Self-assembled monolayers of alkanesulfonic and -phosphonic acids on amorphous iron oxide nanoparticles

We have functionalized amorphous Fe2O3 nanoparticle s with alkane sulfonic and octadecanephosphonic acids. TEM reveals nanoparticles 5-10 nm in diamet er. FTIR spectra suggest that while in all cases the alkyl chains are packe d in a solid-like arrangement, packing disorder increased with decreasing c hain length. TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles sh ows that moieties started to decompose and desorb from the iron oxide surfa ce at about 260 degrees C. In the case of the octadecanephosphonic acid (OP A)-functionalized Fe2O3, moieties started to decompose and desorb at 340 de grees C. It is suggested that free Fe-OH groups can serve as proton donors to assist in the sulfonic acid desorption process and that because of the d iprotic nature of the phosphonic acid these free surface Fe-OH groups may n o longer be available. Among all, the octadecanesulfonic acid coating displ ays the lowest magnetization, which may be explained by the high packing an d ordering of the alkyl chains on the particle surface. The saturation curv e of the OPA case gives the smallest value of magnetization we have ever me asured for functionalized Fe2O3 nanoparticles. It is suggested that the spi n state of surface Fe3+ ions is affected by the bonded surfactant, through a mechanism of p pi-d pi P-O, and d pi-d pi Fe-P interactions and that the phosphonate empty d orbitals increase magnetic interactions between neighbo ring Fe3+ spins.