How Size and Crystallinity of Nanoparticles (NPs) Affect Their Chemical Activity in Water? Acid-Base Properties of Metal Oxide Nps and Adsorption of Oxy-Anions and Fatty Acids

Particle size effects on (bio)chemical properties of metal (hydr)oxide nanoparticles (NPs) are of fundamental scientific and technological interest. We present results of the first multi-pronged study of the effect of nanoparticle size and crystallinity on acid-base and adsorption properties of hematite (Ą-Fe2O3). Using measurements of point of zero charge, electrokinetic mobility, in situ FTIR spectroscopy, and DFT modeling, we discovered that particle size controls adsorption properties of hematite significantly. In particular, inner-sphere mononuclear monodentate carbonate dominates on 7 nm NPs where as an increase in the particle size to 40 nm (or the lattice order) results in an increase in the relative concentration of the inner-sphere bidentate binuclear complexes. Interestingly, the surface concentration of adsorbed carbonate also increases with such increase in size. No or very little adsorption of carbonate was detected on 60 nm and larger NPs. In the case of sodium laurate, a decrease in either particle size or crystallinity lowers 1) the molecular form of the adsorbed surfactants vs. the ionic form; 2) packing order of the hydrocarbon chains and hence hydrophobicity of the surfactant-coated surface. It was shown that laurate adsorbs as both inner-sphere monodentate complex and water-surface-hydration separated ion pair (outer-sphere bridging bidentate complex). The latter is protonated with increasing particle size, which explains the observed increase in the packing order of the laurate chains. These effects are explained by a decrease in basicity of hematite with increasing particle size due to the quantum size effects, size-induced changes in structure of the NP surface and interior, surface delocalization of electronic charge, and the synthesis route.