Critical behavior of dielectric permittivity in the isotropic phase of nematogens

It has been shown that the temperature behavior of dielectric permittivity (epsilon) in the isotropic phase of nematogens can-he described in the same way as in critical binary solutions. Hence, using a relation with the critical exponent phi=1-alpha=0.5+/-0.03 it was possible to portray the results of epsilon(T) measurements in the isotropic phase of 5-heptyl-2-(4'-cyanobiphenyl)-pyrimid and 4,1-n-octylcyanobiphenyl. The influence of the position of the permanent dipole moment on the results was tested by additional measurements in n-(p-methoxybenzylidene)-p'-butylaniline. It also has been shown that a fluidlike analogy can be applied to the nonlinear dielectric effect (NDE), which describes changes of dielectric permittivity induced by a strong electric field. Measurements were conducted for the lowest frequency used in NDE studies (f=67 kHz), so the condition tau f much less than 1 (with tau the relaxation time) was always fulfilled. Values of discontinuities of the isotropic-nematic phase transition from both the analysis of epsilon(T) measurements and NDE studies are in good agreement.