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 liquid 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 in the isotropic phase is proportional to the chirality in the chiral nematic phase at the phase transition, and may in fact be continuous across the transition. While molecular field and phenomenological theories can explain the strong temperature dependence in the chiral nematic phase, including the helix inversion, these theories predict a strong discontinuity in the chirality at the phase transition that is not supported by experiment. So while a theory that includes short range molecular correlations is called for to understand the behavior of the chirality across the phase transition, theoretical attempts to explain the chirality of a phase from a microscopic level must account for the strong role played by long range orientational order.