A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of aerosils in the liquid crystal $4\text{−}n$-pentylphenylthiol-$4^{\prime }\text{−}n$-octyloxybenzoate $\left(\overline{8}\mathrm{S}5\right)$ as a function of aerosil concentration and temperature spanning the smectic-$C$ to nematic phases. Over this temperature range, this liquid crystal possesses two continuous $XY$ phase transitions: a fluctuation-dominated nematic to smectic-$A$ transition with $\alpha \approx {\alpha }_{XY}=-0.013$ and a mean-field smectic-$A$ to smectic-$C$ transition. The effective critical character of the $N\text{−}\mathrm{Sm}A$ transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the $N\text{−}\mathrm{Sm}A$ transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the $\mathrm{Sm}A\text{−}\mathrm{Sm}C$ transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the $\mathrm{Sm}A\text{−}\mathrm{Sm}C$ transition scales as ${\rho }_S^{-0.5}$ with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus $B$) with silica density.

• Received 10 May 2005

DOI:https://doi.org/10.1103/PhysRevE.72.051716