Metastable systems in thermodynamics: Consequences, role of constraints

Metastable states are not in true equilibrium and so cannot be directly tre ated by thermodynamics and statistical thermodynamics. To circumvent this d ifficulty one can specify an equivalent equilibrated state, equivalent in t he sense that the observed macroscopic features are indistinguishable from those of the metastable state, and equilibrated because of the imposition o f auxiliary constraints which totally block the very slow relaxations that cannot be completed during the time of the experiment. This procedure permi ts one to treat metastable systems consistently within a completely time in dependent and causal thermodynamic framework. It also gives a consistent de scription of the entropy of glassy and similar random metastable systems in which the entropy vanishes as T-->0 K, and it explains the apparent residu al entropy at 0 K obtained in most conventional analyses based upon experim ents carried out over irreversible paths (but not always recognized as such ).