Calorimetric Study of Nematic-Semctic-A Tricritical Behavior in Mixtures of Heptyloxypentylphenylthiolbenzoate and Octyloxycyanobiphenyl

The critical heat capacity associated with the nematic ( N ) to smectic- A  (Sm A ) phase transition has been studied in mixtures of nonpolar heptyloxypentylphenylthiolbenzoate ( 7¯ S5) and polar octyloxycyanobiphenyl (8OCB). In this system, which exhibits both an  N -Sm A -Sm C  multicritical point and an  N -Sm A  tricritical point, the  N -Sm A  transition temperature  T NA  increases very rapidly as the 8OCB mole fraction  X  is increased. Data were obtained for a sample near the tricritical composition, for two compositions where the  N -Sm A  transition is second order, and for three compositions exhibiting weakly-first-order transitions. Because of the large values of  dT NA /dX, Fisher renormalization was observed. The finite cusp in  C p  near the tricritical point yields the renormalized critical exponent  α R =-0.93±0.17. Since  α R =-α/(1-α), this corresponds to  α=0.48±0.05 , in good agreement with the value 0.5 expected at a Gaussian tricritical point.