Studies of critical behaviour at the nematic to smectic A phase transition in C6 and C7 binary mixture system

Abstract

The nematic (N) to smectic A (SmA) phase transitions in binary mixtures of the 6th (D6AOB) and 7th (D7AOB) members of alkylazoxybenzene series have been studied using high-precision calorimetry at 10, 20, 30, 50, 75 and 100 wt% of D7AOB concentrations. The temperature range of the nematic phase on six binary mixtures was wide, and it was dependent on the concentration of D7AOB and the ratio of transition temperatures T-NA/T-NI changes from 0.889 for 10 wt% D7AOB to 0.951 for 100 wt% D7AOB. The ratio of specific-heat amplitudes above and below the nematic to smectic (NA) transition, A(+)/A(-), remains similar to 1.0. The value of specific-heat exponent, a, changes from 0.1 to 0.4, implying variations from three-dimensional Ising type behaviour to mean field tricritical type behaviour, respectively, when the concentration of D7AOB increased from 10 wt% to 100 wt%.