Phase behavior and blue-phase-III-isotropic critical point in (R)-(S) mixtures of a chiral liquid crystal with a direct twist-grain-boundary to blue-phase transition.

We have investigated mixtures of the (R) and (S) enantiomers of a chiral liquid crystal, (R)- or (S)-1-methylheptyl 3'-fluoro-4'-(3-fluoro-4-octadecyloxybenzoyloxy)tolane-4-carboxylate using high-resolution adiabatic scanning calorimetry. The pure (R) compound has a direct transition from the twist-grain-boundary to the blue phase without an intermediary chiral nematic phase. For the blue phases a different kind of phase behavior as a function of enantiomeric excess is observed, most probably related to the presence of a twist-grain-boundary-A instead of a chiral nematic phase below the blue phases. The general form of this phase diagram is compared with traditional blue-phase behavior. Furthermore a blue-phase-III-isotropic phase critical point, analoguous to that of a liquid-gas system, is observed, consistent with experimental and theoretical work recently published in this field. Finally, the effect of changing enantiomeric excess on the latent heats of the different first order phase transitions is measured and discussed.