Rational design of interfacial properties of ferric (hydr)oxide nanoparticles by adsorption of fatty acids from aqueous solutions

Notwithstanding the great practical importance, still open are the questions how, why, and to what extent the size, morphology, and surface charge of metal (hydr)oxide nanoparticles (NPs) affect the adsorption form, adsorption strength, surface density, and packing order of organic (bio)molecules containing carboxylic groups. In this article, we conclusively answer these questions for a model system of ferric (hydr)oxide NPs and demonstrate applicability of the established relationships to manipulating their hydrophobicity and dispersibility. Employing in situ Fourier transform infrared (FTIR) spectroscopy and adsorption isotherm measurements, we study the interaction of 150, 38, and 9 nm hematite (α-Fe(2)O(3)) and ～4 nm 2-line ferrihydrite with sodium laurate (dodecanoate) in water. We discover that, independent of morphology, an increase in size of the ferric (hydr)oxide NPs significantly improves their adsorption capacity and affinity toward fatty acids. This effect favors the formation of bilayers, which in turn promotes dispersibility of the larger NPs in water. At the same time, the local order in self-assembled monolayer (SAM) strongly depends on the morphological compatibility of the NP facets with the geometry-driven well-packed arrangements of the hydrocarbon chains as well as on the ratio of the chemisorbed to the physically adsorbed carboxylate groups. Surprisingly, the geometrical constraints can be removed, and adsorption capacity can be increased by negatively polarizing the NPs due to promotion of the outer-sphere complexes of the fatty acid. We interpret these findings and discuss their implications for the nanotechnological applications of surface-functionalized metal (hydr)oxide NPs.