Iron Stearate Structures: An Original Tool for Nanoparticles Design

Iron carboxylates are widely used as iron precursors in the thermal decomposition process or considered as in situ formed intermediate precursors. Their molecular and three-dimensional (3D)-structural nature has been shown to affect the shape, size, and composition of the resulting iron oxide nanoparticles (NPs). Among carboxylate precursors, stearates are particularly attractive because of their higher stability to aging and hydration and they are used as additives in many applications. Despite the huge interest of iron stearates, very few studies aimed up to now at deciphering their full metal-ligand structures and the mechanisms allowing us to achieve in a controlled manner the bottom-up NP formation. In this work, we have thus investigated the molecular structure and composition of two iron stearate precursors, synthesized by introducing either two (FeSt₂) or three (FeSt₃) stearate (St) chains. Interestingly, both iron stearates consist of lamellar structures with planes of iron polynuclear complexes (polycations) separated with stearate chains in all-trans conformation. The iron content in polycations was found very different between both iron stearates. Their detailed characterizations indicate that FeSt₂ is mainly composed of [Fe₃-(μ₃-O)­St₆·xH₂O]­Cl, with no (or few) free stearate, whereas FeSt₃ is a mixture of mainly [Fe₇(μ₃-O­(H))₆(μ₂-OH)_xSt_12–2x]­St with some [Fe₃(μ₃-O)­St₆·xH₂O]­St and free stearic acid. The formation of bigger polynuclear complexes with FeSt₃ was related to higher hydrolysis and condensation rates within the iron­(III) chloride solution compared to the iron­(II) chloride solution. These data suggested a nucleation mechanism based on the condensation of polycation radicals generated by the catalytic departure of two stearate chains from an iron polycation-based molecule.