Phase transitions from THE isotropic liquid to liquid crystalline mesophases studied by linear and nonlinear static dielectric permittivity

Results of studies of static dielectric permittivity ({ε}) and nonlinear dielectric effect (NDE) in the isotropic phase of 4-n-4^'-isothiocyanatobiphenyl (nBT) homologous series from $n=2$ to $n=10$ exhibiting the isotropic-smectic E (I-SmE) transition, are presented. They are compared with results of similar studies in 4-cyano-4-n-alkylbiphenyls (nCB) from $n=4$ to $n=12.$ In this homologous series isotropic-nematic (I-N) and isotropic-smectic-A (I--Sm-A) transitions take place. Despite significant differences between N, Sm-A, and Sm-E phases the same pretransitional behavior of {ε} and NDE in the isotropic phase, described by critical exponents γ=1 and α=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 nBT and rises in nCB. The influence of pressure on the discontinuity is also discussed.