C 2004) Journal of Solution Chemistry, Vol. 33, No. 3, March 2004 (
Dielectric Relaxation in Glycine–Water and Glycine–Ethanol–Water Solutions Using Time Domain Reflectometry Ajay Chaudhari,1,3,∗ A. G. Shankarwar,2 B. R. Arbad,2 and S. C. Mehrotra1 Received December 15, 2003; revised February 12, 2004 Dielectric relaxation measurements on water–ethanol–glycine ternary system were carried out using time domain reflectometry (TDR) at 25, 30, 35, and 40◦ C in the frequency range from 10 MHz to 10 GHz. Glycine–ethanol–water solutions are prepared with different concentrations of ethanol (0, 5, 10, 15, 20, and 30%) and also for different glycine molar concentrations (0, 0.2, 0.4, 0.6, 0.8, and 1 M). The dielectric relaxation parameters are measured for aqueous glycine solutions also to compare the results with those for the glycine–ethanol–water ternary system. For all the mixtures considered, only one relaxation peak was observed in this frequency range. All the mixtures display a Cole– Davidson dispersion. The relaxation peaks shift to lower frequency with an increase in glycine concentration and also with ethanol concentration. The logarithm of the relaxation time log(τ ) shows a nonlinear relation with glycine concentration which implies a change of relaxation mechanism with glycine concentration. The dielectric strength ε for these mixtures shows a nearly linear relation with glycine molar concentration. KEY WORDS: TDR; dielectric relaxation; glycine–ethanol–water solutions. 1. INTRODUCTION Water is in many aspects a remarkable liquid. Its unique physical and chemical properties, particularly the latent heat of vaporization, high specific heat, maximum density at 4◦ C have evoked interest in its structure and much effort is being expended to elucidate its exact structure. The structural features of liquid water depend on the ability of the water molecules to participate in any number of hydrogen bonds by utilizing its two hydrogen atoms and the two lone pairs on 1 Department of Electronics and Computer Science, Dr. B. A. M. University, Aurangabada 431 004 (M.S.), India. 2 Department of Chemistry, Dr. B. A. M. University, Aurangabada 431 004 (M.S.), India. 3 Present address: Department of Chemistry, National Chung-Cheng University, Ming-Hsiung, Chia-Yi- 621, Taiwan; e-mail: ajaychau5@yahoo.com. 313 C 2004 Plenum Publishing Corporation 0095-9782/04/0300-0313/0
314 Chaudhari, Shankarwar, Arbad, and Mehrotra its oxygen. The high value of dielectric constant may be due to the spontaneous fluctuation of local polarization arising from partial charge separation during the formation of clusters. One of the most fascinating aspects of mixtures of water with other liquids is the effect of an additional component on the structure of water. A solute, polar, or nonpolar, when dissolved in water, may either break the water structure or preserve the water structure through interactions. The equilibrium between normal water (nonbonded water) and bound water is disturbed because of the addition of solute. Thus, solutes of different types can be classified as water structure breakers and water structure makers.(1) Sandhu et al.(2) have measured the apparent molar volumes of L-serine and L-threonine in methanol–water mixtures. They found that the solvation number increases with the addition of methanol for both the aqueous amino acids at all temperatures. Partial molar volumes and expansibilities of some amino acids in water at 35◦ C were studied by Iqbal and Ahmed,(3) who interpreted the amino acid– water interactions from the partial molar volumes. Lokhande et al.(4) studied the dielectric relaxation of aqueous glycine and valine solutions using time domain reflectometry. Partial molar volumes and adiabatic compressibilities of amino acids in dilute aqueous solutions at different temperatures have been studied by Kikuchi et al.