Kinetics of first-order phase transitions in adiabatic systems

Abstract

Based on thermodynamic investigations a general scenario and a kinetic description of the process of first-order phase transitions in adiabatically closed systems, starting from metastable initial states, is developed. It is shown that, in analogy to isothermal constraints, three main stages of the transition may be distinguished: a first stage of dominating nucleation and simultaneous growth of the already formed supercritical clusters, a second stage of independent growth of the clusters, their number being nearly constant, and a third stage of competitive growth, of Ostwald ripening. The change of the temperature of the system due to the latent heat of the transition can be considered hereby as an additional depletion effect. It leads to an increase of the critical size of the clusters and thus to a significant decrease of the nucleation rate, compared with isothermal conditions, especially for relatively large initial supersaturations. Further, it may result also in variations of the stable heterogeneous equilibrium state—that is, configurations of stable clusters in the otherwise homogeneous medium. In particular, for a one-component system under a constant external pressure it makes the existence of such a state possible and results therefore in a qualitative change of the whole course of the phase transition from independent nucleation and growth to the three-stage scenario as characterized above. A theoretical description of the independent growth of the drops and of Ostwald ripening under adiabatic conditions is developed. The results are compared with growth processes in isothermal systems and both quantitative and possible qualitative differences are discussed. Further, they are applied to an interpretation of molecular-dynamics simulations of first-order phase transitions in adsorbed layers.