In this work the 4-(trans- 4'-n -hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements and showed significant influence of the shape and size of the magnetic particles on the magnetic Fréedericksz transition. For the volume concentration phi= 2 x 10(-4) of the magnetic particles, the critical magnetic field was established for the pure liquid crystal, and for liquid crystals doped with spherical, chainlike, and rodlike magnetic particles. The influence of the magnetic field depends on the type of anchoring, which is characterized by the density of anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and director.

The trans-trans configuration of the -NH-CONHgroups in dialkylureas leads to their exceptionally strong association.However, we still lack a clear picture of the association mechanism and the structure of the species formed. Here we propose a model for jym-dialkylurea (DAU) aggregation in which molecules are linked by four hydrogen bonds, analogous to the associated water molecules. The model gives satisfactory predictions of the spectroscopic and static dielectric behavior of the ureas and allows one to explain the simple Debye nature of dielectric relaxation we observed at the megahertz frequency.

Singular behavior of the static dielectric permittivity of n-alkyloxycyanobiphenyls (CnH2n+1O-Ph-Ph-C [triple bond] N, n=6, 7) was studied above and below the nematic clearing point (T(I-N)). On approaching the clearing point, the evolution of principal components of the nematic permittivity tensor, epsilon(parallel) and epsilon(perpendicular), is described by the order parameter exponent beta approximately 0.25. The mean value of the nematic permittivity epsilon(mean)=(epsilon(parallel)+2epsilon(perpendicular))/3 exhibits a singular behavior similar to that observed in the isotropic phase and that for the diameter of the coexistence curve in binary mixtures. The derivative of experimental data d(epsilon)iso(T)/dT and d(epsilon)mean(T)/dT shows the specific-heat-like behavior with universal exponents alpha=alpha' approximately 0.5. Results obtained confirm the hypothesis of the fluidlike, pseudospinodal, and tricritical behavior of the isotropic to nematic phase transition. [A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)].