(5) Viscosity B-coefficients of L-proline and L-hydroxyproline and the effect of added methanol have been studied by Sandhu and Singh,(6) who found that the values of the B-coefficients in the mixed solvent first decrease and then increase after passing through a minimum at about a 20% methanol– water mixture. Molecular interactions in binary mixtures of lysine and adenosine at different temperatures have been studied by Aswar(7) and the results have been discussed in the light of solute–solvent interactions. The ternary systems of guanosin–DMSO–water have been studied by Misra et al.(8) and the transition state treatment has been applied to discuss the viscous flow mechanism in the binary solution. Parmar and Bhardwaj(9) have determined the viscosity B-coefficient of sucrose and urea in DMSO–water mixtures, and remarkable variations in the behavior of the solute are found when the solvent system changes from pure water to a system containing small quantities of organic cosolvent. Partial molar volume, ultrasonic, and viscometric studies of glycine and alanine in DMSO–water mixtures at different temperatures have been studied by Dash and Pasupalak(10) and the results are discussed in terms of ion–ion, ion–solvent interactions and the structural effects of the solvent in the solution. Takashima and Schwan(11) explained the dielectric dispersion of crystalline powders and amino acids, peptides, and proteins. They found that the adsorbed water increases the dielectric constant markedly. Kumazaki and Sugai(12) measured the dielectric dispersion of electrolytic polyamino acids and its various salts in aqueous solutions in the frequency range 1–3 kHz. They studied the dependence of the relaxation time on the molecular weight. Balay(13) was the first to discuss Dielectric Relaxation Using Time Domain Reflectometry 315 in detail the increase in dielectric permittivity of amino acids with increasing concentration. Jones and Zavody,(14) Nobory and White,(15) Wyman and Meekin,(16) Wyman,(17) and Kirkwood(18) have studied aqueous solutions of amino acids and estimated the dipole moment to be approximately 15 and 25 Debye units for glycine and valine, respectively. Bateman et al.(19) have studied aqueous amino acid solutions at a higher frequency range of up to 70 GHz. Dielectric dispersion in the systems of amino acids, peptides, and proteins in aqueous media has received considerable attention.(20) In view of the role of water in the structural and functional properties of macromolecules and their interactions, much attention has been paid to the properties of water in the aqueous binary system.(21) Dielectric properties of water and various alcohols were measured with extremely high accuracy by using the frequency domain method.(22−24) Kaatze(22,23) showed that the dielectric relaxation of pure water measured up to about 60 GHz is well described by the Debye model. The ethanol–water mixtures have been the subject of numerous investigations.(25−29) Various experimental methods such as ultrasonic absorption,(25) mass spectrometry,(26) and dielectric relaxation measurement(27−29) have been employed in these studies. It is well known that the ethanol–water mixtures reveal abnormalities in various properties such as the partial molar volumes of ethanol, chemical shifts of the hydroxyl signal of water, and adiabatic compressibility. Although dielectric measurements on binary systems have been studied extensively, very little attention has been aimed at collecting information on ternary mixtures. This lack of information is hopefully going to be progressively filled by researchers, in view of the fact that the ternary and higher mixtures may conveniently approximate or even themselves constitute complex real systems. Amino acids are the basic building blocks of proteins and are considered to be the model compound of proteins. There are extensive volumetric and thermochemical property studies of aqueous amino acid systems. Recently, investigations of systems containing amino acids in aqueous mixed solvents have aroused much attention.