An ac calorimetric study of binary mixtures of octyloxycyanobiphenyl (80CB) and hexyloxycyanobiphenyl (6OCB) reveals that the magnitude of the $C_p$ peak associated with the nematic—smectic-$A$ (N-$\mathrm{Sm}A$) transition decreases strongly with increasing concentration of 6OCB. These data are consistent with a second-order N-$\mathrm{Sm}A$ transition for which the entropy vanishes with increasing 6OCB concentration. A reentrant mixture containing 27 wt.% 6OCB did not exhibit any critical heat capacity at either the N-$\mathrm{Sm}A$ transition or the $\mathrm{Sm}A$—reentrantnematic transition, both of which were observed visually.

• Received 25 February 1980

DOI:https://doi.org/10.1103/PhysRevB.22.2569