Salt-Induced Liquid–Liquid Phase Separation and Interfacial Crystal Formation in Poly(N-isopropylacrylamide)-Capped Gold Nanoparticles
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Ames National Laboratory is a government-owned, contractor-operated national laboratory of the U.S. Department of Energy (DOE), operated by and located on the campus of Iowa State University in Ames, Iowa.
For more than 70 years, the Ames National Laboratory has successfully partnered with Iowa State University, and is unique among the 17 DOE laboratories in that it is physically located on the campus of a major research university. Many of the scientists and administrators at the Laboratory also hold faculty positions at the University and the Laboratory has access to both undergraduate and graduate student talent.
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Abstract
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.