Abstract: were used to study the molecular interaction

Abstract: The binary (solid–liquid) mixture containing Carbamide (Urea) with N, N-Dimethyl formamide (DMF) was studied using Time Domain Reflectometric (TDR) technique in the frequency range 10MHz to 50 GHz. The study was carried out at four different temperatures (10, 20, 30 and 40°C).

Dielectric parameters such as static dielectric constant (eo) and relaxation time (t) were obtained by analyzing complex permittivity spectra. The high frequency limiting dielectric constant (e¥) was calculated by measuring refractive indices of the mixtures as (e¥= n2). An effective Kirkwood correlation factor (geff) was calculated for the investigation of dipole orientation of the heteromolecular entities present in the binary mixture. A nonlinear least square fit method was used to fit the data. All the derived parameters were used to study the molecular interaction between compounds. Keywords: TDR, solid-liquid binary mixture, dielectric constant, relaxation time, hydrogen bond, Urea.

 Introduction:Dielectric studies are important for investigating interaction properties in the binary mixture of hydrogen-bonded molecules. When two or more compounds are mixed together, modifications may occur at molecular level due to change in the molecular forces. These forces may be inter- or intra-molecular. Such changes influence the interaction phenomenon of polarization and reorientation of the dipoles present in the mixture. Therefore the dielectric study of mixed solvents is mandatory for the prediction of chemical stability and solubility which are crucial parameters in pharmaceuticals and other industrial processes 1. The techniques which are used to measure dielectric parameters are non-destructive and have gained great importance for monitoring specific properties of the materials undergoing dynamic changes. A large number of studies have been made on the intermolecular interactions taking place in liquid-liquid systems 2-4, however very less attention is paid for solid-liquid mixtures, especially the organic polar liquids with organic solid compounds such as Carbamide (Urea).

 Some studies of Urea compound with water, alcohols and water+DMF mixture were carried out by some researchers 5,6, but no attempt was made on the direct studies of Urea+DMF mixture. Thus the binary mixture of Urea + DMF has been the content of our study.Urea an important compound having significant usage in agriculture and many industrial processes. Also, it plays a crucial role in cure of Hyponatremia 7 hence it is useful in pharmaceuticals 8,9 Recent trend of study of urea and its properties with various solvent and for various applications is observed worldwide 10,11.

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 N, N-dimethyl formamide (DMF) is a well known organic solvent having potential applications in chemical industry and in pharmaceuticals as well. It is a polar aprotic, hydrophilic in nature, having large dipole moment ([email protected]) 12. Its electron donating property helps in deciding its strength for many metallurgic applications too 13. For Urea and DMF, the carbonyl functional group (C=O) is common. Their amide group may be responsible for hydrogen bonding and dipole-dipole interactions 14. The selective information gained by investigation of intermolecular interactions may be useful in exploiting their use in the fields mentioned above.  Experimental:  Materials:The HPLC spectroscopic grade DMF (99.

5%) obtained from S. D. Fine-Chem. Ltd.

Mumbai. Urea (99.5%) was obtained from the Molychem Mumbai. Both chemicals were used without further purification.

Using DMF as a solvent a set of binary mixtures of varying weight fraction of Urea was prepared. The study is carried out over the range of 0.1 to 1.0 gm of Urea added to 10ml DMF in the steps of 0.1 gm. The solubility of Urea in DMF is taken under consideration and was kept under solubility limit. Solutions containing Urea may get contaminated and conductivity may rise over the time 15, hence before mixtures will get contaminated with considerable change in conductivity they were characterized within a period of three hours.

All experiments were carried out at atmospheric pressure for different temperatures (10, 20, 30 and 40°C).  Measurements:The dielectric spectra for the Urea (compound-1) with N, N-Dimethyl formamide (compound-2) system were obtained by Time Domain spectroscopic technique. The frequency range for the said experiment was 10MHz to 50GHz. Tektronix DSA8300 sampling oscilloscope mainframe with dual channel sampling module 80E10B has been used for Time Domain Reflectometry.

The sample was kept in sampling bottle in constant temperature surrounding and a high precision thermometer was used for monitoring temperature. A fast-rising repetitive pulse of 12 ps period was sent to sample via coaxial line of the impedance of 50W. Incident and reflected pulses were recorded using sampling oscilloscope. Time windows of 5ps with 2000 digitized points were used in measurements.

Pulse without and with the sample was named as R1(t) and Rx(t) respectively and stored in the mainframe. Stored data using compact disc was transferred to personal computers for further analysis. High frequency limiting dielectric constant (e¥) was calculated by measuring refractive index (n) of mixtures. e¥ plays a very crucial role in the analysis of effective Kirkwood correlation factor (geff) and have a strong impact on (geff) values 16.  Data Analysis:Experimental data, in forms of recorded pulses, were analyzed to obtain dielectric parameters such as static dielectric constant (eo) and dielectric relaxation time (t). For analysis, recorded pulses R1(t) and Rx(t) were added and subtracted to get p(t) and q(t) as described by equations(1a) and (1b) p(t) = R1(t) + Rx(t)                                                                    (1a)q(t) = R1(t) – Rx(t)                                                                     (1b) The Fourier transform p(w) and q(w) of equation 1 were obtained using Samulon method 17 over frequency range  10MHz – 50GHz. Complex reflection coefficient r*(w) was determined by equation (2).  Where “c” is the speed of light, “w” is angular frequency, “d” is effective pin length (=0.

