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.  

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