Nucleational Energy Traps and Their Role in Crystal Structure Formation.

Abstract:

Crystallization has been one of the most important topics in a wide array of fields because of its importance to biology, atmospheric science, material science, etc. Unlike vapor-liquid nucleation, crystallization has an added level of complexity because the nucleation process can form anything from a perfectly ordered single crystal to an amorphous solid. 

When a nucleating cluster consists of a few dozen molecules, a large majority of the particles are exposed on the surface of the cluster unlike in the bulk phase where the majority of the particles are contained within the interior. As a consequence a cluster must attempt to minimize the surface area while simultaneously maximizing the intermolecular interactions within the cluster in order to maintain stability. The ideal bulk phase configuration is often less stable due to these factors and will not form directly from the gas/liquid phase. Instead the clusters can adopt many non-crystalline structures due to their incredibly high stability at these small cluster sizes. Once these non-crystalline clusters aggregate enough molecules they can attempt to transition from the non-crystalline structure into an ordered crystalline structure; however, if there is a sizable free energy barrier for this crystalline transition the cluster can become trapped in these states and subsequently the formation of an ordered crystal will be unfeasible. This of course can yield physical properties that are significantly different from that of the bulk.

This talk will discuss the simulational study of the underlying nucleation mechanics and their role in the formation of atmospheric ice. The focus will be on the formation of energy traps at small cluster sizes and how these traps are avoided by reducing the conformational degrees of freedom.