Reentrant transition enthalpies of liquid crystals: The frustrated spin-gas model and experiments

Specific-heat--versus--temperature curves are calculated across the reentrant (N-Ad-N-A₁) phase diagram of liquid crystals, using the microscopic ``spin-gas'' model. The transition enthalpies are found to be much weaker on each side of the partial bilayer smectic phase (Ad) than at the onset of the monolayer smectic phase (A₁). This qualitative difference, which agrees with experimental results obtained on octyloxybenzoyloxycyanostilbene (T8), is readily explained by the dipolar frustration reentrance mechanism of our model.