Effect of silica aerosil particles on liquid-crystal phase transitions

High-resolution ac calorimetric studies show that the dispersion of 70-{}-diameter hydrophilic silica spheres (aerosils) has a substantial effect on several liquid-crystal transitions in butyloxybenzlidene octylaniline (4O.8). The weakly first-order nematic $(N)$--isotropic $(I),$ the second-order nematic $(N)$--smectic-A (SmA), and the strongly first-order smectic-A(SmA)--crystal-B(CrB) freezing transition all exhibit shifted transition temperatures and substantial changes in the shape of excess heat capacity peaks. Power-law fits show an evolution of the N--SmA critical exponent {α} from α=0.135 in bulk 4O.8 toward α≈αXY=-0.007 in 4O.8+aerosil suspensions with silica densities ρₛ≈0.08gcm^-3. For ρₛ>~0.11gcm^-3, the N--SmA ΔCₚ peaks are rounded in a manner qualitatively like those for 4O.8 confined in a high-porosity aerogel, one of which was also studied.