Macroscopic Description of Compensated Cholesteric and Chiral Smectic Liquid Crystals

We present a macroscopic description of compensated cholesterics and chiral smectics, i.e., for systems in which the pitch of the helix goes to infinity on a surface in phase space spanned by pressure, temperature and concentration. The macroscopic consequences of this phenomenon in the vicinity of this surface are investigated and an explanation of the experiments by Eber and Janossy for compensated cholesterics and by Beresnev et al. for compensated smectic C* is suggested. An analogy to singular non-Boussinesq effects in binary fluid mixtures is pointed out.