Dielectric relaxation study of dipropylsulfoxide/water mixtures

The complex dielectric spectra of dipropylsulfoxide (DPSO)/water mixtures in the whole concentration range have been measured as a function of frequency between 100 MHz and 20 GHz at four temperatures between 298.15 K and 328.15 K. The dielectric parameters, static dielectric constant (ε_s), relaxation time (τ) and relaxation strength (Δε) have been obtained by the least squares fit method. The relaxation in these mixtures can be described by two Debye functions, whereas for pure DPSO Cole-Davidson type is valid. The relaxation times of the mixtures show a maximum at about x(DPSO) ≈ 0.3. In the concentration range where a maximum appears, the interaction of DPSO with water is presumably the result of hydrogen bonding between water and the sulfonyl group of the sulfoxide molecule. The concentration and temperature dependent excess dielectric constant and effective Kirkwood correlation factor of the binary mixtures have been determined. The excess permittivity is found to be negative for all concentrations.     

Dielectric relaxation study of poly(ethylene glycols) using TDR technique

Temperature dependent dielectric relaxation and thermodynamic properties of polyethylene glycols HO[CH₂CH₂O)_nH with number average molecular weight 200 (n = 4), 300 (n = 7), 400 (n = 9) and 600 (n = 14) g mol^− 1 have been studied using Time Domain Reflectometry (TDR) in the frequency range 10 MHz to 20 GHz. The frequency dependence of the complex dielectric permittivity is analyzed by the Havriliak-Negami expression. The static permittivity ε₀, high frequency limiting static permittivity ε_∞, average relaxation time τ₀ and thermodynamic energy parameters such as free energy, enthalpy of activation and entropy of activation have been determined. The average free energy of activation ΔF_τ for PEG molecules was found to be in the range 4-5 kcal mol^− 1. The values of entropy ΔS_τ for PEG-200, PEG-400 and PEG-600 molecules were found to be positive while entropy ΔS_τ for PEG-300 molecules was found negative, which confirms that the configuration of PEG-300 involved in the dipolar orientation has an activated state, which is more ordered than the normal state compared to PEG-200, PEG-400 and PEG-600 molecules.     

Dielectric relaxation of chlorinated polyvinyl chloride (CPVC) stabilized with cyanoguanidine

Dielectric spectra of CPVC stabilized with cyanoguanidine were studied in the temperature range 300–450 K and frequency range 10 kHz to 1 MHz. In these conditions, only one clear dielectric relaxation band (α-type) associated with dipolar polarization was observed. Dielectric losses was found to directly proportional to the number of dipoles (N) which reflects the orientational distribution of polymer chains in the amorphous region, at which dielectric losses concerned. Calculations of the dielectric modulus M′(T) at low temperature indicate that there is a role of the electrode polarization in the relaxation process. Dielectric loss data were used to calculate the activation enthalpy by two different methods, the obtained value was 450 kJ/mol.     

Dielectric relaxation and ac conductivity of sodium tungsten phosphate glasses

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77–420 K. Complex relative permitivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (σ_m(ω)) has been found to be higher than σ_dc at low temperatures whereas at high temperature σ_m(ω) becomes equal to σ_dc at all frequencies. The ac conductivity obeys the relation σ_ac(ω)=Aω ^S over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(E _F)) have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 (Å)^?1. Values of N(E _F) have the order 10²⁰ which are well within the range suggested for localized states. Present values of N(E _F) are smaller than those for tungsten phosphate glasses.     

Dielectric and conductivity relaxation in mixtures of glycerol with LiCl

We report a thorough dielectric characterization of the α relaxation of glass-forming glycerol with varying additions of LiCl. Nine salt concentrations from 0.1 to 20mol% are investigated in a frequency range of 20Hz-3GHz and analyzed in the dielectric loss and modulus representation. Information on the dc conductivity, the dielectric relaxation time (from the loss) and the conductivity relaxation time (from the modulus) is provided. Overall, with increasing ion concentration, a transition from reorientationally to translationally dominated behavior is observed and the translational ion dynamics and the dipolar reorientational dynamics become successively coupled. This gives rise to the prospect that, by adding ions to dipolar glass formers, dielectric spectroscopy may directly couple to the translational degrees of freedom determining the glass transition, even in frequency regimes where usually strong decoupling is observed.     

