How crystals form: A theory of nucleation pathways

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Classical nucleation theory often fails to capture complex, nonclassical pathways observed in systems like macromolecular solutions.
• Predicting nucleation phenomena requires accurate theoretical models that can handle diverse particle interactions and system dynamics.

Purpose of the Study:

• To develop a unified theoretical framework for nucleation, including crystallization, based on fundamental particle interactions.
• To predict nonclassical nucleation pathways without relying on predefined collective variables.
• To investigate diffusion-limited nucleation in macromolecular solutions, specifically for liquid-liquid separation and crystallization.

Main Methods:

• Integration of classical density functional theory with stochastic process theory and rare event techniques.
• Formulation of theory directly in terms of the density field, avoiding collective variables.
• Application to diffusion-limited nucleation of macromolecules in solution.

Main Results:

• The theory successfully predicts nonclassical nucleation pathways for both liquid-liquid separation and crystallization.
• Crystallization proceeds via a two-step mechanism: dense-solution droplet formation followed by core ordering.
• The free-energy surface exhibits shallow minima during ordering, linked to liquid-to-solid shell freezing.

Conclusions:

• The developed theory provides a robust method for predicting nucleation phenomena from first principles.
• The findings elucidate the mechanism of nonclassical crystallization and offer insights into the metastability of nanoscale clusters.