  We have studied the effects of quenched random disorder created by dispersed
aerosil nanoparticle gels on the nematic to smectic-A (N-SmA) and smectic-A to
reentrant nematic (SmA- RN) phase transitions of thermotropic liquid crystal
mixtures of 6OCB (hexyloxycyanobiphenyl) and 8OCB (octyloxycyanobiphenyl).
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-6OCB+aerosil system with those for butyloxybenzilidene-octylaniline
(4O.8)+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.
