Njelama Sanga: Self-assembly of fluorocarbon amphiphiles and their interactions at interfaces with a seed protein

Abstract

Perfluorinated surfactants, known as per- and polyfluoroalkyl substances (PFAS,) are persistent and potentially toxic pollutants found widely in the environment.  Understanding how counterions affect their self-assembly and dispersion, as well as how these surfactants interact with natural materials at interfaces is crucial.  This thesis investigates the physiochemical properties of perfluorooctanoate salts dispersed in water, with the aim of understanding their micellar structure, factors that influence their micellar characteristics, and their interactions with environmentally relevant materials.  Small-angle scattering and neutron reflection were the primary tools used to study these properties.

This study is divided into two parts. The first section presents structural information on the perfluorooctanoate micelles in the presence of different counterions, focusing on the structure and hydration of magnesium perfluorooctanoate and comparing it with monovalent salts using small-angle X-ray and neutron scattering.  The monovalent (Li+, Na+, NH4+) perfluorooctanoate micelles formed highly charged, cylindrical micelles whose elongation increases with perfluorooctanoate concentration.  Sodium perfluorooctanoate forms shorter, slightly ellipsoidal micelles with a dense fluoroalkyl core and hydrated shell containing seven water molecules.  In contrast, magnesium perfluorooctanoate formed more elongated micelles with a denser fluoroalkyl core and a hydrated shell containing ten water molecules per perfluorooctanoate.  Their length increases significantly with temperature.  A pronounced isotope effect was observed at low temperatures: micelles were five times longer in D2O compared to H2O.  In the presence of an added salt, magnesium perfluorooctanoate precipitates and remained insoluble even at elevated temperature.

The second part studies the adsorption of two fluorocarbon surfactants, sodium perfluorooctanoate and perfluorohexanoate, onto Moringa oleifera seed protein on a silica surface using neutron reflection. The perfluorooctanoate and perfluorohexanoate were found to adsorb to the previously bound protein forming a mixed layer of protein, surfactant and water that expanded to incorporate the extra material.  Interestingly, almost equal amounts of the surfactant and protein were present in the layer.  The surfactant, but not protein, was displaced by rinsing with water.  These results will be of interest for remediation of PFAS contamination as well as enhancing the understanding of their transport in the environment and health implications