SMECTIC-C SMECTIC-I CRITICAL-POINT IN A LIQUID-CRYSTAL MIXTURE - STATIC AND DYNAMIC THERMAL-BEHAVIOR

The theoretically predicted smectic-C (Sm-C)-smectic-I (Sm-I, a tilted hexatic phase) critical point has been discovered in a racemic mixture of methylbutyl phenyl octylbiphenyl-carboxylate (8SI) and the octyloxybiphenyl analog (8OSI). High-resolution ac calorimetry and nonadiabatic scanning calorimetry show an evolution from a first-order Sm-C-Sm-I transition in 8SI to continuous supercritical behavior in 8OSI (no thermodynamic transition). The critical composition is Xc75, where X is the weight percent 8OSI. The static critical heat capacity Cp(T,X=75) is characterized by a critical exponent x=1.06±0.08 that corresponds to the exponent γ due to the path of approach to the critical point. This is a mean-field value of γ rather than the value associated with the new universality class that includes Sm-C-Sm-I, Sm-C*-Sm-C*, and Sm-Ad-Sm-A2 (partial bilayer-bilayer smectic) critical points. It is proposed that mean-field behavior is observed due to long bare correlation lengths (Ginzburg criterion). Note that Cp(T,X) measured along the path X=Xc is effectively a susceptibility, like Cp for a pure fluid near its liquid-gas critical point. Data obtained close to Tc show a systematic frequency dependence, and these Cp(T,ω) data are discussed in terms of critical slowing down and dynamic scaling behavior. © 1995 The American Physical Society.