The effect of electrostatic interaction and wettability of Fe3O4 nano-composites on demulsification of O/W (hexadecane) nanoemulsion

ABSTRACT This paper delves into the impact of electrostatic interaction and wettability of MNPs on the efficiency of demulsifiers. Fe3O4 magnetic nanocomposites (Fe3O4/HA/PDDA/OLA MNPs, S1–S5) were synthesized with varying wettability and zeta potential by adjusting the surface modification ratio of poly-dimethyl-diallyl-ammonium chloride (PDDA) and oleylamine (OLA). S1–S5 were then used to demulsify O/W (hexadecane) nanoemulsions. The oil/water separation efficiency (SE) initially increased (S1–S4) and then decreased (S4–S5) with their increasing water contact angles (WCA). S4 with a WCA about 90° exhibited the maximum SE, although S4 has a negative charge and shows electrostatic repulsion with oil droplets, indicating that the wettability of MNPs, rather than electrostatic interaction, played a dominant role in demulsification. Adsorption energy of S1–S5 at the water–oil interface aligned with the variation in their demulsification efficiency. Based on previous reports, a possible demulsification mechanism was proposed. Hydrogen bond and hydrophobic long chains present in PDDA and OLA increased the interfacial activity of S4, which facilitated the spontaneous migration of MNPs to the interface of oil and water. With oil droplet aggregation by magnetic recovery, a good oil–water separation performance was achieved. The study provides a theoretical foundation for the design and synthesis of magnetic demulsifiers.