Critical heat capacity near the nematic—smectic-Atransition in octyloxycyanobiphenyl in the range 1-2000 bar

An ac calorimetric technique has been used to study the nematic—smectic-$A$ ($N-\mathrm{Sm}A$) and nematic-isotropic ($N-I$) transitions in octyloxycyanobiphenyl (8OCB) along a series of isobars between 1 atm and 2 kbar. The excess heat capacity $\Delta C_p\mathrm{}\left(\mathrm{NA}\right)\mathrm{}$ associated with the essentially second-order $N-\mathrm{Sm}A$ transition is found to diminish rapidly with pressure and is not observed for $p>\mathrm{}1.5\mathrm{}$ kbar. The variation in $\Delta C_p\mathrm{}\left(\mathrm{NA}\right)\mathrm{}$ with reduced temperature is not consistent with a logarithmic singularity but can be characterized by a critical exponent $\alpha ={\alpha }^{\prime }=0.25\mathrm{}\pm 0.05$ for all the pressures studied. The excess heat capacity at the $N-I$ transition is almost independent of pressure; these data suggest but do not clearly establish quasitricritical behavior. Details of the experimental method and the fitting procedures are given.