Temperature dependence of the electrical conductivity of imidazolium ionic liquids.

The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10^7 Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.ionic liquids; molten salts;

Adiabatic scanning calorimeter

The present invention generally relates to an adiabatic scanning calorimeter for simultaneous measurements of the temperature dependence of heat capacity and enthalpy of liquids and solids and phase transitions therein. Moreover, the invention allows for an accurate separation between pretransitional enthalpy variations and true latent heats at first-order or weakly first-order phase transitions. In addition, the invention relates to calorimeters for controlling temperature differences and heat fluxes in different modes of operation.status: publishe

Differential adiabatic scanning calorimeter

The present invention generally relates to a differential adiabatic scanning calorimeter (dASC) for simultaneous measurements of the temperature dependence of heat capacity and enthalpy of an unknown sample (liquid or solid) from a comparison with the heat capacity and enthalpy of another known sample (liquid or solid). In particular, the invention allows the determination of small differences in heat capacity and enthalpy of an unknown sample and of a known very similar reference sample. Moreover, the invention allows for an accurate separation between pre-transitional enthalpy variations and true latent heats at first- order or weakly first-order phase transitions of the unknown sample. In addition, the invention relates to calorimeters for controlling temperature differences and heat fluxes in different modes of operation.status: publishe

Determination of some thermophysical parameters of liquids from thermal waves generated by the Peltier effect

This paper discusses the feasibility to use commercial Peltier elements as a periodic heat source in experimental setups for the determination of the thermal diffusivity and the heat capacity. The diffusivities of three liquids are reported, obtained from different approaches to the depth profile and frequency dependence of the thermal wave. Also the temperature dependence of the amplitude of the thermal wave, sensitive to changes in the heat capacity, is measured near phase transitions, showing the feasibility of the approach for ac calorimetry. Theory, advantages, problems and future perspectives are discussed.status: publishe

Confinement effects on strongly polar alkylcyanobiphenyl liquid crystals probed by dielectric spectroscopy

Dielectric spectroscopy has often been used to study confinement effects in alkylcyanobiphenyl liquid crystals. In this paper, we highlight some of the effects that have been discovered previously and add new data and interpretation. Aerosil nanoparticles form a hydrogen bonded random porous network. In dispersions of alkylcyanobiphenyls with aerosils, an additional slow process arises, that we ascribe to the relaxation of liquid crystal molecules in close interaction with these nanoparticles. Their relaxation is retarded by a hydrogen bond interaction between the cyano group of the liquid crystals and an aerosil surface hydroxyl group. A similar surface process is also observed in Vycor porous glass, a random rigid structure with small pores. A comparison of the temperature dependence of the relaxation times of the surface processes in decylcyanobiphenyl and isopentylcyanobiphenyl is made, both for Vycor and aerosil confinement. In decylcyanobiphenyl, the temperature dependence for the bulk and surface processes is Arrhenius (in a limited temperature range above the melting point), except in Vycor, where it is a Vogel–Fulcher–Tamman dependence (over a much broader temperature range). In bulk and confined isopentylcyanobiphenyl, the molecular processes have a Vogel–Fulcher–Tamman dependence, whereas the surface processes have an Arrhenius one. Another effect is the acceleration of the rotation around the short molecular axis in confinement, and particularly in aerosil dispersions. This is a consequence of the disorder introduced in the liquid crystalline phase. The disorder drives the relaxation time towards a more isotropic value, resulting in an acceleration for the short axis rotation.status: publishe

Accelerated dielectric relaxation in the isotropic phase of associating liquid crystals dispersed with aerosils

The dielectric response of liquid crystals in their nematic phase shows an acceleration of the relaxation associated with the rotation around the short molecular axis when dispersed with aerosils. However, in the isotropic phase, this acceleration is only seen for certain liquid crystalline molecules. In this paper, an associating liquid crystal 5CB and a shorter monotropic liquid crystalline member of the same homologous series 4CB, a liquid crystal that does not show association 5NCS and a nonassociating liquid crystal 5¯O5 have been studied by dielectric spectroscopy in the isotropic phase. Both 4CB and 5CB show a constant acceleration in the isotropic phase, observed over a broad temperature range. 5NCS and 5¯O5 do not show an acceleration in their isotropic phase. It seems that the disturbance of the dipole-dipole association process for the associating molecules is an important factor in the explanation of the acceleration of the relaxation in the isotropic phase.status: publishe