Polarization-dependent NEXAFS study of adsorption of long-chain surfactants on mechanically milled iron powder

- Plate-like particles modified with surfactant molecules were obtained under high-energy ball milling.
- Adsorption layers were studied with polarization-dependent NEXAFS spectroscopy.
- For the first time, arrangement of surfactants molecules on the powdered metal surface has been determined.
- Tails of surfactant molecules (CF/CH) are shown to be oriented perpendicular to the particle surface.
- Arrangement of carboxylate groups on the particle surfaces is discussed.

In this work we have demonstrated the possibility of using the polarization-dependent NEXAFS spectra to study the structure of organic layers at the surface of powdered materials with plate-like shaped particles. The polarization dependence of the NEXAFS spectra may be easily obtained by just changing the angle between the X-ray beam direction and the substrate onto which the powder particles are set. For the first time, we have carried out a detailed study of the surfactant layers (n-perfluorononanoic and stearic acid), which are formed at the surface of iron plate-like particles under mechanical milling of iron powder with an addition of corresponding surfactants. The surfactant molecules are predominantly oriented perpendicular to the surface of the mechanically milled particles. Such orientation is similar to the arrangement of the molecules in the layers formed under equilibrium conditions, e.g. deposition from solutions. The changes in the chemical environment occurring in the molecule tails (defluorination or dehydrogenation) under mechanochemical treatment, do not result in a significant change in the molecular orientation and disordering of the adsorbed layer.