Kinetic phase diagram for two-step nucleation in colloid-polymer mixtures

Area of Science:

• Physical Chemistry
• Materials Science
• Soft Matter Physics

Background:

• Two-step crystallization, involving a metastable intermediate phase, is often considered outside classical nucleation theory (CNT).
• Understanding non-classical nucleation mechanisms is crucial for controlling material properties and synthesis.
• Colloid-polymer mixtures provide a model system for studying complex phase transitions.

Purpose of the Study:

• To investigate two-step crystallization in colloid-polymer mixtures using an intermediate liquid phase.
• To develop and validate a kinetic phase diagram based on CNT for predicting nucleation pathways.
• To differentiate between classical and non-classical crystallization mechanisms within a unified theoretical framework.

Main Methods:

• Utilized classical nucleation theory (CNT)-based seeding simulations to construct a kinetic phase diagram.
• Identified regions where the critical nucleus is either liquid or crystalline.
• Validated predictions using transition path sampling simulations across nine distinct state points.

Main Results:

• The kinetic phase diagram successfully predicted the nature of the critical nucleus (liquid or crystalline).
• When the critical nucleus is liquid, crystallization occurs stochastically during the growth phase.
• When the critical nucleus is crystalline, crystallization occurs pre-critically at a fixed nucleus size.

Conclusions:

• Classical nucleation theory-based kinetic phase diagrams are effective for understanding and predicting non-classical crystal nucleation.
• The study demonstrates that seemingly non-classical processes can be rationalized within an extended CNT framework.
• This approach offers a powerful tool for controlling crystallization pathways in complex fluid systems.