Results of studies of static dielectric permittivity (ɛ) and nonlinear dielectric effect (NDE) in the isotropic phase of 4-n${-4\mathrm{}}^{\prime }$-isothiocyanatobiphenyl $\left(n\mathrm{BT}\right)$ homologous series from $n=2\mathrm{}$ to $n=10\mathrm{}$ exhibiting the isotropic-smectic E $(I-\mathrm{S}\mathrm{m}E)$ transition, are presented. They are compared with results of similar studies in 4-cyano-4-n-alkylbiphenyls $\left(n\mathrm{CB}\right)$ from $n=4\mathrm{}$ to $n=12.\mathrm{}$ In this homologous series isotropic-nematic $\mathrm{}(I-N)\mathrm{}$ and isotropic-smectic-A $(I–\mathrm{Sm}-A)$ transitions take place. Despite significant differences between N, $\mathrm{S}\mathrm{m}-A,$ and $\mathrm{Sm}-E$ phases the same pretransitional behavior of ɛ and NDE in the isotropic phase, described by critical exponents $\gamma =1\mathrm{}$ and $\alpha =0.5,$ was found. It has been shown that when the length of the alkyl chain of a compound increases the discontinuity of the transition drops in $n\mathrm{BT}$ and rises in $n\mathrm{CB}.$ The influence of pressure on the discontinuity is also discussed.

• Received 2 September 1999

DOI:https://doi.org/10.1103/PhysRevE.61.5355