Aqueous Aggregation and Surface Deposition Processes of Engineered Superparamagnetic Iron Oxide Nanoparticles for Environmental Applications

Engineered, superparamagnetic, iron oxide nanoparticles (IONPs) have significant potential as platform materials for environmental sensing, imaging and remediation due to their unique size, physicochemical and magnetic properties. To this end, controlling the size and surface chemistry of the materials is crucial for such applications in the aqueous phase, and in particular, for porous matrixes with particle-surface interaction considerations. In this study, superparamagnetic, highly monodispersed 8 nm IONPs were synthesized and transferred into water as stable suspensions (remaining monodispersed) by way of an interfacial oleic acid bilayer surface. Once stabilized and characterized, particle–particle and model surface interactions (deposition and release) were quantitatively investigated and described systematically as a function of ionic strength (IS) and type with time-resolved dynamic light scattering (DLS), zeta potential, and real-time quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The critical coagulation concentration (CCC) for oleic acid bilayer coated iron oxide nanoparticles (OA-IONPs) were determined to be 710 mM for NaCl (matching DLVO predictions) and 10.6 mM for CaCl₂, respectively. For all conditions tested, surface deposition kinetics showed stronger, more favorable interactions between OA-IONPs and polystyrene surfaces compared to silica, which is hypothesized to be due to increased particle-surface hydrophobic interactions (when compared to silica surfaces)