Multiple pathways in NaCl homogeneous crystal nucleation.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• The established view of sodium chloride (NaCl) crystal nucleation from metastable solutions assumes a single-step mechanism with simultaneous growth and ordering.
• Recent experimental findings suggest complex ion-ion correlations and the emergence of NaCl crystals from disordered intermediates, contradicting the traditional model.

Purpose of the Study:

• To investigate and characterize NaCl crystallization pathways from metastable solutions using molecular dynamics simulations.
• To analyze the mechanisms and timescales of nucleation, particularly focusing on deviations from the single-step model.

Main Methods:

• Employed biased and unbiased molecular dynamics simulations to explore crystallization pathways.
• Utilized a Markov state model to analyze nucleation mechanisms and timescales from unbiased trajectories.
• Calculated committor probabilities and estimated nucleation rates.

Main Results:

• Observed the emergence of large, liquid-like NaCl clusters as solution concentration increases.
• Identified a dominant two-step nucleation pathway where crystalline order develops within dense liquid regions, contrasting with the one-step model.
• Characterized these clusters as transient, differing from stable prenucleation species found in other systems.

Conclusions:

• The 'critical' nucleus concept should encompass an ensemble of clusters with diverse structures, including disordered domains.
• Alternative crystallization pathways beyond the single-step mechanism exist and can be influenced by factors like carbon surfaces.
• Findings necessitate a revised understanding of NaCl nucleation, incorporating multi-step processes and disordered intermediates.