Does the characteristic value of the discontinuity of the isotropic-mesophase transition in n-cyanobiphenyls exist?

Results of the extended Landau?de Gennes model analysis and experimental studies of the isotropic?nematic (I?N) and isotropic?smectic-A (I?SmA) phase transitions in rod-like liquid crystalline n-alkylcyanobiphenyls are presented. Experiments were carried out as a function of temperature and pressure using the static dielectric permittivity and its ?nonlinear? (strong electric field related) counterpart?the low-frequency nonlinear dielectric effect. Precise estimations of the values of the discontinuity of the isotropic?mesophase transitions (?T) for nCB from n?=?3?14 have been obtained. It is suggested that for each nCB a unique, characteristic minimal value of ?T, associated with the I?N?SmA triple point, exists. For ?shorter? nCBs it can be hidden in the negative pressures domain. The possibility of the extension of the ?melting curve? into the negative pressures region as well as the appearance of the ?melting inversion? at high enough pressures is indicated.