Effects of F-, Cl-, Br-, NO3- and SO42- on the colloidal stability of Fe3O4 nanoparticles in the aqueous phase

The effect of ions on the colloidal behavior of magnetic nanoparticles (MNPs) is an important factor for determining the dispersibility of MNPs. Compared with the effects of cations and organic matter, the effect of anions on MNPs has rarely been studied. Hence, in this study, the effect of anions on the aggregation of Fe3O4 MNPs in the aqueous phase was investigated using F-, Cl-, Br-, NO3- and SO42-. The results indicated that the effect of anions on the colloidal behavior of the MNPs varied widely depending on their valence state, concentration, hydration ability, solution pH, and the magnetic force between the MNPs. Specifically, at pH 5.0, the anions were mainly adsorbed on the particle surface by electrostatic attraction, decreasing the electrostatic repulsion between the MNPs and causing an aggregation of the particles in the order of SO42- > F- > Br- > Cl- approximate to NO3-. At pH 9.0, anions strengthened the suspension of the MNPs at low ionic strength (IS) (55); however, with increasing IS, an aggregation of the MNPs in the following order was formed: NO3- > Cl-> Br- >= F- > SO42-. This was a result of the combined effects of the IS of solution, hydrability, and polarizability of the anions. Furthermore, the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory can explain the colloidal behavior of MNPs in the presence of magnetic forces, but it fails to differentiate the MNP behaviors between monovalent anions because the effects of ionic hydrability and polarizability are not considered. Distinctively, the secondary minimum between the MNPs particles were induced via magnetic attraction and played a critical role in adjusting the colloidal stability of the MNPs. Overall, these results indicate that specific ionic effects and magnetic attraction are important for interpreting the colloidal stability of MNPs in aqueous conditions. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the effect of different anions on the aggregation of Fe3O4 MNPs in the aqueous phase, and found that the type, concentration, hydration ability, solution pH, and magnetic force of the anions differed, leading to varying colloidal behaviors of MNPs.