Critical heat capacity near the nematic—smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>transition in octyloxycyanobiphenyl in the range 1-2000 bar

Abstract

An ac calorimetric technique has been used to study the nematic---smectic-$A$ ($N\ensuremath{-}\mathrm{Sm}A$) and nematic-isotropic ($N\ensuremath{-}I$) transitions in octyloxycyanobiphenyl (8OCB) along a series of isobars between 1 atm and 2 kbar. The excess heat capacity $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ associated with the essentially second-order $N\ensuremath{-}\mathrm{Sm}A$ transition is found to diminish rapidly with pressure and is not observed for $p&gt;1.5$ kbar. The variation in $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ with reduced temperature is not consistent with a logarithmic singularity but can be characterized by a critical exponent $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=0.25\ifmmode\pm\else\textpm\fi{}0.05$ for all the pressures studied. The excess heat capacity at the $N\ensuremath{-}I$ 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.