How Crystals Nucleate and Grow in Aqueous NaCl Solution

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Understanding crystal nucleation and growth is crucial for various chemical and materials processes.
• Classical nucleation theory provides a framework but may not capture all microscopic details.

Purpose of the Study:

• To investigate the microscopic mechanism of sodium chloride (NaCl) crystal nucleation and growth in a supersaturated aqueous solution.
• To compare the observed process with Ostwald's rule of stages and classical nucleation theory.

Main Methods:

• Large-scale molecular dynamics simulations utilizing 64,000 particles.
• Simulations conducted at 300 K and 1 atm in a slightly supersaturated NaCl solution.
• Observation and analysis of early-stage nucleation and subsequent crystal growth over approximately 140 nanoseconds.

Main Results:

• Observed early-stage nucleation within regions of locally high salt concentration.
• Characterized the initial nucleus as a loosely ordered ion arrangement with significant water content.
• Documented the slow removal of residual water during crystal growth towards an anhydrous state.
• Demonstrated that Ostwald's rule of stages better describes the nucleation and growth process than classical nucleation theory.

Conclusions:

• The microscopic mechanism of NaCl crystallization from a supersaturated solution is complex and involves a loosely ordered, hydrated intermediate.
• Ostwald's rule of stages offers a more accurate description of this process compared to classical nucleation theory.
• The findings provide insights into the fundamental steps of crystallization relevant to solution chemistry and materials science.