Adiabatic Scanning Calorimetric Investigation of the Smectic-A to Hexatic-B Phase Transition in the Liquid Crystal 65OBC.

Adiabatic scanning calorimetry (ASC) was used to investigate the smectic-A to hexatic-B (SmA -- HexB) phase transition in the liquid crystal n-hexyl-4'-n-pentyloxybiphenyl-4-carboxilate (65OBC). The high- resolution ASC technique allows one to measure simultaneously the enthalpy and specific heat capacity as a function of temperature and separate the true latent heat at (weakly) first-order phase transitions from pretransitional enthalpy changes. We were able to prove in a direct way that the SmA -- HexB transition in 65OBC is very weakly first-order with a latent heat of 0.04 +/- 0.02 J/g. Our experiments also confirm the high value of 0.64 +/- 0.05 for the critical specific heat capacity exponent earlier reported in literature, suggesting closeness of the transition to a tricritical point.