Salt-Induced Liquid–Liquid Phase Separation and Interfacial Crystal Formation in Poly(N-isopropylacrylamide)-Capped Gold Nanoparticles

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.