The hydrolysis of iron(III) ions can yield different iron (oxyhydr)oxide phases varying in crystallinity and composition, and thus in physical and chemical properties. Which phase is formed depends on many parameters such as pH, the presence of counter- and spectator ions, or temperature.<br/>For the understanding of the different paths leading to the final solid phases, the mechanisms of nucleation and the underlying chemical reactions have to be elucidated. The species present at the early stages, their composition and nature are of particular interest, as they represent the basic precursors. Corresponding investigations are challenging due to the complex chemistry, the dynamic character and the small size of the early species.<br/>We demonstrate that distinct nucleation stages can be accessed using a titration setup that provides reproducible and homogeneous reaction conditions. Numerous on-line and <i>in situ </i>analytical techniques have been applied to these stages, avoiding isolation artefacts. This shows that nucleation in different iron(III) systems proceeds according to the notions of the Pre-Nucleation Cluster (PNC) pathway. In the early reaction stage, thermodynamically stable olation clusters are observed, which meet the definition of PNCs. The onset of oxolation reactions within the clusters lead to a more rigid structure and decreased cluster dynamics. The development of an interface with the surrounding solvent upon oxolation is the molecular basis of the phase separation event, further supported by the observations made under differing reaction conditions. The model explains the role of ions, additives and temperature in the directed formation of specific phase identities.

Quantitative analyses of the mechanisms of iron (oxyhydr)oxide nucleation

253rd National Meeting (2017)

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