Determination of critical cholesteric pitch exponents in pure cholesteryl myristate and its mixtures with cholesteryl benzoate all showing a cholesteric/smectic-a tricritical point

Abstract

The liquid-crystal phase transition cholesteric (Ch) to smectic-A (S _A) can be associated with a rapid increase of the pitch of the cholesteric helix. This pretransition effect has been considered as a critical phenomenon, although no regard has been taken so far, as to whether the pitch really turns to infinity at the phase transition point Ch/S_A .

We now give the pressure-temperature-molar fraction conditions, where the pitch of Cholesteryl myristate and its mixtures with Cholesteryl benzoate diverge to infinity. This divergence of the pitch corresponds to a change of the phase transition Ch/S_A from first to second-order. As long as the transition is of first order the coexisting cholesteric phase has a helix with a finite pitch. Consequently critical pitch exponents can be only determined when the phase transition is of second order.

Key Words:

• Liquid crystals
• phase transition cholesteric/smectic-A
• tricritical point
• cholesteric pitch
• critical exponent
• crossover effect