18mm for the present study). Resulting complex permittivity spectra r*(w) were obtained by applying bilinear calibration methods as described by Cole 18. Figure 1, indicates observed spectra of frequency dependent dielectric permittivity (e’) and dielectric loss (e”) obtained by analysis of complex reflection coefficient for Urea+ DMF mixture at 30°C.

 Result and Discussion:Dielectric relaxation for the binary mixture of Urea + DMF was obtained by using Havriliak-Nigami equation 19. All symbols have their usual meanings and empirical parameters a and b having values between 0 and 1, depends on the model used. The magnitude of empirical parameters indicates the distribution of relaxation time. The basic Havriliak-Nigami equation includes three relaxation models viz. The Debye model (a=0 and b=1) implies a single relaxation time while Cole-Cole model (0

Obtained static dielectric constant (eo) and dielectric relaxation time (t) are tabulated in table 1. Static Dielectric Constant and Relaxation Time:Static dielectric constant and relaxation time are important for studies of structural properties of binary mixtures. These parameters at four temperatures with the varying weight fraction of a Urea are tabulated in table 1 and are graphically represented in figure 2 and 3 respectively. Non-linear variation of eo indicates the presence of interactions between solute and solvent; hence conclude that interaction occurred in the solute-solvent mixture.

As weight fraction of urea increases, dielectric constant also increases. However, as temperature increases, the dielectric constant of systems becomes decreases. Same graphical nature was observed for relaxation time.   Figure 2: static dielectric constant (eo) versus weight fraction of Urea for Urea+DMF mixture at 10, 20, 30 and 40oC. Figure 3: Dielctric relaxation time (t) versus weight fraction of Urea for Urea+DMF mixture at 10, 20, 30 and 40oC.Increase in dielectric constant and dielectric relaxation time with the increase in urea weight fraction is may be due either hydrogen bonding or dipole-dipole interaction between constituent molecules. This may be attributed to the fact that, the addition of urea increases the number of amide (-CONH) groups in the mixture, with a possibility of the creation of multimers.

This may be the cause of the increase in dielectric constant and relaxation time with the increase in weight fraction of Urea. This indicates that there is an enhancement of molecular interaction between compounds. Relaxation time is a measure of the ability of the molecules to reorient as per applied frequency of the incident pulse. It gets influenced by functional groups which have the capability of formation of hydrogen bonding. Other factors which govern relaxation time are concentration, temperature, etc.

here observed results support to the fact that, with an increase in number of functional groups, the relaxation time increases. Decrease in dielectric constant and relaxation time with increase in temperature may be due to thermal agitations which results from decrees in numbers of polarizable entities per unit volume 21.  Kirkwood Correlation Factor:Kirkwood correlation factor (g) is the important parameter for the study of solute-solvent interactions in binary mixtures. The value of (g) indicates relative orientation of electrical dipoles parallel or anti-parallel in neighboring molecules of the mixture in presence of applied electric field 22.   It is a shape dependent parameter, which reveals information about the degree of short-range intermolecular forces that leads to dipole-dipole interactions in mixtures 23. For pure components, Kirkwood correlation factor g is given by the expression as-  Where, k is Boltzmann constant, m is dipole moment, NA is Avogadro’s number, r is the density of solution at temperature T and M is a molar mass of the pure compound.

For the study of binary mixtures, Kirkwood-Frohlich theory must be applied while considering hydrogen bond contribution or dipole-dipole interactions 24. Therefore, to get an insight of associating effects on molecules of mixtures an effective Kirkwood correlation factor geff is necessary. This can be calculated by using modified Kirkwood-Frohlich equation 25 as-  Where, f1 and f2 are weight fractions of compounds 1 and 2.  If geff is unity, it implies that no interaction is taking place. Deviation of geff from unity shows net change in dipole ordering of constituents of the mixtures due to hydrogen or any other complex formation inside mixtures. That is the variation of ‘geff’ with concentration reflects upon dipole alignment indicating nature of the association of multimerization. In case of Urea+DMF binary mixtures, results obtained for geff are given in table 2. Values of geff less than unity indicate anti-parallel alignments of dipoles and greater than unity indicates parallel alignment of dipoles in the mixture.

The variation observed for all mixtures at all temperatures are from anti-parallel to parallel reorientation of electrical dipoles. This confirms the presence of intermolecular interaction amongst molecules of binary mixtures. The observed trend suggests that as urea weight fraction increases in the mixtures, heterogeneous interaction occurs between molecules of different constituents to form multimers 26. Conclusion:The dielectric study for binary mixtures containing Urea and DMF for molecular association was carried out successfully. Static dielectric constant, dielectric relaxation time and Kirkwood correlation factor have been analyzed. Studies were carried out with various weight fractions and at four different temperatures (10, 20, 30 and 40°C) using the Time domain Reflectometric technique. Derived parameters have been interpreted in terms of molecular interactions among molecular species of the binary mixture. Values of dielectric constant and relaxation time were observed to be increasing with increase in weight fraction of Urea in all the mixtures.

This can be attributed to the possibility of the increase in hydrogen bonding in mixture due to an addition of (-CONH) groups. Deviation of effective Kirkwood correlation factor (geff) from less than to greater than unity indicates changes in dipole orientation and hence confirms solute-solvent interaction. The dipoles get aligned such that effective dipole becomes larger as compared to pure liquid.

This confirms the presence of interaction amongst molecules of binary mixtures, a possibility of hydrogen bonding and multimerization. Acknowledgement:One of the authors, Mr. H.N. Thorat, is thankful to UGC, New Delhi for Rajiv Gandhi National Fellowship.