Effect of Surface Ligands on Colloidal Stability, Shape and Sedimentation of Apolar Nanoparticles

Abstract

Understanding how nanoparticles interact with one another and their surroundings is critical to controlling their colloidal stability and assembly behaviour, and surface ligands play a vital role in determining the inter-particle forces both during and after synthesis. How- ever, our ability to predict the effect of these molecules on how nanoparticles behave in solution is currently poor. This thesis presents a theoretical study of the effect of surface ligands on the colloidal stability, sedimentation and shape deformation of apolar nanoparticles. In particular, inspired by recent experimental results, we develop models of Au and CdSe nanoparticles coated with apolar ligands in apolar solvents and use molecular dynamics simulations to study the conformational and energetic state of the ligand shell and to characterise the interaction between nanoparticles in solution. This work is divided in two parts. In Part I, we characterise and explain the effect of surface ligands on the colloidal stability of apolar nanoparticles. We show that agglomeration in solution can be induced either by the van der Waals attraction between the cores or the attractive interaction between ordered ligand shells, depending on the particle size. We find that in the shell-dominated case, stability depends strongly on the difference in free energy between the ordered and disordered states of the ligands, being affected by even small changes in ligand and solvent structure. In Part II, we show that ligands can strongly affect other properties of nanoparticles in solution, which we do by studying the sedimentation and shape deformation of apolar CdSe nanoparticles. Overall, our results provide a microscopic description of the forces induced by surface ligands and explain why classical colloid theories often fail to explain the colloidal stability of apolar nanoparticles.