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We report on the colloidal stability of nanoparticles with alkanethiol shells in apolar solvents. Small angle X-ray scattering and molecular dynamics simulations were used to characterize the interaction between nanoparticles in linear alkane solvents ranging from hexane to hexadecane, including \SI{4}{\nano\meter} gold cores with hexadecanethiol shells and \SI{6}{\nano\meter} cadmium selenide cores with octadecanethiol shells. We find that the agglomeration is enthalpically driven and that, contrary to what one would expect from classical colloid theory, the temperature at which the particles agglomerate increases with increasing solvent chain length. We demonstrate that the inverted trend correlates with the temperatures at which the ligands order in the different solvents, and show that the inversion is due to a combination of enthalpic and entropic effects that enhance the stability of the ordered ligand state as the solvent length increases. We also explain why cyclohexane is a better solvent than hexane, despite having very similar solvation parameters to hexadecane.
Self-assembly of nanoscale structures at liquid-solid interfaces occurs in a broad range of industrial processes and is found in various phenomena in nature. Conventional theory assumes spherical particles and homogeneous surfaces, but that model is oversimplified, and nanoscale in-situ observations are needed for a more complete understanding. We used liquid phase scanning transmission electron microscopy (LP-STEM) to examine the interactions that direct the self-assembly of superlattices formed by gold nanoparticles (AuNPs) in non-polar liquids. Varying the molecular coating of the substrate modulated short-ranged attraction and led to switching between a range of different geometric structures including hexagonal close-packed (hcp), simple hexagonal (sh), dodecahedral quasi-crystal (dqc), and body-centered cubic (bcc) lattices, as well as random distributions. Langevin dynamics simulations explain the experimental results in terms of the interplay between nanoparticle faceting, ligand shell structure, and substrate-NP-interactions.
