Use of ferrocenyl surfactants of varying chain lengths to study electron transfer reactions in native montmorillonite clay

A series of ferrocenyl surfactants was tested as model compounds to study electron transfer reactions involving structural Fe(III) in clay minerals. The surfactants contain trimethylammonium headgroups, ferrocene tail groups, and intervening hydrocarbon chain lengths of one, six, or 11 carbons. Two factors considered to be decisive for electron transfer were addressed: (1) physical access of the surfactant ferrocene to the reactive sites through hexagonal holes in the clay lattice by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) and (2) thermodynamic favorability of the overall oxidation/reduction reaction based on experimentally determined oxidation/reduction potentials. In suspensions of clay with the longer chain surfactants, (ferrocenylhexyl)trimethylammonim (FHTMA(+)) and (ferrocenylundecyl)trimethylammonium (FUTMA(+)), where electron transfer may be expected to be favored by both factors, physical accessibility, and thermodynamic favorability, ferroecene oxidation was observed by diffuse reflectance infrared spectroscopy (DRIFT), ultraviolet-visible spectroscopy (UV-vis), and visual color changes. In contrast, the shorter chain length surfactant, (ferrocenylmethyl)trimethylammonium (FMTMA(+)), did not participate in electron transfer with the clay, as substantiated by UV-vis and no visible color changes. Rigid conformation and/or higher oxidation/reduction potential than clay Fe can account for the lack of reaction. The utility and limitations of using these surfactants as model compounds is discussed.