Dielectric study of acetophenone and its derivatives

Complex permittivity measurements on acetophenone and its derivatives o-hydroxybenzaldehyde, o-methylacetophenone, and o-hydroxyacetophenone are performed at frequencies between 1 MHz and 20 GHz at temperatures from 273 to 323 K. The parameters obtained from the fitting of the complex permittivity are analyzed in order to study the effects of the hydroxyl group within a molecule on the dielectric relaxation phenomenon in these liquids. The analysis indicates that dynamical properties are affected not only by the intermolecular hydrogen bond but also by the slight change in molecular structure. This conclusion differs from those obtained from the results of other experiments.     

Dielectric relaxation studies of aqueous sucrose in ethanol mixtures using time domain reflectometry

Time domain reflectometry method has been used in the frequency range of 10 MHz to 10 GHz to determine dielectric properties of aqueous sucrose in ethanol. The dielectric parameters, i.e., static dielectric constant and relaxation time were obtained from the complex permittivity spectra using the non-linear least squares fit method. The Luzar theory is applied to compute the cross-correlation terms for the mixtures. It adequately reproduces the experimental values of static dielectric constants. The Bruggeman model for the non-linear case has been fitted to the dielectric data for mixtures.     

Dielectric relaxation of poly(ethylenglycol)- b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers above the glass transition temperature

The complex dielectric permittivity has been measured for three poly(ethylenglycol)-b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers with different content of poly(ethylenglycol) (15%, 33% and 80%), and increasing degree of crystallinity (0%, 10% and 20%, respectively). Only the non-crystalline sample shows the normal mode relaxation together with the segmental (α-relaxation) and the Johari-Goldstein (β-relaxation) modes. The crystalline samples show also polarization contributions due to the existence of interfaces between the crystallites and the amorphous phase. The relaxation times of the (α and normal modes can be described by a VFT equation with the same value of T₀. There is a slowing-down of the segmental mode due to the presence of crystallites. The temperature dependence of the α and β relaxations in the copolymers is very similar to that found in pure PPG, while there are significant differences in the case of the normal mode of the non-crystalline sample. The size of the cooperatively rearranging regions CRR, and the width of the glass transition region increase slightly with the degree of crystallinity. The temperature dependence of the size of CRRs is compatible with the prediction of fluctuation theory. No systematic effect of the degree of crystallinity on the β-relaxation has been found. Near T _g the β-relaxation time is close to the primitive time of the coupling model. Received: 31 May 2000     

Dielectric relaxation and ac conductivity of double perovskite oxide Ho2ZnZrO6

Double perovskite oxide holmium zinc zirconate Ho₂ZnZrO₆ (HZZ) is synthesized by solid state reaction technique under a calcination temperature of 1100 °C. The crystal structure has been determined by powder X-ray diffraction, which shows monoclinic phase at room temperature. The variation of dielectric constant (ε′) and loss tangent (tan δ) with frequency is carried out assuming a distribution of relaxation times. The frequency corresponding to loss tangent peak is found to obey an Arrhenius law with activation energy of 89.7 meV. The frequency-dependant electrical data are analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary electric modulus shows the temperature-independent nature of the distribution of relaxation times. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions.     

Study of heterogeneous interaction in binary mixtures of 2-methoxyethanol-water using dielectric relaxation spectroscopy

The dielectric relaxation measurements on binary mixtures of 2-methoxyethanol with water have been carried out over entire concentrations and at temperature range of 0 °C to 25 °C using a picosecond time domain reflectometry technique. The complex dielectric permittivity spectra of 2-methoxyethanol/water mixtures were fitted using Havriliak-Negami equation. The static dielectric constant and relaxation time for all concentrations were obtained using least square fit method. The principal relaxation time is small if compared to that of corresponding alcohol/water mixtures this may be due to the hydrogen bonding ether oxygen in the 2-ME-water system. Excess dielectric properties, Kirkwood correlation factor, thermodynamic properties and Bruggeman factor are also determined and the results are interpreted in terms of heterogeneous interactions among the unlike molecules due to hydrogen bonding.     

