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 ${\mathrm{cm}}^{\mathrm{-}3}$). 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 Δ${\mathit{C}}_{\mathit{p}}$(N-I) and Δ${\mathit{C}}_{\mathit{p}}$(N–Sm-A) are observed to undergo distinctly different changes as a function of aerogel density. The changes in peak height h≡Δ${\mathit{C}}_{\mathit{p}}$(max) and peak position ${\mathit{T}}_{\mathrm{peak}}$ 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.

• Received 20 July 1994

DOI:https://doi.org/10.1103/PhysRevE.51.2157