Surface-sensitive X-ray reflectivity and grazing incidence small-angle X-ray scattering reveal the structure of polymer-capped-gold nanoparticles (AuNPs that are grafted with poly( N-isopropylacrylamide); PNIPAM-AuNPs) as they self-assemble and crystallize at the aqueous suspension/vapor interface. Citrate-stabilized AuNPs (5 and 10 nm in nominal diameter) are ligand-exchanged by 6 kDa PNIPAM-thiol to form corona brushes around the AuNPs that are highly stable and dispersed in aqueous suspensions. Surprisingly, no clear evidence of thermosensitive effect on surface enrichment or self-assembly of the PNIPAM-AuNPs is observed in the 10-35 °C temperature range. However, addition of simple salts (in this case, NaCl) to the suspension induces migration of the PNIPAM-AuNPs to the aqueous surface, and above a threshold salt concentration, two-dimensional crystals are formed. The 10 nm PNIPAM-AuNPs form a highly ordered single layer with in-plane triangular structure, whereas the 5 nm capped NPs form short-range triangular structure that gradually becomes denser as salt concentration increases.

We report on the dynamic response of aqueous solution containing poly­(N-isopropylacrylamide)-capped gold (pNIPAM-capped Au) nanoparticles to the introduction of NaCl. The addition of NaCl increases the density of the solution and prompts the liquid–liquid phase separation process, confining the polymer to a lower-density salt-deficient aqueous phase. As the pNIPAM-occupied aqueous phase becomes excluded from the higher-density NaCl-rich bulk solution, the pNIPAM-capped Au nanoparticles follow liquid–liquid phase separation and reside on the surface of the formed pNIPAM-filled globes at the interface between the NaCl-rich bulk solution and the pNIPAM-containing solution, exhibiting a hexagonal packing with interparticle distance of ∼23 nm. Driven by the minimization of hydrophobic interactions, the buoyant Au-decorated globular assemblies filled with aqueous pNIPAM solution escape to the air/water interface, collapse at the interface, and form planar hexagonal crystalline domains of different sizes, depending on NaCl concentration. At low NaCl concentrations, the collapse of the Au-decorated aqueous pNIPAM-filled globes at the air/water interface produces an interfacial two-dimensional (2D) hexagonal lattice of pNIPAM-capped Au nanoparticles with an interparticle distance of 25–27 nm. The increase in NaCl concentration leads to a formation of smaller globes escaping to, and collapsing at the air/water interface and yielding smaller two-dimensional hexagonal domains.