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VV2.02 - An In Situ View of Direct and Two-Step Nucleation Dynamics 

In the classical picture of nucleation, density fluctuations that are inherent at finite temperature form unstable clusters of the new phase through monomer-by-monomer addition. Clusters transition from unstable to stable if they exceed a critical size, beyond which the free energy cost of creating the new phase boundary is compensated by the drop in chemical potential. In recent years, hierarchical pathways involving assembly of species more complex than monomers have been proposed for numerous systems. Amongst these pathways, a “two-step nucleation” process was proposed whereby macromolecular crystals nucleate within monomer-rich, non-crystalline clusters. However, reports of such pathways are almost entirely based on computational models or interpretations of indirect observations. Moreover, little is known about two-step nucleation dynamics, and whether the monomers in the clusters are one and the same as those that comprise the crystal nucleus or are act instead to provide an environment for heterogeneous nucleation is uncertain, as is the extent to which two-step pathways are general features of either macromolecular or inorganic materials. To address these knowledge gaps, we have used in situ TEM and AFM to investigate nucleation in numerous systems. To examine nucleation pathways of macromolecules, we synthesized a biomimetic polymer sequence that forms 2D ordered structures and used in situ AFM to observe nucleation. Our results show that the nucleation occurs along a two-step pathway that begins with creation of disordered clusters containing ~ 10-20 molecules. These clusters transform directly into ordered nuclei that grow via molecule-by-molecule addition, with the kinetics of transformation strongly dependent on Ca concentration. However, when a small aggregation-promoting hydrophobic region is deleted, even though the same final structure is obtained, nucleation occurs in a single step and the kinetics are dramatically altered. To investigate nucleation of simple inorganic materials, we used in situ TEM to observe nucleation of CaCO3. Formation pathways are confirmed in most cases by collecting diffraction information of the observed phases. We find that amorphous calcium carbonate (ACC), as well as the three predominant crystalline phases: calcite, vaterite, and aragonite, can form directly even under conditions in which ACC readily forms. In addition to these direct formation pathways, we observe two-step nucleation of aragonite and vaterite from ACC. Here, ACC transforms directly to the crystalline phases through distinct nucleation events on or just beneath the surface followed by consumption of the parent ACC particle. The results demonstrate that two-step pathways are possible in both inorganic and macromolecular systems, but are not universal. They can be accompanied by direct nucleation pathways and, in the case of macromolecules, their existence can depend on the specific sequence of the molecule. 

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