Computer simulation of early-stage crystal growth and dissolution

The growth and dissolution of small crystals in a liquid are studied using non-equilibrium Molecular Dynamics techniques. Two systems were studied in detail: an atomic system (with particles interacting via the Lennard-Jones potential, and an aqueous system, in which the water interactions are simulated using the extended simple point charge (SPC/E) model.; For the Lennard-Jones system at a temperature 27% under the melting point at zero pressure, it is found that the critical cluster size for the crystal is 170 particles. Calculations of the probability for growth as a function of size show that there is a very narrow band of sizes for which crystals can either grow or dissolve. Rates of crystal growth as a function of crystal size are non-monotonic, suggesting the existence of particularly stable crystals. The crystallites adopt shapes in which the close-packed layers form the facets. The free energy of the crystals is smaller than predicted by the capillarity approximation using bulk properties for the liquid and the solid.; Simulations of ice and gas hydrate crystal dissolution at 273 K reveal large differences in the kinetics between these two cases. An attempt to grow hydrate at 220 K produced mixed results. Initially, growth is observed. After a molecular layer has grown over the seed crystal, growth stops. At 240 K the size of the hydrate crystals is stable. Calculation of the phase diagram for the SPC/E model of water shows that the model is unable to predict correctly the properties of ice. The solid which is produced upon freezing of the water model is a high density ice polymorph. The melting point of this crystal is 295 K. A new potential to model water interactions is necessary if ice or hydrates interfacial systems are to be studied.; The techniques developed to determine the crystal size and the location of the interface are presented. They are based on structural quantities; no dynamical information of the system is necessary. A new method based on thermodynamic integration, is developed and used to calculate the free energies of ice and hydrate crystals.