5/17/2018 - They Call It “Free Energy” So, Hey, Why Pay?


Time: 3:30 PM - 4:30 PM
Location: Schlichter 3853

Presented By:

Jim De Yoreo, PNNL


Nucleation is the seminal process in the formation of minerals. Recent observations have revealed a rich set of hierarchical pathways of nucleation involving higher-order species ranging from multi-ion clusters to dense liquid droplets to transient amorphous or crystal-line phases. Despite their complexity, a holistic framework for understanding such path-ways based on classical concepts emerges when the effects of complexities in free energy landscapes and kinetic factors are considered. I illustrate that framework using in situ TEM and AFM studies on calcium carbonate, gibbsite and hematite. The results show that introduction of size-dependent phase stability or high driving force coupled with the exist-ence of metastable polymorphs leads to true two-step pathways characterized by the initial appearance of a bulk precursor. However, the introduction of surfaces that reduce the in-terfacial energy lead back to classical one-step pathways. On the other hand, introducing impurities that reduce molecular mobility can freeze in the metastable precursors for pure-ly kinetic purely reasons. Creation of micro-states representing local free energy minima likely stabilized by configurational factors also drives hierarchical pathways, but the in-termediates can only exist as transient microscopic entities. When high driving force cre-ates many nuclei, subsequent growth proceeds by interaction and coalescence of primary particles. Taken together, these findings provide a common basis for understanding the wide variety of nucleation pathways observed in mineral systems.

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