Calorimetric study of nematic to smectic-Atricritical behavior

Abstract

High-resolution heat capacity measurements have been made on six mixtures of 40.8 and 60.8, the butyl and hexyl homologs of alkoxybenzylidene octylaniline. The nematic (N)–smectic-A (Sm-A) transition becomes tricritical close to X=0.35, where X is the mole fraction of 60.8. The $C_p$ data for a mixture with X=0.35 are well described by the tricritical exponent α=0.5 and an amplitude ratio $A^{\mathrm{-}}$/$A^{+}$=1.6, which is not a universal quantity for tricritical measurements over the accessible range of reduced temperatures. The small value of $A^{\mathrm{-}}$/$A^{+}$ means that N–Sm-A tricriticality is further from classical Landau tricritical behavior than is the ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He system. Analysis of $C_p$ data over an extended range shows the importance of several correction terms to the leading singularity. In particular, there seems to be a small step discontinuity at $T_c$, which does not violate scaling at a tricritical point as it would for a second-order critical point.