Evolution of the isotropic to nematic phase transition in binary mixtures of octylcyanobiphenyl and n-hexane.

High-resolution calorimetry has been performed to study the effect of dilution by a nonmesogenic, low-molecular weight solvent (linear n-hexane) on the isotropic-nematic (I-N) phase transition in the liquid crystal (LC) octylcyanobiphenyl (8CB) as a function of n-hexane concentration. Numerous temperature scans were performed without continuous mixing for pure 8CB and all mixture samples of n-hexane mole fraction ranging from xhex=0.02 to 0.12. The I-N specific heat peak remains first-order for all samples and shifts toward lower temperature nonlinearly while the two-phase I+N coexistence width broadens linearly with increasing xhex. Multiple heating and cooling scans are reproducible and indicate phase separation, if it occurs, must be on very short length scales and is reversible. These results may be a consequence of a competition between random dilution effects and the tendency to phase separate. It is shown that solvent dilution of a LC, if miscible and depending on solvent structure, can lead to a controlled altering of the intermolecular potentials and softening of the LC viscoelastic properties. [ABSTRACT FROM AUTHOR]