On the colloidal stability of apolar nanoparticles: The role of particle size and ligand shell structure

TL;DR

This study investigates how particle size and ligand shell structure influence the colloidal stability of apolar nanoparticles, revealing that ligand ordering or core attraction dominate stability depending on particle size.

Contribution

It provides a microscopic understanding of nanoparticle interactions, showing the transition from ligand shell dominance to core attraction with increasing particle size.

Findings

Ligand shell ordering affects stability in smaller particles.

Core van der Waals forces dominate larger particle agglomeration.

Classical colloid theory does not account for these microscopic effects.

Abstract

Being able to predict and tune the colloidal stability of nanoparticles is essential for a wide range of applications, yet our ability to do so is currently poor due to a lack of understanding of how they interact with one another. Here, we show that the agglomeration of apolar particles is dominated by either the core or the ligand shell, depending on the particle size and materials. We do this by using Small-Angle X-ray Scattering and molecular dynamics simulations to characterize the interaction between hexadecanethiol passivated gold nanoparticles in decane solvent. For smaller particles, the agglomeration temperature and interparticle spacing are determined by ordering of the ligand shell into bundles of aligned ligands that attract one another and interlock. In contrast, the agglomeration of larger particles is driven by van der Waals attraction between the gold cores, which eventually becomes strong enough to compress the ligand shell. Our results provide a microscopic description of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.