Characterization of surface hydrophobicity of engineered nanoparticles. 
The surface chemistry of nanoparticles, including their hydrophobicity, is a key determinant of their fate, transport and toxicity.  Engineered NPs often have surface coatings that control the surface chemistry of NPs and may dominate the effects of the nanoparticle core.  Suitable characterization methods for surface hydrophobicity at the nano-scale are needed.  Three types of methods, surface adsorption, affinity coefficient and contact angle, were investigated in this study with seven carbon and metal based NPs with and without coatings.  The adsorption of hydrophobic molecules, Rose Bengal dye and naphthalene, on NPs was used as one measure of hydrophobicity and was compared with the relative affinity of NPs for octanol or water phases, analogous to the determination of octanol-water partition coefficients for organic molecules.  The sessile drop method was adapted for measuring contact angle of a thin film of NPs.  Results for these three methods were qualitatively in agreement.  Aqueous-nC(60) and tetrahydrofuran-nC(60) were observed to be more hydrophobic than nano-Ag coated with polyvinylpyrrolidone or gum arabic, followed by nano-Ag or nano-Au with citrate-functionalized surfaces.  Fullerol was shown to be the least hydrophobic of seven NPs tested.  The advantages and limitations of each method were also discussed.