Influence of magnesium on the structure and hydration of perfluorooctanoate micelles: comparison with sodium salt

The influence of counterions such as magnesium on the assembly and dispersion of perfluorooctanoate, which is an environmentally persistent fluorocarbon surfactant, is important for its transport in the environment. In this study, the structure and hydration of magnesium perfluorooctanoate micelles were investigated as a function of temperature using small-angle scattering techniques. Multiple water contrasts using isotopes for neutron studies, complemented by X-ray scattering, provide data for fitting detailed structural models. The magnesium perfluorooctanoate formed elongated rod-like micelles with a dense core of 1.86 g cm^-3 and a hydrated shell containing ∼10 water molecules per perfluorooctanoate molecule. The length of the micelles increased at lower temperatures while the number of water molecules per head group remained almost constant. A pronounced isotope effect was observed at low temperatures: micelles were approximately five times longer in D₂O compared to H₂O. In the presence of added salt, magnesium perfluorooctanoate precipitated and remained insoluble at elevated temperatures. Sodium perfluorooctanoate formed shorter ellipsoidal micelles with a length of 18 Å and a hydrated shell containing approximately seven water molecules, with a less dense core of 1.64 g cm^-3. The presence of counterions such as magnesium and sodium influences the transport and dispersion of perfluorooctanoate in the environment. The results provide valuable insight into remediation strategies, including salt-induced precipitation to concentrate the surfactant before subsequent degradation.