# On the colloidal stability of apolar nanoparticles: The role of ligand length

## TL;DR

This study investigates how ligand length influences the colloidal stability of apolar nanoparticles, revealing an unexpected inversion in stability regimes linked to core size and ligand interactions, challenging classical colloid theories.

## Contribution

It provides a microscopic understanding of ligand length effects on nanoparticle stability, combining experimental and simulation approaches to explain stability inversion phenomena.

## Key findings

- Longer ligands increase stability in core-dominated regime
- Shorter ligands favor stability in ligand-dominated regime
- Classical colloid theory does not fully explain observed behaviors

## Abstract

Inorganic nanoparticle cores are often coated with organic ligands to render them dispersible in apolar solvents. However, the effect of the ligand shell on the colloidal stability of the overall hybrid particle is not fully understood. In particular, it is not known how the length of an apolar alkyl ligand chain affects the stability of a nanoparticle dispersion against agglomeration. Here, Small-Angle X-ray Scattering and molecular dynamics simulations have been used to study the interactions between gold nanoparticles and between cadmium selenide nanoparticles passivated by alkanethiol ligands with 12 to 18 carbons in the solvent decane. We find that increasing the ligand length increases colloidal stability in the core-dominated regime but decreases it in the ligand-dominated regime. This unexpected inversion is connected to the transition from ligand- to core-dominated agglomeration when the core diameter increases at constant ligand length. Our results provide a microscopic picture of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.