Bo Gestblom

Dielectric relaxation study of alcohol/diol(s) mixtures

Abstract The dielectric relaxation of binary and ternary diol(s)/alcohol mixtures has been studied. Diols of different chain length namely 1,2 ethanediol, 1,3 propanediol, 1,4 butanediol and 1.5 pentanediol are used. The monohydric alcohols used are benzyl, n-hexyl, cyclopentyl, cyclohexyl and cycloheptyl alcohol. The relaxation spectra have been analyzed in one Cole-Davidson term. An apparent dipole moment term and the effective relaxation time of the main dispersion region are given as function of the number density of dipoles N D .

Comparison of the Dielectric Relaxation Data for the Isotropic and Nematic Phases of Three Homologous Series: nCb, nOCB and nPCH, (n =5÷8)

Abstract Results of dielectric studies of three homologous series: n-alkyl-cyanobiphenyls - nCB, n-alkyloxy-cyanobiphenyls - nOCB, and trans-4-(4′-n-alkyl-cyclohexyl)-cyanobenzene - nPCH, with n from 5 to 8, are presented. The parameters characterizing the rotational motions of the molecules around their short axes in the nematic and isotropic phases were derived (dielectric increments, relaxation times, activation enthalpies, retardation factors, nematic potentials, order parameters). Comparative analyses of the parameters in respect to the lengths of the terminal groups and the structures of the molecular cores are given.

Dielectric relaxation studies of monoalcohol solutions with 1,2-diols

Dielectric relaxation processes in solutions of monoalcohols with ethanediol, 1,2-propanediol, and 1,2-butanediol have been studied using the dielectric time domain spectroscopy technique. The dielectric spectrum has been analyzed in terms of two dispersions. The influence of varying content of diol, and of chainlength of the alcohol, on the static dielectric constant ∈s and on the main dispersion is reported. For the long-chain alcohols ∈s is lowered and the main relaxation time successively shortened with the addition of diol to the solution. The results are compared to these found for aqueous alcohol solutions and discussed in terms of a structural model with rupturing of predominantly linear complexes on addition of water, diols, or on increasing the temperature.

Dielectric spectroscopy and electrooptic studies of new MHPOBC analogues

Two new MHPOBC analogues, exhibiting ferroelectric and antiferroelectric phases, have been studied by means of frequency domain dielectric spectroscopy, time domain spectroscopy and electrooptic me ...

Structural changes in the microemulsion system didodecyldimethylammonium bromide/water/dodecane as investigated by means of dielectric spectroscopy

Abstract The microemulsion system didodecyldimethylammoniumbromide (DDAB)/dodecane/water (electrolyte) has been investigated by means of phase diagrams and dielectric time domain spectroscopy (TDS). The microemulsion phase (L2) will gradually decrease upon electrolyte addition until only a narrow channel exists. The original microemulsions are built up by closed aggregates and bicontinuous structures depending on the location in the L2 phase. Dielectric parameters indicate that the electrolyte will gradually change the shape of the aggregates from elongated structures to spherical ones until the one-phase region vanishes. At 0.03 m NaBr only bicontinuous microemulsions exist.

A dielectric spectroscopic study of some ionic and nonionic microemulsions

Abstract Dielectric spectra between 50 MHz and 11 GHz of the organic solutions in the systems sodium octanoate-decanol-water and tetraethylene glycol nonylphenol ether (Triton N42)-dodecane-water have been measured by time domain spectroscopy. The permittivity is discussed in relation to models consisting of rather well-defined aggregates. Several dielectric dispersion regions are observed, and the relaxation modes are attributed to the relaxation mechanisms of the different chemical compounds within the microemulsion.

The signs of the side-chain spin couplings in furanaldehydes as determined by nuclear magnetic triple resonance

Abstract The signs of the two long-range aldehyde couplings J CHO-4 and J CHO-5 in the 2- and 3-furanaldehydes have been determined with respect to the ring coupling constant J 45 . Only the coupling J CHO-4 in 3-furanaldehyde was found to be of opposite sign to J 45 . A triple resonance method for the determination of relative signs in weakly coupled spin systems is described and the implications of the results are discussed.

Relative signs of nuclear spin coupling constants by means of transitory selective irradiation experiments

Abstract A new method for the determination of energy level arrangements in nuclear spin systems is described. In this method, a transitory selective irradiation (TSI) of some chosen line is used to produce a redistribution of the spin level populations within a time that is short compared with the spin lattice relaxation time T 1 . These population changes give rise to intensity changes for lines which have an energy level in common with the irradiated line—and only for such lines—when the spectrum is recorded within a time that is short compared with T 1 . The application of the TSI method for the determination of the relative signs of spin coupling constants is described. As an illustration the method has been used for the determination of the relative signs of the ethylenic coupling constants in the simple three-spin systems of styrene and vinylbromide. In three-spin spectra with only two observable coupling constants the relative signs of these cannot be determined by double resonance experiments, but with an extension of the TSI method this problem can be solved. In this particular case three transitory irradiations are required. The procedure to be followed is described, and as an application we have studied the case of trans -cinnamic aldehyde where it was found that the two vicinal coupling constants in the side chain have the same sign.

On the derivation of the nematic order parameter from the dielectric relaxation times

The dielectric relaxation times characterizing the rotations around the molecular short axis in the nematic (τ‖) and isotropic (τis) phases of several liquid crystalline cyanophenyl compounds are re-analysed with the aim of obtaining the retardation factor g‖ and then the nematic potential q and the order parameter S. Two theoretical approaches are employed, those of Meier and Saupe and of Coffey etal., which lead to substantially different S values. The calculated order parameters as functions of temperature and pressure are compared with those measured by several other experimental techniques. On this basis it is concluded that the Coffey etal. theory better relates the relaxation times with the main parameters characterizing the nematic state (nematic potential, order parameter). The quantities obtained are used to check some theoretical predictions, especially at the isochoric condition.

Dielectric studies of trans-4-n-octyl-(4-cyanophenyl)cyclohexane (8PCH) at ambient and high pressure

Dielectric relaxation studies have been performed on trans -4- n -octyl-(4-cyanophenyl)is cyclohexane (8PCH) at ambient and high pressure (0.1-175MPa). Two experimental set-ups were applied: a time domain spectrometer (TDS) covering the frequency range 10 MHz-5 GHz, was used to study the relaxation processes in the isotropic phase (at ambient pressure); an impedance analyser (1 kHz-13MHz) was used for high pressure measurements on both the nematic (N) and isotropic (I) phases. The low frequency (l.f.) relaxation process connected with molecular rotations about the short axis is hindered by the activation enthalpy of 70 kJ mol-1 and 32.6kJ mol-1 in the N and I phases, respectively, whereas the high frequency process (rotations about the long axis) has an activation enthalpy of 22.6kJ mol-1 (isotropic phase). From the pressure and temperature dependencies of the l.f. relaxation time tau, the activation volume, enthalpy and energy were calculated. It was found that the energy barrier hindering the molecular ...