Multistep nucleation compatible with a single energy barrier: catching the non-classical culprit

Area of Science:

• * Materials Science
• * Chemical Engineering
• * Physical Chemistry

Background:

• * Understanding the fundamental mechanisms of crystal nucleation is crucial for controlling material properties.
• * Classical nucleation theory (CNT) and non-classical pathways, such as prenucleation clusters, offer different perspectives on nucleation.
• * Reconciling these views within a single theoretical framework remains a challenge.

Purpose of the Study:

• * To develop a cohesive model integrating classical and non-classical nucleation views for strontium sulfate (SrSO₄) precipitation.
• * To explain the experimentally observed dual nucleation mechanisms (driven by neutral species vs. free ions).
• * To investigate the role of kinetics and thermodynamics in determining the nucleation pathway.

Main Methods:

• * Experimental co-titration of SrCl₂ and Na₂SO₄ at varying dosing rates.
• * Time-resolved measurements of turbidity, conductivity, and ion-specific data.
• * Development and application of a mesoscopic nucleation theory (MeNT) model incorporating thermodynamics and kinetics.

Main Results:

• * Slow dosing rates favor nucleation driven by neutral species (prenucleation clusters).
• * High dosing rates lead to nucleation dominated by free ion consumption (classical nucleation).
• * The MeNT model explains these transitions via an effective energy barrier with a kinetically influenced intermediate minimum.

Conclusions:

• * A single, unified model based on MeNT can capture both single-step (classical) and multi-step (non-classical) nucleation pathways.
• * The observed nucleation mechanisms are dependent on experimental conditions, specifically dosing rates.
• * The model highlights the interplay between thermodynamics and kinetics in determining nucleation pathways.