Initial Aggregation Process of Thickener in Lithium Soap Greases: All-Atom Molecular Dynamics Simulation

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Grease is an organogel formed by self-assembling low molecular mass gelators.
• Lithium soaps, such as lithium stearate (LiS) and lithium 12-hydroxystearate (Li12HS), are key components in grease formulations.
• Understanding their aggregation behavior is crucial for grease performance.

Purpose of the Study:

• To investigate the aggregation process of lithium stearate (LiS) and lithium 12-hydroxystearate (Li12HS) in hexane using molecular dynamics simulations.
• To elucidate the role of the hydroxy group in Li12HS on aggregation pathways and final structures.
• To explore the impact of temperature on the aggregation dynamics and structural transitions.

Main Methods:

• All-atom molecular dynamics simulations were employed.
• Simulations were conducted using LiS and Li12HS in a hexane solvent.
• Analysis focused on aggregation structures, bonding interactions, and temperature-dependent effects.

Main Results:

• LiS formed plate-like reverse micelles, consistent with experimental observations.
• Li12HS formed a metastable network structure at room temperature due to hydroxy group interactions.
• Specific hydroxy group bonds hindered crystal formation, while inter-hydroxy bonds promoted it.
• High temperatures and thermal cycling facilitated the transition from network to crystal-like structures.

Conclusions:

• The hydroxy group in Li12HS significantly influences aggregation, leading to network formation.
• Thermal cycling plays a critical role in altering aggregation pathways and promoting crystal-like structures.
• These findings offer molecular-level insights into lithium soap aggregation and grease rheology.