Molecular tail lengths, dipole pairings, and multiple reentrance mechanisms of liquid crystals

Employing the “spin-gas” model which incorporates the inherent microscopic frustration of dipoles, observed sequences of reentrant nematic and smectic phases are reproduced. In particular, the quadruple reentrance (nematic ↔ smecticAd ↔ nematic ↔ smecticAd ↔ nematic ↔ smecticA1) is explained. The molecular tail length theoretically required for this phenomenon agrees with experiment. In the theory, associations if correlated triplets of molecules propagate smectic order. By contrast, dipole pairings giving rise to dimers of molecules frustrate smectic order and favor the nematic phase. Calculated dimer concentrations are insensitive to N ↔ Ad transitions, consistently with dielectric measurements, whereas dimers break up at the N ↔ A1 transition, causing the large transition enthalpy.