ACS PRF | ACS
All e-Annual Reports
41933-G9
Probing Thermodynamics and Kinetics of Crystallization with Simulation
Activities:
In the second year of this grant, in the technical aspect we extended the AVUS-HR (developed during the first year of this grant) to the study of crystal nucleation events in cluster systems. Also a new cluster-based simulation approach was formulated based on the AVUS-HR technique that allows efficient evaluation of the solvation effects that are critical to many chemical and biological phenomena (e.g., folding of bio-molecules).
Findings:
These technical extensions greatly expanded the application scope of the aggregation-volume-bias-based method and enable the study of crystal nucleation and the solvation-induced conformational transition events. In the application of this approach to the crystal nucleation of the Lennard-Jonesium clusters, we found that this process proceeds initially via a vapor-liquid-like aggregation followed by the formation of crystals inside the aggregates, even at deep undercooling conditions. Another surprising finding is that the classical nucleation theory, despite its simplicity, provides remarkably good predictions on the crystal nucleation barrier heights. However, it systematically overestimates the barrier for the initial aggregation process. In the solvation project, we applied this cluster-based approach to the study of the potentials of mean force between ions in a solvation environment. It was found that an attractive interaction between two anions or two cations can be induced by the addition of an appropriate number of water molecules. This result corroborates a recent experimental report of an intriguing folding of a dianionic polymer into a more compact structure with addition of water molecules in gas-phase as well as previous theoretical findings of possible attraction between like-ion pairs in bulk aqueous phases. In addition, we extended the application of the AVUS-HR approach for the first time to a ternary system water, n-nonane, and 1-butanol. This ternary mixture has been used as a model system for atmospheric nucleation by Viisanen and Strey in their nucleation pulse chamber experiment. Our simulations demonstrate a more complex nucleation mechanism than previously thought, where critical nuclei with different compositions are present even for a given vapor-phase composition.
Impact:
The nucleation method developed by the PI by combining aggregation-volume-bias Monte Carlo (AVBMC) with umbrella sampling has allowed simulations to study long time-scale events (such as those nucleation phenomena encountered in phase transitions) using realistic, atom-based force fields. The PI has continued along this method development and made several important improvements, including (i) the extension of this technique to the study of crystal nucleation events; and (ii) the conjunction of the AVUS-HR technique with a thermodynamic cycle to allow efficient evaluation of the solvation effects that are critical to many chemical and biological phenomena (e.g., folding of bio-molecules). Part of this work has been presented at several National/regional Meetings and as invited talks at several universities/institutions. In terms of human resource development, this projects outlined in this grant continue to attract aspired young students into my research group. Hyunmi Kim is a new group member and is actively involved in the crystal nucleation project. Two undergraduate students (Mike DeLee and Lawrence Tauzin) have joined our group, which provided the supervising opportunities for gradute students. All students are encouraged to travel to national and local conferences to present their research accomplishments.
