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 (ScCO2). Because of the high CO2 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 ScCO2 were measured as a function of pressure (from 3 10 to 120 bar) under isothermal conditions (35 degreesC). 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 ScCO2 at 35 degreesC, the square-well potential minimum decreases from -0.6kT to -1.3kT as the pressure is lowered from 310 to 120 bar.