Dielectric Spectroscopic Studies of Propylene Glycol/Aniline Mixtures at Temperatures Between 303K to 323K

Dielectric spectra of propylene glycol, aniline and their binary mixtures with different concentrations were studied at 303K-323K by using Coaxial cable method in the microwave frequency range 20 MHz-20 GHz. The relaxational response of the propylene glycol, aniline and their binary liquid mixtures over the entire composition range is analysed by using Cole-Cole relaxation model. Dipole moments obtained from the Higasi’s method are compared with the quantum mechanical HF and DFT calculations. From the experimental datadipole moment, Bruggeman parameter, Kirkwood g factor, excess dielectric and thermodynamic parameters have been calculated. The obtained data have been analysed in terms of the parallel and anti parallel orientation of the dipoles, chain length and hydrogen bond interaction in the mixture composition. Keywords--Relaxation Time, Dipole Moment, Excess Dielectric And Thermodynamic Parameters


1.INTRODUCTION
Dielectric relaxation spectroscopy (DRS) is an effective method to explain the structure and molecular dynamics of the liquids and nature of the intermolecular interactions [1][2][3][4][5][6][7][8][9][10][11][12]. Depending upon the nature of the liquid samples under investigation, DRS may provide sufficient information about the thermodynamics, kinetic and structural features of the solutions. The high susceptibility of DRS to molecular interactions makes this method a valuable tool to get a depth understanding into the liquid state properties which governs with the forces. The dielectric studies of liquid mixtures containing the varying amounts of interacting components helps to investigate the structure of the complexes formed. Hydrogen bonding considerably alerts the dielectric properties of liquids, understanding Hydrogen bonding remains a complex task due to the uncertainty to recognise the particular bonds and the elements are involved [13]. Further the thermodynamic properties of liquids and their liquid mixtures have been used to know the molecular interaction between the constituents involved in the liquid mixture and also for engineering applications related to heat energy transfer, mass transfer, activation energy, enthalpy and entropy of the polar molecules [14]. Relaxational response of liquids depends not only upon intra-and intermolecular interaction but also on the profound features like molecular size and shape, these geometric factors are important to elucidate the structural behaviour of liquid mixtures in which weaker intermolecular interactions, mainly of dipolar nature, are present. The first systematic dielectric dispersion studies of pure poly (propylene glycol)s of different molecular weight in the glass transition region measured by Baur and Stockmayer [15] and dielectric relaxation spectra of propylene glycols studied as a function of temperature and pressure by Suzuki et al [16]. The complex dielectric permittivity of viscous propylene glycol is studied by impedance methods and observed that there exist two distinct nonlinear features in the super cooled liquid near its glass transition temperature [17]. Park et al [18,19] explained the liquid glass transition and α relaxation in terms of the thermal and dielectric properties of propylene glycol and polypropylene glycol with different molecular weights. Navarkhele et al [20] studied the dielectric relaxation behaviour of formamide-propylene glycol binary mixture in the frequency range 10 MHz-20 GHz by using TDR technique and explained the Kirkwood angular correlation factor (g eff ) is more than one in formamide rich region and less than one in propylene glycol region. Mali et al [21] have reported the dielectric relaxation of poly ethylene glycol in aqueous medium and their results shows that intermolecular homogeneous and heterogeneous hydrogen bonding vary significantly with increase in concentration of poly ethylene glycols in aqueous solution medium. In this article, an attempt has made to investigate the molecular interaction between the self associative propylene glycol and non self associative aniline molecules and also in their mixtures of different molar concentration levels by determining the complex dielectric permittivity and relaxation times. Complex dielectric permittivity of these liquid mixtures were measured in the frequency range 20MHz -20 GHz by considering open-ended coaxial probe method [22,23] at different temperatures i.e. 303K, 308K, 313K, 318K and 323K. The experimental dipole moments of propylene glycol, aniline and their equimolar binary mixtures were calculated by using Higasi's method [24]. The theoretical dipole moments were also calculated by using Quantum mechanical Hatree-Fock and Density Functional Theory (B3LYP) calculations with 6-311G+, 6-311G++ basis sets by using Gaussian software [25][26][27][28][29]. The relaxational response of the propylene glycol, aniline and their binary liquid mixtures over the entire composition range is analysed by using Cole-Cole relaxation model [30,31]. By using Eryings rate equation [32,33], the thermodynamical parameters such as enthalpy of activation ΔH*, entropy of activation ΔS* are determined and also effective Kirkwood 'g' factor is obtained from the Kirkwood-Frohlich equation [34]. The long range and short range interactions between dipoles is obtained from the excess Helmholtz energy ( calculations [35]. The obtained experimental data of the binary mixtures of propylene glycol and aniline were interpreted in terms of the parallel and anti parallel orientation of the dipoles, chain length and Hydrogen bond interaction in the liquid mixture composition.

