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 ρS. The excess specific heat peaks associated with the nematic-isotropic (N-I) and the nematic endash 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 ρS is increased. Two distinct regimes of ρS-dependent behaviors are observed with a crossover between them at ρS congruent 0.1thinspgthinspcm-3. For lower silica densities, sharp second-order Cp peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with ρS from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order Cp 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 Cp 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. copyright 1998 The American Physical Society