 K T=313 K T=318 K T=33 K Fig. 5 Relaxation tie (τ) with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures Table 5 Values of Gibb s energy of activation ( G* kjol 1 ) with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures X T=303 K T=308 K T=313 K T=318 K T=33 K Fig. 6 Relaxation tie (τ) with ole fraction (X ) of ethyl benzoate in propan-1-ol at different teperatures bond with the benzoate olecules. Thus, dipoledipole interaction occurs in such a way that the effective dipole oent gets increased and linear α-ultiers are fored 5. At low concentration of 1PN in the ixtures, there are only a sall nuber of alcohol olecules to enable dipole-dipole interaction through hydrogen bonding with the non-associative benzoate olecules. As a result, weak interolecular interaction occurs. The relaxation tie increases with increasing alkyl chain length of benzoate indicating that the degree of cooperation for reorientation of the olecules increases with increasing length and the bulk of cluster increases. So the foration of hydrogen bonding between hydroxyl group ( OH) of 1PN and CH group of benzoates restricts the free internal rotation of the olecules. The values of therodynaic paraeter, Gibb s energy of activation ( G*) exhibited positive deviations, increased with decreased concentrations of benzoates (MB and EB) in propan-1-ol at different teperatures are presented in Tables 4 and 5, respectively. The increasing positive values of G* suggest the existence of olecular associations between unalike olecules in the ixture 3. In the present study, G* values of syste are greater than that of syste 1 indicating the strength of bond foration in syste is high copared to syste 1. The olar enthalpy of activation ( G*) and olar entropy of activation ( S*) for various ole fractions of benzoates (MB and EB) in propan-1-ol are also calculated and are given in Table 6. The positive value of H* gives the aount of heat liberated in the process of dielectric reorientation 14,34. The olar enthalpy values for both the systes (1 and ) are

7 674 INDIAN J PURE & APPL PHYS, VOL 48, SEPTEMBER 010 positive indicating the bond foration. The negative values of S*, in both the systes, indicate that the activated coplexes have lower entropy than the reactants, since entropy is a easure of disorder or chaos of the reaction. This is the situation encountered with ost biolecular reactions because the two olecules initially in rando situation ust coe together with the resulting loss of entropy 33. These values also indicate that there are fewer configurations possible in the activated state and for these configurations the olecules are ordered to higher extent than in the noral state 34. Observing the FT-IR spectra for the equi olar ixture of 1PN+MB (syste 1), there is a shift of 163 c 1 wave nuber in the position of OH and 11 c 1 wave nuber in the position of CH for the ixture copared with the pure spectrus of 1PN and MB, respectively. Siilarly, the FT-IR spectra for the equiolar ixture of 1PN+EB (syste ), there is a shift of 199 c 1 wave nuber in the position of OH and 1 c 1 wave nuber in the position of CH for the ixture copared with the pure spectrus of 1PN and EB, respectively. These shifts are caused by the strong interaction between the high electro-negative charge of oxygen in 1PN and hydrogen of the benzoates (MB and EB). Thus, the IR analysis convinces interolecular hydrogen bonding of the equi olar binary ixtures (syste 1 and syste ) effectively with proportionate variations in stretching frequencies of OH and CH copared to their respective pure systes 35. The coparison of experiental and theoretical FT-IR values are provided in Table 7 and the obtained theoretical values are in good agreeent with the experiental values 38. The Optiized geoetrical structures for the foration of hydrogen bonding in 1PN + MB and 1PN + EB, which are obtained fro Hailtonian quantu echanical calculations using Spartan Modeling software are shown in Figs 7 and 8, respectively. Table 6 Values of olar enthalpy ( H*) and olar entropy ( S*) for various ole fractions (X ) ethyl benzoate and ethyl benzoate in propan-1-ol X H* (kjol 1 ) S* (kjol 1 K 1 ) 1PN+MB 1PN+EB 1PN+MB 1PN+EB Copound Fig. 7 Optiized converged geoetrical structure of hydrogen bonded propan-1-ol and ethyl benzoate. (red: oxygen, black: carbon, white: hydrogen) Fig. 8 Optiized converged geoetrical structure of hydrogen bonded propan-1-ol and ethyl benzoate. (red: oxygen, black: carbon, white: hydrogen) Table 7 Experiental and theoretical FT-IR analysis for the pure and binary ixtures of 1PN, MB and EB Band Experiental v (c 1 ) v (c 1 ) v (c 1 ) Theoretical AM1 PM3 MNDO v v v v (c 1 ) (c 1 ) (c 1 ) (c 1 ) v (c 1 ) 1PN OH MB CH EB CH PN+MB OH CH OH OH OH OH 963 1PN+EB OH CH CH 199-OH 1-CH CH 161-OH 10-CH CH 61-OH 63-CH CH 16-OH 17-CH

