Small-angle neutron scattering study on the molecular packing and hydration in magnesium perfluorooctanoate micelles

Counter-ions have a large effect on the assembly and dispersion of fluorocarbon amphiphiles including the per-and polyfluoroalkyl substances (PFAS) that are widespread pollutants in the environment. Micelles play a crucial role in dispersion of these fluorocarbon materials in the environment. Understanding their physiochemical properties is also valuable in the development of the novel chemical processing that are being developed for their remediation. The calcium salt will be the subject of future studies as X-ray studies reveal that the overall phase behaviour is apparently more complex . The study focusses on magnesium perfluorooctanoate which has a low critical micelle concentration and is typical of divalent ions in the environment. It will investigate the molecular packing in micelles and the extent of their hydration using small-angle neutron scattering with contrast variation. Small-angle X-ray scattering shows that the magnesium perfluorooctanoate forms elongated micelles, contrasting with the short cylinders of sodium perfluorooctanoate. Temperature is seen to influence the shape and interactions. Combined analysis of the different contrasts is essential to develop and validate detailed models.