(30,31) The reason for the study of amino acids in mixed solvents is not only that aqueous mixed solvents are extensively used in chemistry and related fields to control factors such as the solubility, reactivity and stability of systems, but also because biological fluids are ultimately not pure water. In this paper, we report the temperature-dependent dielectric relaxation of the water–ethanol– glycine ternary system. Glycine–water–ethanol mixtures were prepared at room temperature with different concentrations of ethanol in water, viz., 0, 5, 10, 15, 20, and 30%, and also for different glycine molar concentrations, viz., 0, 0.2, 0.4, 0.6, 0.8, and 1 M. Solutions with 0% ethanol in the glycine–ethanol–water mixture are nothing but aqueous solutions of glycine and are therefore binary systems. We have measured the dielectric relaxation spectra for these solutions at 25, 30, 35, and 40◦ C using time domain reflectometry in the frequency range of 10 MHz–10 GHz. 316 Chaudhari, Shankarwar, Arbad, and Mehrotra 2. EXPERIMENT Ethanol (spectroscopic grade) and glycine (S.D. Fine chemicals, India) were obtained commercially and used without further purification. Water was carefully purified by deionization and double distillation, and was used in the measurements immediately after the purification. Solutions containing different molar concentrations of glycine were prepared by dissolving accurately known weights of glycine in water–ethanol mixtures. The dielectric measurements were carried out using TDR. A detailed explanation of the apparatus and the experimental procedure have already been reported in previous papers.(32,33) 3. RESULTS AND DISCUSSION Only one relaxation peak is observed in the complex permittivity spectra of all the solutions. Every relaxation curve observed in these experiments can be described well by the Cole–Davidson equation as, ε∗ (ω) = ε∞ + [ε/(1 + jωτ )β ] (1) where ε*ω is the complex permittivity spectra at an angular frequency ω, ε∞ is the permittivity at ω = ∞, ε = (εs − ε∞ ) is the dielectric relaxation strength, εs is the static permittivity, and β (0 < β ≤ 1) is the shape parameter of an asymmetric relaxation curve. The relaxation parameters in Eq. (1) determined by a leastsquares fit procedure are listed in Table I for water–ethanol–glycine solutions. Glycine–ethanol–water solutions can be described with a Cole–Davidson function as shown in Fig. 1. As can be seen from Table I, with increasing glycine molar concentration the values of the static permittivity and relaxation time increase. The increased values of relaxation time indicate that the absorption peak ( f max ) shifts to lower frequency with increasing glycine molar concentration. An increase in temperature would allow faster molecular motions that are consistent with the shifts in f max to higher values (decreasing relaxation time values) with increasing temperature. Both dielectric strength and relaxation time values decrease with increasing temperature whereas ε∞ showed essentially no variation with temperature. With an increase in ethanol concentration, ε values decrease and the dielectric spectra are observed to be asymmetric and broader on the high frequency side. The f max values decrease with increasing ethanol concentration suggesting slower molecular motion during relaxation. From Table I, for the glycine–water binary system, a value of the relaxation time greater than that for water arises from intermolecular hydrogen bonding, and tends to correlate additively with the dipole moments of neighboring water molecules. With increasing temperature the dielectric strength decreases. This typical behavior of relaxation reflects the fact that the orientational motions of Dielectric Relaxation Using Time Domain Reflectometry Table I. 317 Dielectric Relaxation Parameters in Eq. (1) for Water–Ethanol–Glycine Solutions for Various Concentrations at 25, 30, 35, and 40◦ C Glycine molar concentration ε log(τ ) ε∞ β ε log(τ ) ε∞ β 5.1 