Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
John M. Eggebrecht
Maurice A. Larson
Thermodynamic and kinetic studies of the vapor/liquid phase transition of a Lennard-Jones model fluid are presented which challenge the conventional interpretation of the mechanism of the condensation process. We have lifted the usual approximations (incompressible phases, constant surface tension, and non-depleting vapor phase) applied in classical nucleation theory to investigate the nature of the barrier to condensation. Based on the 1[superscript]st Yvon-Born-Green integro-differential equation we have developed a thermodynamically consistent molecular theory which accurately predicts the radially dependent surface tension and the location of the surface of tension of microscopically small droplets. The droplet size dependence of the inter-facial free energy is sufficiently strong that the free energy barrier to the nucleation is absent;Computer simulations of ionic and neutral fluids have been performed to study the dynamical behavior of the fluid during the phase separation. We find that phase transitions in the metastable region for both systems are characterized by the instantaneous formation of concentration fluctuations. In the vapor phase of the Lennard-Jones fluid nearly spherical, disjoint, high density regions are formed, whereas in the ionic vapor a network of charged chains is observed. The connectivity between the clusters and their linearity diminishes with charge asymmetry. The short induction time where small clusters are spontaneously formed is followed by two rate determining regimes. First, the clusters absorb surrounding atoms and smaller clusters. During this regime, the evolution of the number of atoms in the cluster is linear in time and can be described by a modified Lifshitz-Slyozov theory. Second, the clusters undergo Brownian motion and further growth is mainly driven by coalescence. The Brownian character of this motion is due to unsymmetric internal motion near the surface. This is in contrast to the usual interpretation of the origin of Brownian motion as environmental noise;These results support our earlier conclusions that the free energy of formation of a spherical droplet is irrelevant to a description of vapor condensation, but require an alternative mechanism for the persistence of metastability. We have constructed a new model for the vapor/liquid phase transition, which regards the phase separation as a cascade of Brownian walkers whose mass grows linearly in time. The nucleation rate is then simply determined by the sum over the first passage times required for the binary coalescence event. These intervals scale like time to the 11/6 and supersaturation to the -2/3 providing an explanation of the sensitivity of nucleation rate to supersaturation.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Guenther Herbert Peters
Peters, Guenther Herbert, "A contribution to the understanding of crystallization " (1991). Retrospective Theses and Dissertations. 9675.