Calorimetric and small angle x-ray scattering study of phase transitions in octylcyanobiphenyl-aerosil dispersions

High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Angstrom-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 (N-1) and the nematic-smectic-A (N-SmA) 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)congruent to 0.1 g cm(-3). For lower silica densities, sharp second-order C-p peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with ps 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-SmA 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. [S1063-651X(98)11711-7].