4.8 4.8 4.7 4.6 4.5 1.000 0.999 0.997 0.996 0.993 0.992 −11.161 −11.060 −11.006 −10.954 −10.868 −10.807 5.1 4.9 4.8 4.7 4.6 4.4 1.000 0.997 0.996 0.995 0.993 0.991 0% ethanol (glycine–water) 0.0 0.2 0.4 0.6 0.8 1.0 73.02 79.08 83.58 89.03 93.46 95.94 25◦ C −11.090 −10.853 −10.805 −10.759 −10.697 −10.644 5.1 4.9 4.8 4.7 4.6 4.5 1.000 0.998 0.996 0.995 0.993 0.991 70.76 76.58 81.77 86.36 91.56 94.10 35◦ C 0.0 0.2 0.4 0.6 0.8 1.0 30◦ C −11.107 −10.865 −10.837 −10.807 −10.752 −10.713 40◦ C 69.98 75.52 80.90 84.43 89.41 91.90 −11.134 −11.022 −10.926 −10.860 −10.799 −10.752 5.0 4.8 4.7 4.7 4.6 4.5 1.000 0.998 0.996 0.995 0.994 0.992 66.30 73.78 78.97 83.28 88.23 89.64 0.0 0.2 0.4 0.6 0.8 1.0 71.70 77.42 82.88 87.42 91.67 94.18 25◦ C −10.938 −10.729 −10.682 −10.633 −10.587 −10.554 4.8 4.7 4.6 4.5 4.4 4.3 0.984 0.982 0.980 0.977 0.975 0.973 69.78 75.59 80.70 84.26 89.73 92.15 30◦ C −10.979 −10.764 −10.709 −10.663 −10.634 −10.610 4.9 4.8 4.6 4.5 4.4 4.3 0.986 0.981 0.979 0.978 0.976 0.974 0.0 0.2 0.4 0.6 0.8 1.0 67.02 72.40 77.23 81.07 84.12 89.85 35◦ C −10.992 −10.798 −10.760 −10.712 −10.684 −10.634 4.8 4.7 4.6 4.4 4.3 4.3 0.985 0.982 0.981 0.978 0.976 0.974 65.78 68.93 75.50 77.84 82.30 87.80 40◦ C −11.020 −10.827 −10.782 −10.745 −10.702 −10.659 4.8 4.7 4.6 4.5 4.4 4.4 0.985 0.982 0.980 0.977 0.976 0.975 68.83 76.03 80.67 83.93 89.18 92.82 25◦ C −10.862 −10.667 −10.628 −10.512 −10.461 −10.440 67.03 73.71 78.84 82.14 87.52 90.79 30◦ C −10.897 −10.699 −10.653 −10.541 −10.480 −10.454 4.6 4.5 4.5 4.4 4.2 4.1 0.958 0.956 0.953 0.951 0.950 0.949 64.88 71.67 76.88 80.07 35◦ C −10.939 −10.718 −10.677 −10.559 63.26 67.41 70.94 75.66 40◦ C −10.955 −10.725 −10.695 −10.587 4.5 4.4 4.4 4.2 0.957 0.956 0.954 0.951 5% ethanol 10% ethanol 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 4.6 4.5 4.4 4.3 4.2 4.1 4.6 4.5 4.4 4.3 0.956 0.955 0.953 0.952 0.950 0.949 0.958 0.955 0.954 0.952 318 Chaudhari, Shankarwar, Arbad, and Mehrotra Table I. Continued 0.8 1.0 82.13 88.14 −10.492 −10.467 4.1 4.0 0.951 0.950 0.0 0.2 0.4 0.6 0.8 1.0 67.63 71.41 73.32 77.72 83.66 90.70 25◦ C −10.791 −10.632 −10.545 −10.492 −10.438 −10.409 4.5 4.4 4.3 4.2 4.0 3.9 0.947 0.945 0.944 0.942 0.940 0.939 0.0 0.2 0.4 0.6 0.8 1.0 64.76 68.18 70.75 74.95 79.41 86.28 35◦ C −10.849 −10.671 −10.586 −10.522 −10.465 −10.433 4.5 4.3 4.4 4.1 4.0 3.9 0.997 0.946 0.944 0.943 0.940 0.938 0.0 0.2 0.4 0.6 0.8 1.0 67.28 70.06 72.86 77.91 80.28 89.24 25◦ C −10.751 −10.598 −10.498 −10.436 −10.398 −10.346 4.3 4.2 4.1 3.9 3.8 3.7 0.937 0.936 0.935 0.933 0.932 0.931 0.0 0.2 0.4 0.6 0.8 1.0 62.76 67.04 69.33 73.09 76.05 84.85 35◦ C −10.832 −10.634 −10.554 −10.496 −10.455 −10.400 4.3 4.2 3.9 3.7 3.6 3.7 0.936 0.934 0.935 0.933 0.931 0.930 61.66 68.75 70.92 74.96 78.22 84.41 25◦ C −10.707 −10.528 −10.422 −10.406 −10.372 −10.341 55.86 61.40 65.49 71.36 72.77 80.38 35◦ C −10.804 −10.593 −10.471 −10.450 −10.412 −10.400 79.03 85.39 −10.499 −10.478 4.1 4.1 0.951 0.949 66.31 69.66 71.98 75.93 81.07 88.41 30◦ C −10.823 −10.656 −10.566 −10.505 −10.454 −10.428 4.4 4.4 4.3 4.1 4.1 4.0 0.948 0.945 0.945 0.943 0.941 0.940 62.97 65.82 69.99 72.00 77.03 84.64 40◦ C −10.890 −10.711 −10.626 −10.542 −10.496 −10.442 4.4 4.3 4.3 4.2 4.1 4.0 0.947 0.945 0.944 0.942 0.940 0.938 65.33 68.43 70.71 74.29 78.37 87.26 30◦ C −10.783 −10.617 −10.537 −10.475 −10.431 −10.363 4.2 4.1 3.9 3.7 3.6 3.6 0.936 0.935 0.935 0.934 0.933 0.931 62.09 64.40 68.32 71.05 74.18 83.88 40◦ C −10.883 −10.661 −10.587 −10.529 −10.475 −10.430 4.2 4.3 4.0 3.8 3.6 3.6 0.937 0.935 0.934 0.933 0.932 0.930 58.90 66.08 69.25 72.86 74.79 82.08 30◦ C −10.756 −10.568 −10.456 −10.425 −10.405 −10.367 4.1 4.1 3.9 3.8 3.8 3.5 0.930 0.928 0.928 0.926 0.924 0.923 53.97 58.09 61.56 66.60 69.72 78.40 40◦ C −10.846 −10.633 −10.502 −10.480 −10.431 −10.410 4.2 4.1 4.0 3.9 3.6 3.7 0.930 0.929 0.928 0.925 0.925 0.924 15% ethanol 20% ethanol 30% ethanol 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 4.2 4.1 4.0 3.8 3.7 3.5 4.2 4.0 3.9 3.9 3.7 3.6 0.930 