Interactions of Ligand-Coated Nanoparticles at a Liquid Surface*

The pair interaction potential of ligand-coated silica nanoparticles (NPs) at a liquid surface were determined by scanning electron microscopy, exploiting the nonvolatility of ionic liquids to stabilize the liquid specimens against microscope vacuum. Even at near contact, individual, two-dimensionally well-dispersed NPs were resolved. The potential of mean force, reduced to the pair interaction potential for dilute NPs, was extracted with good accuracy from the radial distribution function as both NP diameter and poly(ethylene glycol) ligand length varied. While NP polydispersity broadened the core repulsion, the pair potential well-approximated a hard sphere interaction. For short (5 kDa) ligands, a weak (<kT) long-range attraction was discerned, and for ligands of identical length, the potentials overlapped for NPs of different diameter; the attraction is suggested to arise from ligand-induced menisci. To probe the factors underlying the potential, NP surface-binding energies were measured by interfacial tensiometry, and NP contact angles were assessed by new atomic force microscopy and transmission electron microscopy methods.