Heat-capacity study of nematic-isotropic and nematic-smectic-A transitions for octylcyanobiphenyl in silica aerogels.

Abstract

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 \ensuremath{\rho}=0.08--0.60 g ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). 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 \ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(N-I) and \ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(N--Sm-A) are observed to undergo distinctly different changes as a function of aerogel density. The changes in peak height h\ensuremath{\equiv}\ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(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.