Behaviour of the pitch of the cholesteric and chiral smectic C helix near and at the cholesteric-smectic A-chiral smectic C multicritical point

A cholesteric-smectic A-chiral smectic C multicritical point was established in binary mixtures of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate with cholesteryl myristate or cholesteryl benzoate. At this point the cholesteric phase, smectic A phase, and chiral smectic C phase become indistinguishable. Whereas the pitch of the cholesteric helix at the S_A-Ch phase transition is infinite already in the vicinity of the multicritical point, the pitch of the cholesteric helix at the S*_C-Ch transition becomes infinite only at this point. In accord with the theory of Beresnev the pitch of the chiral smectic C helix remains finite at the multicritical point. Additional high pressure experiments support the results obtained at atmospheric pressure.