Characterization of the smectic-A-hexatic-B transition in 650BC by adiabatic scanning calorimetry.

We investigated the smectic-A-hexatic-B (SmA-HexB) transition in the liquid-crystal n-hexyl-4'-n-pentyloxybiphenyl-4-carboxilate (650BC) with adiabatic scanning calorimetry. We were able to prove in a direct way that this transition is indeed very weakly first order, as was already suggested in the literature. The latent heat at the transition was determined to be deltaHL = 0.04 +/- 0.02 J/g. Our experiments confirm the high value for the heat capacity critical exponent earlier reported, yielding alpha = 0.64 +/- 0.05.