High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Å-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density ${\rho }_S.$ The excess specific heat peaks associated with the nematic-isotropic $\mathrm{}(N-I)\mathrm{}$ and the nematic–smectic-A $(N-\mathrm{Sm}A)$ transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as ${\rho }_S$ is increased. Two distinct regimes of ${\rho }_S$-dependent behaviors are observed with a crossover between them at ${\rho }_S\cong 0.1{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{-}3}.$ For lower silica densities, sharp second-order $C_p$ peaks are observed at the $N-\mathrm{Sm}A$ transitions, characterized by effective critical exponents that decrease monotonically with ${\rho }_S$ from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order $C_p$ features are observed at the $N-I$ transition. For higher silica densities, both the $N-\mathrm{Sm}A$ and the $N-I$ transitions exhibit a single rounded $C_p$ peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CB+aerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silica densities above and below the crossover density.

• Received 13 May 1998

DOI:https://doi.org/10.1103/PhysRevE.58.5966