Molecular-dynamics simulations of urea nucleation from aqueous solution

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Nucleation from solution is crucial across science and engineering but remains poorly understood at the molecular level.
• Molecular simulations offer a powerful approach to investigate nucleation phenomena.
• Urea nucleation from aqueous solution serves as a model system for studying solution-based phase transitions.

Purpose of the Study:

• To elucidate the molecular mechanisms of urea nucleation from aqueous solution.
• To compute the free-energy profile of urea nucleation using advanced simulation techniques.
• To compare simulation-derived nucleation pathways with classical nucleation theory.

Main Methods:

• Combined theoretical analysis with molecular simulations.
• Employed well-tempered metadynamics to calculate the free-energy landscape of nucleation.
• Analyzed simulation trajectories to identify key events and intermediates in the nucleation process.

Main Results:

• The free-energy profile of urea nucleation exhibits significant finite-size effects, which were successfully addressed.
• Nucleation is characterized by a predominant two-step process, initiated by large concentration fluctuations.
• Disordered urea clusters precede the formation of crystal-like nuclei, indicating a deviation from classical nucleation theory.
• Competition between two distinct polymorphs was observed during the early stages of nucleation.

Conclusions:

• Urea nucleation from aqueous solution follows a complex pathway involving pre-nucleation clusters and concentration fluctuations.
• Classical nucleation theory does not fully capture the observed nucleation mechanism for urea.
• Molecular simulations provide valuable insights into the intricacies of solution-based nucleation processes.