Calorimetric study of nematic–smectic-Atricritical behavior in mixtures of heptyloxypentylphenylthiolbenzoate and octyloxycyanobiphenyl

The critical heat capacity associated with the nematic (N) to smectic-A (SmA) phase transition has been studied in mixtures of nonpolar heptyloxypentylphenylthiolbenzoate (7¯S5) and polar octyloxycyanobiphenyl (8OCB). In this system, which exhibits both an N-SmA-SmC multicritical point and an N-SmA tricritical point, the N-SmA transition temperature $T_{\mathrm{NA}}$ 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-SmA transition is second order, and for three compositions exhibiting weakly-first-order transitions. Because of the large values of ${\mathrm{dT}}_{\mathrm{NA}}$/dX, Fisher renormalization was observed. The finite cusp in $C_p$ near the tricritical point yields the renormalized critical exponent ${\alpha }_R$=-0.93±0.17. Since ${\alpha }_R$=-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.