Narasimha H. Ayachit

On the effect of temperature on the dielectric relaxation time of some benzene derivatives and certain of their binary mixtures

Dielectric relaxation behaviour of polar molecules in a non-polar solvent, or mixtures of these substances at different microwave frequencies and over a range of temperatures and concentrations give an idea about inter- and intra-molecular forces. Also such studies enable one to calculate thermodynamic parameters such as, the change of activation energy for dipole orientation (Δ G*), the enthalpy (Δ H*) and entropy (Δ S*) of activation. Such studies in the case of binary, ternary, etc. mixtures of polar molecules in pure liquid phase or in dilute solution phase of them in a non-polar solvent help in drawing certain quantitative conclusions regarding their relaxation behaviour as to whether a single component is responsible for observed microwave absorption, or a cooperative phenomenon (average) by all the dipoles of the mixture contribute to it. An experimental investigation is here performed on typical systems. With this in view, systematic dielectric measurements in a range of temperatures are carried out at a single microwave frequency on a single weight fraction in benzene of the four substituted phenols, namely, p-fluorophenylacetonitrile, p-bromonitrobenzene, m-bromonitrobenzene and 2-chloro-6-fluoro-benzaldehyde and on binary (1 : 1) mixtures of [p-2-chloro-6-fluoro-benzaldehyde + o-ethylphenol] and [p-fluorophenylacetonitrile + 2-n-butyl phenol] in benzene as solvent at different temperatures. The results are presented and discussed.

Characterization of Some Liquid Crystals through Sound and Thermodynamic Parameters

The structural changes arising from transition between the mesophases of liquid crystalline materials can be better understood by ultrasound in conjunction with specific volume properties. In particular, ultrasonic studies are found to give clear information regarding the nature of the phase transition, pretransitional effects, and molecular ordering. For example, temperature corresponding to the maximum value of acoustic impedance, adiabatic compressibility, and temperature corresponding to the minimum value of ultrasonic velocity, Raos number, order parameter, and molar compressibility, in thermotropic liquid crystals are very useful in understanding their structural behavior. In view of these facts, it was thought worthwhile to study ultrasonic velocity with data available for density measurements on pure samples of nematics, namely, 4,4′-azoxy anisole and 4,4′-dibutoxy azoxy benzene. The results of this work, are presented in this article and discussed.

A study on thermodynamic and sound parameters of some nematics

Measurements of ultrasonic velocity and density of solutes in dilute solutions as a function of concentration and temperature can be used to obtain several thermodynamic and acoustical parameters and their excess adiabatic compressibility. The physical parameters that can be determined through these are: molar sound velocity or Raos number (R), molar compressibility or Wadas constant (B), characteristic acoustic impedance (Z), intermolecular or free length (L f), free volume (V f), available volume (V a) and adiabatic compressibility (β). These parameters play an important role in the phenomenon associated with intermolecular interaction and hence in sound transmission. For example, the parameter Z that is determined by the product of density (d) and ultrasonic velocity (c) has a greater significance as a characteristic property of the medium than does either the density or velocity individually so also in case of sound transmission the parameter of more concern is the molar sound volume (V m) rather than molar sound velocity or Rao’ number. Such studies on liquid crystals around phase transition temperature in their pure state yield important information and so also in their dilute solutions. In view of above facts, the ultrasonic velocity with density measurement studies on pure samples of nematics, namely, Butyl-p-(p-ethoxy phenoxy carbonyl)phenyl carbonate, p-(p-ethoxy phenyl azo)phenyl undecylenate and p-[N-(p-methoxy benzylidene)amino]phenyl benzoate in dilute solutions was taken up both as a function of mole fraction (in benzene) and temperature. The results of the above work is presented in this article and discussed.

Excited state polarizabilities from solvatochromic shifts

A new method is proposed to estimate the polarizability (αe) of a molecule in an excited state using solvatochromic shift measurements and McRaes equation. In the earlier methods the contribution due to polarizability was not considered. In view of this, the proposed method is also expected to give a better estimation of excited state electric dipole moment (μe) and the (θ) angle between excited and ground state electric dipole moments, μe and μg apart from giving values of polarizability of the molecules in the excited state. This method has been applied in the case of the La band of p-nitro aniline and the results for all the parameters are found to be satisfactory and of right order in comparison with that reported in literature.

