Enantiotropic hydrogens

A similar approach with 4-nitrophenol also generated mesomorphic systems. This time, the mesophases were slightly more stable and some phases appeared to be enantiotropic (i.e., they became more stable than their crystal phase) (69). However, the beauty of the hydrogen-bonding approach to liquid crystals is that it is not always necessary to synthesize complete moleciiles. Thus, it was realized that if the 4-substituted phenols lowered the anisotropy as indicated in Fig. 53, the anisotropy ought to recover to give much more stable systems if the 3-substituted systems were used (Fig. 54) (69). That this approach was viable was... 

Another empirical rule is the Heat of Fusion Rule, which states that if the higher melting form has a lower heat of fusion relative to the lower melting form, then the two forms bear an enantiotropic relationship. Less well obeyed is the Density Rule, which states that the most dense form will be the most stable at absolute zero. Strictly speaking, the Density Rule is only properly applied to polymorphs of molecular solids where intramolecular hydrogen bonding is not a significant factor. 

If the remaining hydrogen atom in the iV-methylamide group is also replaced by a second methyl group, the solvent dimethylformamide results, which does not possess any hydrogen bond donor atoms. In the contact region of this solvent with C50, only an enantiotropic N phase and a monotropic lamellar L phase appear (Fig. 5.13). 

Oligooxyethylene esters of this acid also display enantiotropic hexagonal columnar mesophases [312]. Brienne etal. [313] found hexagonal columnar phases in binary systems of non-mesogenic compounds, due to the formation of hydrogen bonded complexes. 

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