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) over bar 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 (2) over bar 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.