Equilibrium Relations in the Case of Liquid Crystals.—Since, now, we have seen that we are dealing here with substances in two crystalline forms (which we may call the solid and liquid[[96]] crystalline form), which possess a definite transition point, at which, transformation of the one form into the other occurs in both directions, we can represent the conditions of equilibrium by a diagram in all respects similar to that employed in the case of other enantiotropic substances, e.g. sulphur (p. [35]).
In Fig. 12 there is given a diagrammatic representation of the relationships found in the case of p-azoxyanisole.[[97]]
Although the vapour pressure of the substance in the solid, or liquid state, has not been determined, it will be understood from what we have already learned, that the curves AO, OB, BC, representing the vapour pressure of solid crystals, liquid crystals, isotropic liquid, must have the relative positions shown in the diagram. Point O, the transition point of the solid into the liquid crystals, lies at 118.27°, and the change of the transition point with the pressure is +0.032° pro 1 atm. The transition curve OE slopes, therefore, slightly to the right. The point B, the melting point of the liquid crystals, lies at 135.85°, and the melting point is raised 0.0485° pro 1 atm. The curve BD, therefore, also slopes to the right, and more so than the transition curve. In this respect azoxyanisole is different from sulphur.
The areas bounded by the curves represent the conditions for the stable existence of the four single phases, solid crystals, liquid crystals, isotropic liquid and vapour.
The most important substances hitherto found to form liquid crystals are[[98]]:—
| Substance. | Transition point. | Melting point. |
| Cholesteryl benzoate | 145.5° | 178.5° |
| Azoxyanisole | 118.3° | 135.9° |
| Azoxyphenetole | 134.5° | 168.1° |
| Condensation product from benzaldehyde and benzidine | 234° | 260° |
| Azine of p-oxyethylbenzaldehyde | 172° | 196° |
| Condensation product from p-tolylaldehyde and benzidine | 231° | — |
| p-Methoxycinnamic acid | 169° | 185° |