A Critical Re-examination of Constrained Equilibrium Hypothesis in Classical Nucleation Theory

The first order phase transition, the nucleation process, of a thermodynamic system is one of the basic physical phenomena and it has significant relevance on several scientific fields. Despite the importance of the nucleation process, the theoretical understanding is still imperfect. The emergence of a new phase, liquid or solid cluster, in the metastable gas phase is mainly treated with classical nucleation theory (CNT) by using known macroscopic thermodynamic properties of the studied substance, but the theory often fails in predicting the nucleation process adequately. The failure of describing the nucleation event by CNT has shifted the theoretical focus on molecular-level nucleation studies to improve the prediction and understanding of the origin of the failure. This thesis examines one of the key assumptions behind CNT, the constrained equilibrium hypothesis, by approaching it from statistical mechanics and thermodynamic point of view. The main tools in this work are computational: both Monte Carlo (MC) and molecular dynamics (MD) simulations have been used to simulate the homogeneous nucleation processes of Lennard-Jones argon. Two separate studies are presented: At first we compare the nucleation rates obtained by MC (based on thermodynamic equilibrium) and molecular dynamics simulations using the nonisothermal nucleation theory and then the constrained equilibrium hypothesis is invalidated by studying the kinetics of Lennad-Jones argon clusters from size of 4 up to 31 molecules at 50 K. In addition to the actual study, the thesis includes a systematic overview of the theoretical treatment of homogeneous nucleation from thermodynamic liquid drop model to applicable molecular-level simulation techniques.