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

Abstract

High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-\AA{}-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density ${\ensuremath{\rho}}_{S}.$ The excess specific heat peaks associated with the nematic-isotropic $(N\ensuremath{-}I)$ and the nematic--smectic-A $(N\ensuremath{-}\mathrm{Sm}A)$ transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as ${\ensuremath{\rho}}_{S}$ is increased. Two distinct regimes of ${\ensuremath{\rho}}_{S}$-dependent behaviors are observed with a crossover between them at ${\ensuremath{\rho}}_{S}\ensuremath{\cong}0.1{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ For lower silica densities, sharp second-order ${C}_{p}$ peaks are observed at the $N\ensuremath{-}\mathrm{Sm}A$ transitions, characterized by effective critical exponents that decrease monotonically with ${\ensuremath{\rho}}_{S}$ 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 $N\ensuremath{-}I$ transition. For higher silica densities, both the $N\ensuremath{-}\mathrm{Sm}A$ and the $N\ensuremath{-}I$ 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.