a.Materials
The chemicals used in this work such as propylene glycol, aniline and benzene were supplied by Merck, Germany (purity 99 %, AR Grade). These liquids were further purified by double distillation under reduced pressure and only middle fractions were collected [36]. Before use, the chemicals were stored over 4Å molecular sieves for 48 hrs to avoid water content and were then degassed. Initially dilute solutions of polar liquids (Solute) are prepared over a concentration range of 0 to 1 ml in 10 ml of non-polar solvent benzene in order to evaluate the dipole moments of the pure and equimolar binary liquids of propylene glycol and aniline by considering the Higasi's method in the temperature range 303K-325K.

c. Dielectric Measurements
Measurements of static dielectric constant (εs) and optical refractive indices (n) of the above dilute systems i.e., propylene glycol and aniline in benzene and their equimolar binary mixtures are carried out by using digital capacitance meter (820 Hz) and Abbe refracto-meter in the temperature range 303K-323K with a temperature variation of ± 0.1K. The complex dielectric permittivity (*=ε'-j") of pure liquids of propylene glycol, aniline and the different molar concentration levels of aniline in propylene glycol is measured in the microwave frequency range (20MHz -20 GHz) by using the open-ended coaxial probe method. The detailed analysis and procedure of the open ended coaxial probe method and determination of excess dielectric parameters such as excess permittivity (ε E ), Bruggeman factor (fB) , excess inverse relaxation time (1/τ) E , Gibbs free energy of activation ΔG*, Kirkwood correlation factor (g eff ) were explained previously in our published manuscript [2,3]. The maximum errors in the evaluated values of static dielectric constant (εs) and refractive indices (n) are ± 1% and real (ε') and imaginary part of dielectric permittivity (") are ± 2% and ± 2-3% respectively.
The excess Helmholtz energy ( parameter to determine the interaction between the constituents in the liquid mixture through breaking mechanism of hydrogen bond [35] and expressed as 12 or rr and Vr is the molar volume of the components and NA is the avagadros number. The parameters εr, ε∞r and εm represents the dielectric permittivity values at static (820 Hz) and optical frequencies of the pure liquids, binary mixtures and g1 and g2 are the effective g factors of the pure liquid samples respectively.

