Helix inversion in the chiral nematic and isotropic phases of a liquid crystal

Measurements of the chirality (2 pi/pitch) in the chiral nematic phase and of a structural constant proportional to the chirality in the isotropic liq uid for a system in which a helix inversion line crosses the chiral nematic to isotropic phase transition line are reported. While the chirality shows a strong temperature dependence in the chiral nematic phase, it loses all temperature dependence in the isotropic phase. In addition, the chirality i n the isotropic phase is proportional to the chirality in the chiral nemati c phase at the phase transition, and may in fact be continuous across the t ransition. While molecular field and phenomenological theories can explain the strong temperature dependence in the chiral nematic phase, including th e helix inversion, these theories predict a strong discontinuity in the chi rality at the phase transition that is not supported by experiment. So whil e a theory that includes short range molecular correlations is called for t o understand the behavior of the chirality across the phase transition, the oretical attempts to explain the chirality of a phase from a microscopic le vel must account for the strong role played by long range orientational ord er.