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 crys tal phase transitions for several dispersions of 70-Angstrom-diam sili ca spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of sili ca density rho(S) The excess specific heat peaks associated with the n ematic-isotropic (N-1) and the nematic-smectic-A (N-SmA) transitions b oth exhibit shifts to lower temperatures, decreases in the specific he at 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 obs erved at the N-SmA transitions, characterized by effective critical ex ponents that decrease monotonically with ps from the pure 8CB value to ward the three-dimensional XY value, and two closely spaced but distin ct first-order C-p features are observed at the N-I transition. For hi gher silica densities, both the N-SmA and the N-I transitions exhibit a single rounded C-p peak, shifting in temperature and decreasing in t otal enthalpy in a manner similar to that observed in 8CB+aerogel syst ems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for ae rosil aggregates that differ for samples with silica densities above a nd below the crossover density. [S1063-651X(98)11711-7].