Solvent Density-Dependent Steric Stabilization of Perfluoropolyether-Coated Nanocrystals in Supercritical Carbon Dioxide

Gold nanocrystals, 5 nm in diameter, were synthesized with coatings of thiolated perfluoropolyether (PFPE) ligands. These nanocrystals are dispersed in supercritical carbon dioxide (scCO 2 ). Because of the high CO 2 compressibility, the particle dispersibility is a strong function of solvent density, with higher interparticle attraction and increased aggregation at lower density. Using small-angle X-ray scattering (SAXS), the interparticle interactions between PFPE-coated nanocrystals in scCO 2 were measured as a function of pressure (from 310 to 120 bar) under isothermal conditions (35 °C). A small peak, which increases in intensity with decreasing pressure, appears at low q in the scattering curve because of interparticle clustering. A model that approximates the nanocrystal interactions with an attractive square-well potential and accounts for the scattering contribution from clustering particles provides excellent agreement with the data. For 5.25-nm-diameter gold nanocrystals dispersed in scCO 2 at 35 °C, the square-well potential minimum decreases from −0.6 kT to −1.3 kT as the pressure is lowered from 310 to 120 bar.