Colloidal stability of aqueous ferrofluids with iron oxide nanoparticles sterically stabilized by phosphorylated polyethylene glycol

Abstract

The magnetic density separation of plastics requires new aqueous ferrofluids with excellent colloidal stability. Here, we studied the stabilization of iron oxide nanoparticles with poly ethylene glycol as a neutral adsorbed polymer. The goal was to correlate the adsorption to colloidal stability. Maghemite nanoparticles of 8 nm in diameter were synthesized via an adapted Massart synthesis. Coating of the particles with phosphorylated polyethylene glycol (PPEG) was found to be reversible, presumably via chemisorption of the phosphate headgroup to the iron oxide surface yielding sterically stabilized colloidal dispersions that were stable at more than 1 M NaCl concentrations. Magnetic susceptibility measurements of centrifuged samples were used to quantify the stability of the ferrofluids as function of the amount of PPEG added. Quantitative KBr-IR measurements were used to plot an adsorption isotherm and combined with the stability measurements it was found that PPEG with 550 Da molar weight yields stable ferrofluids at less than 10% surface coverage. PPEG of 5 kDa molar weight showed no plateau value in adsorbed molecules on the surface, indicating a different adsorption mechanism alongside phosphate headgroup chemisorption. Aqueous ferrofluids coated with 550 PPEG can provide a benchmark for near optimal colloidal stability at elevated salt concentrations that can be used for comparison in future colloidal stability studies.