Calorimetric study of the smectic-Ad–smectic-A2critical point in a binary mixture of homologs

High-resolution calorimetry measurements have been performed in the vicinity of the smectic-${\mathit{A}}_{\mathit{d}}$–smectic-${\mathit{A}}_2$ (Sm${\mathit{A}}_{\mathit{d}}$-Sm${\mathit{A}}_2$) critical point in mixtures of 10OPCBOB and 11OPCBOB, where nOPCBOB is alkyloxyphenyl-cyanobenzyloxy benzoate. The variation of the heat capacity ${\mathit{C}}_{\mathit{p}}$ for three near-critical mixtures with 11OPCBOB mole % X=40.7, 42.2, and 43.5 is well described by a simple power law over a temperature range as large as ${\mathit{T}}_{\mathit{c}}$±10 K. The critical exponent γ/Δ has the value 0.45±0.08, which agrees with neither the mean-field nor the three-dimensional Ising model. This experimental exponent value supports the theoretical prediction that the Sm${\mathit{A}}_{\mathit{d}\mathrm{-}}$Sm${\mathit{A}}_2$ critical point belongs to a new universality class different from that of Ising systems. A significant distortion of the ${\mathit{C}}_{\mathit{p}}$(T) peak very close to its maximum was observed for all three samples. This is an intrinsic effect that is not presently well understood; it may represent a crossover behavior for samples with a concentration X slightly different from the critical value ${\mathit{X}}_{\mathit{c}}$.