Viscoelastic studies of some phenols from dielectric measurements

The variation of dielectric relaxation time with the viscosity of the medium is being exploited in drawing certain quantitative conclusions regarding molecular motion and intermolecular forces in liquids, liquid mixtures, dilute solutions and multi-component polar solutes in dilute solution. With no perfect empirical or theoretical equation in place for the variation of dielectric relaxation time with viscosity, the experimental investigations on different systems only can give an insight. In the present study the results of dielectric measurements carried out on pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, and 2,6-butylphenol in dilute solutions in different mixed solvents (benzene + paraffin) and on binary mixtures (1 : 1) of p-2-n-butylphenol + 4-n-butylphenol and p-bromonitrobenzene + 2,6-butylphenol, are reported. For comparison, the results on p-bromonitrobenzene + m-bromonitrobenzene as an example of mixture of non-associative liquids was also carried out and the results are presented. Different parameters determined using these dielectric measurements are also presented using different models and these studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the system under study.

Excited state electric dipole moments of two exalite dyes from solvatocromic shift measurements

The determination of excited state electric dipole moment through solvatochromic shifts of two Exalite dyes, namely, E-392A, E-398 have been carried out using fluorescence shifts and have been found to be less than those calculated through absorption shifts. It is felt that the one calculated using shifts in emission are more reliable. The results are presented and discussed. A qualitative estimate of the orientation of dipole moments in ground and excited state are also presented and discussed.

On the Viscoelastic and Dielectric Behavior of Some Organic Compounds and Their Binary Mixtures

The dependence of dielectric relaxation time on the viscosity of the medium is being extensively used to draw certain quantitative conclusions regarding molecular motion and inter-molecular forces in liquids, liquid mixtures, dilute solutions, and multi-component polar solutes in dilute solution. In the absence of proper empirical or theoretical equations for the variation of dielectric relaxation time with viscosity, only the experimental investigations on different systems can give an insight. In the present study, the results of dielectric measurements carried out on pure samples of bromohexane, bromooctane and bromodecane in dilute solutions in different mixed solvents (benzene + paraffin) and on binary mixtures (1 : 1) of (bromohexane + bromodecane); (bromodecane + propyl alcohol) and (propyl alcohol + methyl alcohol) are reported. For comparison, the results of bromodecane + propyl alcohol and propyl alcohol + methyl alcohol are chosen as they form examples of mixture of non-associative + associative and associative + associative liquids, respectively. Different parameters determined using these dielectric measurements are also presented using different models. These studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the systems under study.

Dielectric studies on some mesogenic, non-mesogenic and organic molecules

Two important molecular parameters, namely, the dielectric relaxation time τ and electric dipole moment are highly useful in having insight into the molecular structure, size, shape, apart from the inter- and intra-molecular forces, etc. With these in view dielectric measurements in benzene at room temperature on the pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, 2,6-dimethoxyphenol and 3,4-difluorophenol were carried out at a frequency of 9.98 GHz by employing concentration variation method. Similar measurements, on a single weight fraction of each of them at 9.98 GHz and also at 8.74 GHz are carried out. Measurements on a single weight fraction in benzene of each of the liquid crystal samples, namely, EPCP.car (Butyl-p-(p-ethoxy phenoxy corbonyl)-phenyl carbonate), PPPB (p-pentylphenyl-p-propyl benzonate), EPAP.Hp (p-(p-ethoxyphenyl azo) phenyl heptenate) and EPAP. Und (p-(p-ethoxyphenyl azo) phenyl undecylenate were also carried out at the said two frequencies. Using the obtained values of ε′, ε′′ relaxation time and dipole moment were determined using different methods. The obtained results are compared with earlier results wherever available.

Dielectric studies on some substituted indoles

Dielectric behavior of some substituted indole compounds, namely, 5-bromoindole, 5-methoxyindole, 5-fluoroindole, 2,3-dimethylindole, 2,5-dimethylindole at room temperature have been studied at two frequencies 8.55 and 9.65 GHz. These studies have been carried out in dilute solutions because such studies have the advantage over pure liquids that the strong dipole–dipole interactions in the dilute solution phase are much reduced.