A comprehensive modeling approach for polymorph selection in Lennard-Jones crystallization.

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Polymorph selection, predicting crystal structures during crystallization, is vital for manufacturing targeted solid properties.
• Understanding nucleation thermodynamics and kinetics is key to polymorph selection.
• Post-nucleation events like crystal growth and transformation also influence final crystal structures.

Purpose of the Study:

• To investigate the nucleation kinetics of the Lennard-Jones (LJ) fluid, focusing on the competition between FCC and HCP crystal structures.
• To determine when polymorph selection occurs during the crystallization process.
• To develop a comprehensive population balance modeling (PBM) approach for polymorph selection.

Main Methods:

• Employed molecular dynamics (MD) simulations to study LJ fluid nucleation.
• Utilized 2D free energy calculations to analyze nucleation pathways.
• Developed and applied a population balance modeling (PBM) approach incorporating nucleation and post-nucleation kinetics.

Main Results:

• Polymorph selection occurs after nucleation, when cluster sizes exceed the critical size, contradicting classical nucleation theory.
• A structure-dependent nucleation rate was accurately estimated using 2D free energy surfaces and MD-derived diffusion coefficients.
• The proposed PBM accurately predicted the polymorphic distribution in LJ system crystals generated via MD simulations.

Conclusions:

• The classical nucleation mechanism, assuming independent nucleation of ideal polymorphs, is insufficient for systems exhibiting non-classical nucleation.
• Post-nucleation kinetic events play a critical role in determining the final crystal structures, especially in non-classical nucleation scenarios.
• The developed PBM offers a robust framework for predicting polymorph selection by integrating nucleation and post-nucleation kinetics.