We have studied the effects of quenched random disorder created by dispersed aerosil nanoparticle gels on the nematic to smectic-$A$ (N-$\text{Sm}A$) and smectic-$A$ to reentrant nematic ($\text{Sm}A$-RN) phase transitions of thermotropic liquid-crystal mixtures of hexyloxycyanobiphenyl (6OCB) and octyloxycyanobiphenyl (8OCB). These effects are probed using high-resolution synchrotron x-ray diffraction techniques. We find that the reentrant characteristics of the system are largely unchanged by the presence of the aerosil gel network. By comparing measurements of the smectic static structure amplitude for this 8OCB-$\text{6OCB}+\text{aerosil}$ system with those for butyloxybenzilidene-octylaniline $\left(4\text{O}.8\right)+\text{aerosil}$ gels, we find that the short-range smectic order in the smectic-$A$ phase is significantly weaker in the reentrant system. This result is consistent with the behavior seen in pure 8OCB-6OCB mixtures. The strength of the smectic ordering decreases progressively as the 6OCB concentration is increased. Detailed line shape analysis shows that the high- and low-temperature nematic phases (N and RN) are similar to each other.

• Received 17 November 2007

DOI:https://doi.org/10.1103/PhysRevE.77.031702