HEAT-CAPACITY STUDY OF NEMATIC-ISOTROPIC AND NEMATIC-SMECTIC-A TRANSITIONS FOR OCTYLCYANOBIPHENYL IN SILICA AEROGELS

Quenched randomness and finite size can both have substantial effects on critical behavior at phase transitions. A high-resolution ac calorimeter study has been carried out on octylcyanobiphenyl (8CB) in four silica aerogels of different porosities (mass densities ρ=0.08-0.60 g cm-3). The weakly-first-order nematic-isotropic (N-I) and second-order nematic-smectic-A (N-Sm-A) transitions in bulk liquid crystals belong to different universality classes and have been very well characterized in bulk 8CB. The excess heat capacity peaks ΔCp(N-I) and ΔCp(N-Sm-A) are observed to undergo distinctly different changes as a function of aerogel density. The changes in peak height hΔCp(max) and peak position Tpeak relative to the bulk values are not well represented by finite-size scaling for either transition, and the underlying influence of quenched randomness is discussed as the major cause of the observed effects. © 1995 The American Physical Society.