Effect of silica aerosils on the nematic to isotropic transition: A theoretical treatment.

The effect of silica aerosils on a first-order nematic-isotropic (NI) transition temperature shift DeltaT(NI)(rho(s)) and on the enthalpy jump DeltaH(rho(s)) is phenomenologically described in the framework of the Landau-de Gennes theory. It is shown that the aerosil network dispersed in the liquid crystal phase creates an additional curvature perturbation, resulting in linear and nonlinear slopes both in the transition temperature shift DeltaT(NI)(rho(s))=-T(NI)(0)alpharho(s) and in the enthalpy jump DeltaH(rho(s))=1 / 4DeltaH(0)(1+sqrt[1-kapparho(s)])(2)(1-alpharho(s)), where rho(s) is the aerosil density, and both alpha and kappa are constants of the theory. In the low rho(s)(</=0.2 g/cm(3)) regime, both the calculated and the measured values of DeltaH(rho(s)) for polar liquid crystals, such as 4-n-octyl-4(')-cyanobiphenyl (8CB) and 4-n-heptyl-4(')-cyanobiphenyl (7CB) show a good agreement between theory and experiment. The comparisons between the calculated and measured values of DeltaT(NI)(rho(s)), in the low-rho(s) regime, show a better agreement in the 7CB+aerosil than in the 8CB+aerosil systems.