III. RESULTS AND DISCUSSION
The low frequency dielectric permittivity (εo), dipole moment (μ), relaxation time (τ) values of the pure and equimolar binary systems of propylene glycol and aniline at room temperature (298K) are tabulated in Table 1 and also the variation of dipole moments of the pure and their binary mixtures at different temperatures are reported in Table 2 respectively. The experimentally determined dipole moment values are compared with the theoretical HF, DFT (B3LYP) calculations which are tabulated in Table 3. Experimental dipole moments are determined by diluting the pure  [50] Equimolar binary mixtures of A+B 16.56 ----185.98 ---- † crc handbook of chemistry and physics (1969-1970,) weast rc (ed) (1983-84) hand book of chemistry and physics. 64th edn, crc press, fl 62 Aparicio et al compounds in non-polar solvent benzene using Higasi's method [24]. From the Tables 2 and 3, it is observed a decrease in the dipole moment of equimolar binary mixture when compared to the individual pure systems due to polarization effect [44]. The calculated value Table 2 Experimental dipole moment (μ) and excess dipole moment (Δμ) values for the pure system aniline, propylene glycol and equimolar binary systems-aniline and propylene glycol  (1969)(1970) of Δμ for the above binary system is negative and it represents the absence of charge-transfer effects. If a charge-transfer effect exists, the value of Δμ would be greater and positive value [45]. In the present investigation Δμ values are negative that presence of a polarization effect. Sabesan et al. [46] and Thenappan and co-workers [47,48] have reported similar conclusions on alcohol mixtures. A small deviation in the experimental dipole moment value when compared to the theoretical values and it may be due to the π electron cloud of non polar solvent benzene affecting the dipole moment values of the solute system of propylene glycol and aniline and their binary mixtures. From Table 2, it is noticed that the change in temperature notably influences the experimental dipole moment values of the pure compounds and equimolar binary systems. At low temperatures, the bond lengths between the atoms are very much restricted in their movement, and hence maintain their minimum energy stable conformational structure. This conformational structure permits the cancellation of dipole moments to some extent, resulting in lower dipole moments at low temperatures. As the increase in temperature provides more thermal energy and hence degree of rotation of the individual groups and bond lengths between the atoms also increases, resulting in some changes in the stable structure. The change in the stable structure leads to decrease in the cancellation of the side-group dipole moments and hence consequential increase in the mean dipole moment value. From Fig.1 it is observed that experimental values of the low frequency dielectric permittivity (ε0) which is measured at 20 MHz decreases with increase in temperature as well as increase in mole fraction of aniline in propylene glycol binary system is due to increase in temperature that may cause decrease in the degree of polarization of the dipoles. The increased in thermal energy reduces the alignment of the dipoles in the mixture. The decrease in low frequency dielectric permittivity value with increase in the mole fraction  i.e., aniline (Fig.3) and equi-molar binary mixtures (Fig.5) respectively. The increase in the number of self associated groups formed through hydrogen bonded network in the liquid system takes longer time to attain one equilibrium position to another equilibrium position causing increase in the relaxation time values. The average relaxations times of the pure liquids as well as binary liquid mixtures are determined by using the Cole-Cole relaxation model [31] and which is as shown in Fig.6. From the Fig.6   The excess dielectric parameters like excess permittivity (ε E ); excessive inverse relaxation time ((1/τ) E ) provides the information regarding the molecular interaction between the polar-polar liquid mixtures. From the Fig.7 it is observed that negative values of excess permittivity (ε E ) for all concentrations and temperatures. The negative values of ε E indicates the formation of multimer structures which leads to decrease in the total number of dipoles in the systems and also interaction among unlike molecules which may cause  [49]. The possitive trend of (1/τ) E provides the information about the fast rotations of dipoles in the system. This may be due to the formation of monomeric structure in liquid system. From the Fig.8 it is observed that negative trend of (1/τ) E with respective molar concentration of aniline in propylene glycol at all temperatures and it shows the solute -solvent interaction produces a field such that the effective dipoles rotates slowly in the liquid system [50]. The Kirkwood effective g factor (g eff ) and gf values for various mole fractions of aniline in propylene glycol are represented in Fig.9a and 9b respectively. It is observed that the high values of g eff for the pure glycol system shows that the molecular dipoles have parallel orientation among themselves and the low value of g eff for the aniline indicates the anti-parallel orientation of the electric dipoles or non associative nature. But for the mixture of propylene glycol and aniline, the parameter g eff exhibits a steady decrease as the increase in concentration of aniline as shown in Fig 9a. It leads to the conclusion that heterogeneous interaction between  [51]. The gf values of the above systems are approaching towards one and it indicates that system will be oriented in such a way that the effective dipole moment values will be greater than individual systems. The other dielectric parameter is the Bruggeman parameter (fB), from the Fig. 10 it  Table.4 respectively. From the Table  4 it is observed that Gibbs free energy of activation ΔG* shows a positive value which reveals the existence of interaction between the molecules in the system and also ΔH* value is maximum for propylene glycol and its value decreases with increase in the concentration of aniline. Since the Enthalpy of activation ΔH* depends upon the local environment of the molecules.
The long range and short range interactions among dipoles can be reviewed from the thermodynamic parameter excess  indicates the formation of α clusters. The formation of α clusters increases the effective dipole moment which in turn increases the internal energy. The formation of hydrogen bond between propylene glycol and aniline which is obtained from the minimum energy based geometry optimization procedure by using the DFT (B3LYP) method with 6-311G++ basis set which is represented in Fig.12 respectively. The complex dielectric permittivity spectra of propylene glycol-aniline binary mixtures have been studied using open-ended coaxial probe method in the frequency range 20 MHz-20 GHz at different temperatures. The nonlinear variation of static dielectric constant, dielectric relaxation time and Bruggeman parameter (fB) for all concentrations in the temperature range 303K-323K suggests the heterogeneous interaction between the unlike molecules. The negative trend of excessive inverse relaxation time (1/τ) E with respective molar concentration of aniline in propylene glycol at all temperatures shows the solutesolvent interaction produces a field such that the effective dipoles rotates slowly in the binary liquid system. The negative sign of excess dipole moment values (Δμ) suggests the absence of charge-transfer effect that may be due to a solvent-induced medium effect in the binary system. The values of ΔG* (Gibbs free energy of activation) are positive which represents the presence of molecular interaction between the molecules in the system V. ACKNOWLEDGEMENTS