0.929 0.927 0.926 0.925 0.924 0.931 0.929 0.927 0.925 0.924 0.923 Dielectric Relaxation Using Time Domain Reflectometry 319 Fig. 1. A Cole–Cole plot for 30% ethanol + 70% water with glycine molar concentrations of 0, 0.4, and 0.8 M at 35◦ C. the dipolar water molecules are hindered by interactions with their neighbors, in particular by hydrogen bonding. Thus the effect resulting from water will be largely masked by the dominating relaxation process. High values of the relaxation time have been found for mixtures at high glycine concentrations. The values of dielectric parameters for the glycine–ethanol–water ternary system are given in Table I. It should be noted that the dielectric strength increases with the concentration of glycine in water. When ethanol is added to the aqueous glycine solutions, the dielectric behavior altogether changes from that typical of aqueous glycine solutions. In the higher concentration region, the glycine has a tendency to form multimeric structures. The values of relaxation time increase with the addition of ---- OH groups to the mixture. This means that water molecules and the added ---- OH groups of the ethanol molecules create an environment such that the multimers and then the monomers rotate more slowly resulting in higher relaxation time. The reorientation of the glycine molecules occurs slower than that of water molecules. Water molecules bound to glycine molecules have been found to have higher dielectric relaxation times than the relaxation time of free water molecules. Addition of alcohol to water results in a higher value of the relaxation time.(29,34) With the successive addition of glycine molecules, the relaxation time continues to increase. These glycine molecules cooperate in the formation of bound water that enhances the relaxation time. With increasing temperature the orientational correlation of dipole moments decreases and the values of dielectric parameters decrease. 320 Chaudhari, Shankarwar, Arbad, and Mehrotra Fig. 2. Variation of ε and log(τ ) with glycine molar concentration for different ethanol concentrations at 35◦ C. The dependence of log(τ ) on the molar concentration of glycine shows two linear regions (Fig. 2) for lower concentrations of ethanol. In the low glycine molar concentration region, log(τ ) increases with glycine concentration having a steeper slope than the linear dependence of log(τ ) on the glycine molar concentration in the higher glycine molar concentration region. These two linear regions suggest water–ethanol and water–ethanol–glycine mixtures follow different relaxation mechanisms that dominate the relaxation processes in different glycine concentration ranges. With increasing ethanol concentration in the water–ethanol–glycine system, log(τ ) increases linearly with increasing glycine concentration. The temperature dependencies of the dielectric strength and log(τ ) on glycine molar concentration are shown in Fig. 3. There is a nonlinear relation between log(τ ) and glycine molar concentration, as can be seen from Fig. 3. The dielectric strength follows a nearly linear relationship with the molar concentration of glycine at different temperatures, for the ternary system as shown in Fig. 3, and also with different ethanol concentrations (Fig. 2). 4. CONCLUSIONS The temperature-dependent dielectric relaxation behavior at microwave frequencies was investigated for glycine–ethanol–water solutions. The dielectric spectra for these solutions can be described well by a Cole–Davidson model and all the solutions show only one relaxation peak. Different ethanol concentrations and glycine molar concentrations were investigated. Dielectric strength as well as relaxation time decreases with increasing temperature. The absorption peak shifts Dielectric Relaxation Using Time Domain Reflectometry 321 Fig. 3. Variation of ε and log(τ ) with glycine molar concentration for different temperatures. 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