The van der Waals–Maxwell theory

Metastability is a relevant phenomenon for thermodynamic systems close to a first order phase transition. Examples are supercooled vapours and liquids, super-saturated vapours and solutions, as well as ferromagnets in the part of the hysteresis loop where the magnetization is opposite to the external magnetic field. A metastable state occurs when some thermodynamic parameter such as the temperature, pressure or magnetic field is changed from a value giving rise to a stable state with a unique phase, say X, to one for which at least part of the system should be in some new equilibrium phase Y. Then, in particular experimental situations, instead of undergoing the phase transition, the system goes over continuously into a ‘false’ equilibrium state with a unique phase X′, far from Y but actually close to the initial equilibrium phase X. It is this apparent equilibrium situation that is called a ‘metastable state’. Its properties are very similar to those of the stable equilibrium state; for example for a supersaturated vapour one can determine the pressure experimentally as a function of the temperature and the specific volume. We speak of the ‘metastable branch’ of the isothermal curve.

The distinguishing feature of metastability is that, eventually, either via an external perturbation or via a spontaneous fluctuation, a nucleus of the new phase appears, starting an irreversible process which leads to the stable equilibrium state Y, where the phase transition has taken place.