Dielectric permittivity and electric modulus of polyethylene oxide (PEO)-LiClO4 composite electrolytes

Dielectric permittivity and conductivity relaxation in polyethylene oxide (PEO)-LiClO₄ salt polymer electrolytes have been investigated for different lithium ion concentrations. We have observed that imaginary modulus spectra exhibit asymmetric maxima with peak-width much broader than that of the Debye peak and are skewed toward the high frequency sides of the maxima. The charge carriers for the electrolyte having higher lithium salt concentration relax much faster than that for other electrolytes and produces higher conductivity. The modulus data have been fitted using non-exponential Kohlrausch-Williams-Watts (KWW) function φ(t). We have observed that the value of the non-exponential parameter (β) is fairly low and nearly constant for different salt concentrations. The low value of β suggests a wide distribution of non-exponential relaxation times. Using the scaling of modulus data we have observed that the relaxation dynamics of charge carriers in these PEO-Li salt based electrolytes is independent of temperature and salt concentration.     

Dielectric relaxation studies of some aliphatic alcohols and their mixtures in benzene solutions

Dielectric absorption of four aliphatic alcohols, viz., methyl alcohol, ethyl alcohol, n-propyl alcohol, tert-butyl alcohol and their binary mixtures, mixtures of these alcohols with dimethyl formamide and 2-fluoroaniline has been studied in the 3 cm microwave region at a temperature of 30°C in benzene. In alcohols and their mixtures the tan σ - concentration curve shows a marked increase in the concentration range 0.02 – 0.05 weight fraction. This behaviour has been explained as due to the formation of complex via self and hetero-association. The alcohols + DMF systems show complex formation at a very low concentration range. The study of systems alcohols + 2 - fluoroaniline indicate that the 2-fluoroaniline produces dissociating effects in these alcohols.     

Dielectric relaxation and related studies of 4-ethylphenol-methanol mixtures using time domain reflectometry

The dielectric relaxation studies of 4-ethylphenol-methanol mixtures have been carried out at various temperatures ranging from 10°C to 40°C using time domain reflectometry in the frequency range 10 MHz to 10 GHz. The relaxation mechanism in these systems is explained by Cole-Davidson model. The excess dielectric parameters, Kirkwood correlation factor and activation energy have been calculated and discussed with respect to molecular arrangements, and microdynamics of the binary mixture composed of both the associative type of liquids.     

The dielectric relaxation study of 2(2-alkoxyethoxy)ethanol-water mixtures using time domain reflectometry

Complex permittivity spectra of 2(2-alkoxyethoxy)ethanol-water mixtures for the entire concentration range and at 25 °C have been measured using time domain reflectometry technique up to 30 GHz. The relaxation processes have been obtained by fitting the complex permittivity spectra of the binary mixtures in 2-Debye equation. In glycol ether-water mixtures except for pure water and its rich region the double relaxation times, primary process (τ₁) and secondary process (τ₂) have been observed. The primary relaxation process (τ₁) may be due to cooperative relaxation of the H-bond network of mixture constituents and the secondary relaxation process (τ₂) may be due to its flexible parts attached to the carbon chain. The intra and intermolecular hydrogen bonding of 2(2-alkoxyethoxy)ethanol in pure form as well as in binary mixtures of water have been discussed using Kirkwood correlation factor, Excess dielectric properties and the Bruggeman factor.     

Choice of the reduction and of the representation in centrifugal distortion analysis: A case study of dimethylsulfoxide

The millimeter-wave and submillimeter-wave spectra of dimethylsulfoxide have been measured up to 660 GHz, corresponding to maximum values of J = 70 and K_c = 68. A total of 3331 transitions (corresponding to 2133 distinct frequencies) were fit to a standard Watson Hamiltonian using the S- and A-reductions and the representations I^r and III^r. Although dimethylsulfoxide is an asymmetric oblate top, the combination (A, III^r) gives the worst results. From the point of view of the convergence of the Hamiltonian, the best results are obtained with the combination (S, III^r) but the least-squares system of normal equations is better conditioned with representation I^r. These results are compared to those obtained for a few typical molecules.     

Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material

A polycrystalline vanadium doped lead free dielectric material of Bi(Zn_2/3V_1/3)O₃ (BZV) has been prepared using a standard high-temperature solid state reaction technique. Its temperature and frequency dependent capacitive, conductive and resistive characteristics are outlined though experimental investigation. The formation of single phase compound of BZV material with orthorhombic crystal symmetry is identified through X-ray diffraction data analysis, and the homogeneous distribution of grains are realized through scanning electron micrograph. The acquaintance of frequency–temperature dependent electrical parameters with the obtained micrograph provides the experimental evidence of contributions of grain as well as grain boundary in its capacitive and resistive characteristics. The negative temperature coefficient of resistance behaviour of the material is revealed from impedance characteristic, and non-Debye type relaxation has been realized from the Nyquist plot. The charge carriers of this electronic compound have both long & short range order that has been validated from the complex modulus and impedance analysis. The prepared electronic material substantiate some important dielectric features which props up the material as promising component for electronic devices.     

Dielectric properties and fluctuating relaxation processes of poly(methyl methacrylate) based polymeric nanocomposite electrolytes

Solid polymer nanocomposite electrolytes (SPNEs) consisted of poly(methyl methacrylate) (PMMA) and lithium perchlorate (LiClO₄) of molar ratio C=O:Li⁺=4:1 with varying concentration of montmorillonite (MMT) clay as nanofiller have been prepared by classical solution casting and high intensity ultrasonic assisted solution casting methods. The dielectric/electrical dispersion behaviour of these electrolytes was studied by dielectric relaxation spectroscopy at ambient temperature. The dielectric loss tangent and electric modulus spectra have been analyzed for relaxation processes corresponding to the side groups rotation and the segmental motion of PMMA chain, which confirm their fluctuating behaviour with the sample preparation methods and also with change of MMT concentration. The feasibility of these relaxation fluctuations has been explained using a transient complex structural model based on Lewis acid–base interactions. The low permittivity and moderate dc ionic conductivity at ambient temperature suggest the suitability of these electrolytes in fabrication of ion conducting electrochromic devices and lithium ion batteries. The amorphous behaviour and the exfoliated/intercalated MMT structures of these nanocomposite electrolytes were confirmed by X-ray diffraction measurements.     

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, α_Ac relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, α_c relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and α_a relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for α_Ac and α_c relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.     

Dielectric studies of a bioactive tetramic acid and its complexes with Cu(II) and Co(II)

The 3-acyl tetramic acids constitute a growing class of natural products displaying a range of biological activities. The g , g ' tricarbonyl moiety present in the 3-acyl tetramic acid provides a suitable site for bidentate complexation to a metal, which increases the biological activity. For the dielectric study of N-acetyl-3-butanoyl tetramic acid and a series of its complexes with Cu(II) and Co(II) in symmetric and asymmetric forms, we used the Thermally Stimulated Depolarization Currents (TSDC) technique. The drastic decrease of the intensity of the TSDC peaks of the symmetric and asymmetric complexes, compared to the above mentioned ligand, suggested that the polarizability of the side groups is considerably reduced. This result enhances the proposed complexation mode of the ligand through oxygen next to carbons 3 and 4 of the 5-member ring.     

Artificial neural networks versus partial least squares and multivariate resolution-alternating least squares approaches for the assay of ascorbic acid,rutin, and hesperidin in an antioxidant formulation

Abstract

This study was concerned with the assay of ascorbic acid (ASC), rutin, and hesperidin (HES) in their combined formulation using a multivariate approach. Three chemometric-assisted spectrophotometric models namely: partial least squares, multivariate curve resolution-alternating least squares, and artificial neural networks were developed and validated. The quantitative analyses of all the proposed models were assessed by percentage recoveries, root mean square error of prediction, and standard error of prediction. The proposed models were used in the range of 10.0–70.0, 2.0–10.0, and 2.0–10.0?µg mL^?1 for ASC, rutin, and HES, respectively. In addition, correlation coefficients between the pure and estimated spectral profiles were used for the qualitative analysis of a multivariate curve resolution-alternating least squares model. Artificial neural networks showed higher speed and methodological simplicity over the other two models. These models presented powerful multivariate statistical tools that were applied to the analysis of the combined dosage form in the Australian market. They have the ability to overcome difficulties such as colinearity and spectral overlaps. Statistical comparison between the proposed and reported methods showed no significant difference. The proposed methods can be used for the routine analysis of the studied drugs in quality control laboratories.