Effect of silica aerosil particles on liquid-crystal phase transitions

High-resolution ac calorimetric studies show that the dispersion of 70-Å-diameter hydrophilic silica spheres (aerosils) has a substantial effect on several liquid-crystal transitions in butyloxybenzlidene octylaniline (4O.8). The weakly first-order nematic $\mathrm{}\left(N\right)\mathrm{}$–isotropic $\mathrm{}\left(I\right),$ the second-order nematic $\mathrm{}\left(N\right)\mathrm{}$–smectic-$A$ $\left(\mathrm{Sm}A\right),$ and the strongly first-order smectic-$A\left(\mathrm{Sm}A\right)$–crystal-$B\left(\mathrm{Cr}B\right)$ 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 $N–\mathrm{Sm}A$ critical exponent α from $\alpha =0.135\mathrm{}$ in bulk 4O.8 toward $\alpha \approx {\alpha }_{\mathrm{XY}}=-0.007$ in $4\mathrm{O}.8+\mathrm{aerosil}$ suspensions with silica densities ${\rho }_s\approx 0.08{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{-}3}.$ For ${\rho }_s>~0.11{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{-}3},$ the $N–\mathrm{Sm}A$ $\Delta C_p$ peaks are rounded in a manner qualitatively like those for 4O.8 confined in a high-porosity aerogel, one of which was also studied.