The Colloidal Stability of Apolar Nanoparticles in Complex Solvent Environments

Abstract

Solvent engineering is a powerful and versatile method to tune the colloidal stability and assembly behavior of ligand-coated nanoparticles. To achieve rational control over these properties, it is essential to understand how the structure and dynamics of surface ligands and their interaction with the surrounding solvent affect the stability of the nanoparticles. Motivated by recent experiments, we develop models for gold nanoparticles coated with apolar ligands dispersed in solvent mixtures, a weakly polar solvent, and solvents containing a small amount of additives. Using molecular dynamics simulations, we characterize the structure and energetics of the ligand and solvent in these diverse environments, and their effects on the interaction between the particles, and use this to explain the experimental observations.

Our findings highlight the pivotal role played by molecular intricacies in ligand-solvent

and solvent-solvent interactions. We find that particle agglomeration is dominated by

temperature-dependent ligand order in alkane solvent mixtures, with the temperature at which the ligand shell orders depending on the solvent composition near the ligands. This can differ substantially from the bulk composition, giving rise to nonlinear dependencies on solvent composition. We find that agglomeration in hexanol is instead driven by solvophobic effect with both energetic and entropic contributions that exceeds the influence of ligand order. Considering several thioether additives, we find that they do not influence the ordering of ligand when only present in solution. Instead, we explain their enhancement of the colloidal stability in experiments by their relative affinity for binding to the nanoparticle surface, and find evidence suggesting a random to cluster transition for their distribution on the particle surface. Overall, our results provide understanding between the colloidal stability of ligand-coated nanoparticles and their surrounding solvent environment.