Numerous
cationic magnetic nanoparticles (MNPs) have previously
been developed for demulsifying oil-in-water (O/W) emulsion, and results
showed that the cationic MNPs could effectively flocculate and remove
the negatively charged oil droplets via charge attraction; however,
to the best of our knowledge, there are no research reports regarding
the synergetic influence of both the positive charge density and interfacial
activity of MNPs on the demulsification performance. In this study,
three tertiary amine polymer-grafted MNPs, namely, poly(2-dimethylaminoethyl
acrylate)-grafted MNPs (M-PDMAEA), poly(2-dimethylamino)ethyl methacrylate)-grafted
MNPs (M-PDMAEMA), and poly(2-diethylaminoethyl methacrylate)-grafted
MNPs (M-PDEAEMA), were synthesized and evaluated for their demulsification
performance. Results demonstrated that a high positive charge density
and superior interfacial activity of MNPs could cause partial oil
droplet re-dispersion when excessive MNPs were introduced, leading
to a lower magnetic separation efficiency and narrower demulsification
window. Herein, a demulsification window is defined as a range of
nanoparticle dosages in which the MNPs can effectively demulsify the
O/W emulsion under certain conditions. For highly positively charged
MNPs, their good interfacial activity could aggravate the formation
of a narrower demulsification window. When tertiary amine polymer-grafted
MNPs carried a lower positive charge density or weak interfacial activity,
that is, M-PDMAEA at pH 4.0, M-PDMAEMA at pH 5.0–9.0, and M-PDEAEMA
at pH 9.0–10.0, wide demulsification windows were observed.
Additionally, a recycling experiment suggested that MNPs could maintain
high demulsification efficiency up to at least five cycles, indicating
their satisfactory recyclability. The three tertiary amine polymer-grafted
MNPs can be potentially used for efficient demulsification from surfactant-free
O/W emulsion in various pH ranges.