(5cj) Computational Studies on Nucleation and Growth of Nanoparticles, and on Solubilities and Relative Stability of Solid Polymorphs

Molecular simulations are performed to determine the nucleation and growth of Al nanoparticles from the vapor phase using analytic potential energy functions parametrized to reproduce accurate density functional energies for a range of nanocluster sizes and validated for bulk liquid/vapor equilibrium. Umbrella sampling potentials are utilized to sample a wide range of cluster sizes in a single simulation. At several temperatures above the bulk melting point, the standard state free energy changes relevant to the growth of nanoparticles (i.e., for Al(n)+Al = Al(n+1)) are calculated and the nucleation barrier is determined.

Additionally, several different Monte Carlo simulation techniques are employed to determine relative stability of different polymorphs of organic solids modeled using the Transferable Potentials for Phase Equilibria (TraPPE) force field. Thermodynamic integration along a pseudo-supercritical path yields an accurate normal melting point, as well as the correct stability order of two known low and high pressure polymorphs of benzene. Another technique of relevance for the pharmaceutical industry calculates the solubilities of different polymorphs in, primarily, aqueous solvents. The difference in solubilities gives an estimate of the relative stabilities. In accordance, solubilities of different polymorphs of benzene are studied in water and water/ethanol mixtures using expanded ensemble Monte Carlo simulations. The results obtained agree with the results obtained using thermodynamic integration.