When Like Destabilizes Like: Inverted Solvent Effects in Apolar Nanoparticle Dispersions

Area of Science:

• Colloid and Surface Science
• Materials Chemistry
• Computational Nanoscience

Background:

• Understanding nanoparticle colloidal stability is crucial for their application in various fields.
• Alkanethiol-capped nanoparticles are widely studied but their behavior in apolar solvents remains complex.
• Classical colloid theory often fails to predict stability trends in these systems.

Purpose of the Study:

• To investigate the colloidal stability of alkanethiol-coated nanoparticles in a series of linear alkane solvents.
• To elucidate the driving forces behind nanoparticle agglomeration and the influence of solvent properties.
• To explain the unexpected correlation between solvent chain length and nanoparticle stability.

Main Methods:

• Small-angle X-ray scattering (SAXS) to characterize nanoparticle interactions and aggregation.
• Molecular dynamics (MD) simulations to model nanoparticle-solvent interactions at the molecular level.
• Varying nanoparticle core materials (gold, cadmium selenide) and shell ligands (hexadecanethiol, octadecanethiol) with different alkane solvents (hexane to hexadecane).

Main Results:

• Nanoparticle agglomeration is primarily enthalpically driven.
• The temperature of agglomeration increases with increasing solvent chain length, contradicting classical colloid theory.
• This inverted trend correlates with the ordering of alkanethiol ligands on the nanoparticle surface.
• Both enthalpic and entropic contributions stabilize the ordered ligand state as solvent chain length increases.

Conclusions:

• The colloidal stability of alkanethiol-capped nanoparticles in apolar solvents is governed by ligand ordering and solvent-mediated interactions.
• Classical colloid theory requires modification to account for the observed inverse relationship between solvent chain length and nanoparticle stability.
• The findings provide insights into designing stable nanoparticle dispersions for advanced material applications.