Calorimetric study of the smectic-Ad–smectic-A2critical point

High-resolution heat-capacity measurements are reported for the vicinity of the smectic-$A_d$–smectic-$A_2$ critical point in mixtures of undecyloxyphenyl-cyanobenzyloxy benzoate (11OPCBOB) and nonyloxybiphenyl cyanobenzoate (9OBCB). This type of critical point, which has similarities to the familiar liquid-gas (Ising) critical point, is predicted to belong to a new universality class. The variation of $C_{\mathrm{px}}$ in two near-critical mixtures with 11OPCBOB mole fractions X=0.655 and 0.641 can be characterized by the critical exponent γ/Δ=0.6±0.2. Previously reported smectic layer thicknesses d(T) for a mixture with X=0.642 vary like $t^{1\mathrm{-}(\gamma /\Delta )}$ with γ/Δ=0.62. Extensive rounding of the $C_{\mathrm{px}}$(T) peak near $T_c$, which complicates the critical exponent determination, appears to be an intrinsic effect that may be associated with the unusual x-ray intensity variation near $T_c$.