8 MOHAN et al.: MICROWAVE DIELECTRIC RELAXATION OF HYDROGEN BONDED BINARY MIXTURES Conclusions The dielectric paraeters Kirkwood effective correlation factor, corrective Kirkwood correlation factor, Bruggean paraeter, relaxation tie and therodynaic paraeters Gibb s energy of activation, olar enthalpy of activation and olar entropy of activation are coputed for the pure and binary ixture systes of propan-1-ol with ethyl benzoate and ethyl benzoate at different teperatures for various ole fractions. The foration of hydrogen bond between the ixture systes is identified by studying the variations in the paraeters deterined. The existence of hydrogen bond between OH group of propan-1-ol with CH group of ethyl benzoate and ethyl benzoate is confired through FT-IR spectra. The FT-IR values are deterined theoretically using quantu echanical calculations and are in good agreeent with the experiental values. Acknowledgeent The authors gratefully acknowledge the University Grants Coission Major Research Project No. F: 34-1\008 (SR) dated 30 Deceber 008 and UGC DRS LEVEL III prograe No F530/1/DRS/009 (SAP-I) dated , New Delhi, to the Departent of Physics, IIT Chennai, for providing financial assistance. References 1 Ediger M D, Angell C A & Nagel S R, J Che Phys, 100 (1996) Nath G, Sahu S and Paikaray R, Indian J. Phys. 83 (009) 49 3 Gupta K K, Bnashal A K, Singh P J & Shara K S, Indian J Pure & Appl Phys, 41 (003) Rowlinson J S & Swinton F L, Liquids and Liquid Mixtures, (Butterworth, London), 3 rd Edn, Liakath Ali Khan F, Sivagurunathan P, Raja Mohaed Kail S & Mehrotra S C, Indian J Pure & Appl Phys, 44 (007) Kroeger M K, J Mol Liq, 36 (1987) Sengwa R J, Vinita Khatri & Sonu Sankhla, J Sol Che, 38 (009) Jacobo Troncoso, Claudio Cerdeiriña A, Yolanda Sanaed A, Luís Roaní, & Luís Paulo N Rebelo., J Che Eng Data, 51(5) (006) George A J, An introduction to hydrogen bonding (Oxford University Press, London), Parthipan G, Arivazhagan G & Thenappan T, Philosophical Magazine Letters, 88 (008) Bruggean D A G, Ann Phys (Leipzig), 5 (1935) Cole K S & Cole R H, J Che Phys, 9 (1941) Chaudhari A & Mehrotra S C, Mol Phy, 100 (00) Dharalinga K, Raachandran K, Sivagurunathan P, Prabhakar under B, Khirade P W & Mehrotra S C, Che Pap, 61(4) (007) Nora E Hill, Vaughan E, Price A H & Davies M, Dielectric Properties and Molecular Behavior (Northland Reinhold Copany, London) DR Lide, CRC. Handbook of Cheistry and Physics (CRC Press, Boca Raton) 78 th Edn, Kirkwood J G, J Che Phys, 7 (1939) Puranic S M, Kubharkhane A C & Mehrotra S C, J Mol Liq, 50 (1991) Kaatze U, Z Phys Che, 153 (1987) Eyring H, Cernuschi F, J Che Phys, 7 (1939) Kauzann W, Rev Mod Phys, 14 (194) 1. Chitra M, Subraanya B & Murthy V R K, Indian J Pure & Appl Phys, 39 (001) Sundaraganesan N, Doinic Joshua B, Rajaoorthy M & Gangadhar C H, Indian J Pure & Appl Phys, 45 (007) Spartan version (Wave function Inc, CA: USA) Thenappan T, Prabakar Devaraj A, J Mol Liq, 13 (006) 7. 6 Ajay Chaudhari, Antia Das, Garigipati Raju, Harish Chaudhari, Prakash Khirade, Navinkuar Narain, Suresh Mehrotra, Proc Natl Sci Counc Roc (a), 5 (001) Balaurugan D, Kuar S, Krishnan S, J Mol Liq, 1 (005) Sivagurunathan P, Dharalinga K, Raachandran K, Prabhakar Undre B, Khirade P W & Mehrotra S C, J Mol Liq, 133 (007) Vyas A D, Rana V A, Bhatnagar S P & Vashisth V M, Indian J Pure & Appl Phys, 46 (008) Rajala G E & Crossley J, Candadian J Che, 50 (197) Chitra M, Subraanya B & Murthy V R K, Mol Phys, 99 (001) Patil S P, Chaudari A S, Lokhande M P, Landle M K & Mehrotra S C, J Che Eng Data, 44 (1999) Pal A & Kuar S, J Mol Liq, 109 (004) Shara V, Thakur N, Shara D R, Rangra V S & Negi N S, Indian J Pure & Appl Phys, 46 (008) Bansal R K, Organic Reaction Mechaniss, (Tata McGraw- Hill, New Delhi) Singh A, Misra R, Shukla J P, Saxena M C, J Mol Liq, 3 (1983) Silverstein R M, Bassler G C & Morrik T C, Spectroscopic Identification of Organic Copounds (John Wiley & sons, Singapore) 5 th Edn, Singh H J & Srivastava P, Indian J Pure & Appl Phys, 47 (009) 557.