Phase equilibria and nucleation in condensed phases: A statistical mechanical study.

摘要

Nucleation occurring in condensed phases plays an important role in many industrial applications, however, such nucleation phenomena are not well understood. Despite its prominent use to correlate experimental data, the conventional nucleation theory cannot provide any insight into the nucleation mechanism. Naturally, statistical mechanical approaches such as density functional theory and computer simulation are expected to play an important role in advancing the molecular level understanding of these phenomena.; As a part of this effort, we investigated the mechanism of bubble nucleation in a micellar solution when it is subjected to a negative pressure. We employed a density functional model of a non-ionic surfactant solution, in which the solvent was represented by a single hard-core sphere and the surfactant was represented by two tangent hard-core spheres connected by a rigid bond. The attractive interactions between the particles were modeled by a simple 1/ R6 form, where R is the interparticle distance. We found that the presence of the surfactant molecules lowers the free energy barrier of bubble nucleation. At moderate negative pressures, the stable micelle may evolve to form the critical nucleus and the resulting free energy barrier is expected to be lower than that in the absence of this mechanism. However, kinetic considerations indicated that at lower pressures close to spinodal, the mechanism of micelle-assisted bubble nucleation becomes less effective. The liquid-liquid miscibility of the model system was found to correlate well with the mechanism of bubble nucleation from the stable micelle.; Another important application of nucleation phenomenon is in the crystallization process, in which an understanding of the nucleation mechanism is important to control the composition and the structure of the crystalline products. A computer simulation study of crystal nucleation involving complex molecules of interest is prohibitively expensive and therefore we aimed at understanding the crystal nucleation behavior of binary Lennard-Jones mixtures.; Some earlier molecular level studies indicate that phase behavior strongly affects the nucleation mechanism. In particular, intermediate metastable phases are often seen to interfere with the nucleation process. Thus the calculation of the phase diagrams including the metastable phases becomes crucial to understand crystal nucleation behavior.