Stabilization of Pickering Emulsions by Iron oxide Nano-particles (Fe3O4 NPs)

Abstract

Pickering emulsions are fine dispersions of minute droplets of two immiscible phases stabilized by solid particles adsorbed at the liquid-liquid interface. In the case of oil-in-water (o/w) or water-in-oil (w/o) emulsions, such types of Pickering emulsions have utility in various applications such as the preparation of porous materials due to their ability to self-assemble at the o/w interface. Iron oxide nanoparticles (Fe 3 O 4 NPs) are known to play an important role in stabilizing Pickering emulsions due to their variable functionality and hydrophobicity. The efficiency of Fe 3 O 4 NPs in stabilizing Pickering emulsions depend on the polarity of the oil used in the preparation of the Pickering emulsion. Fe 3 O 4 NPs can be surface modified through coordination via carboxylic acids and silane coupling agents. Non-modified and carboxylic acid-modified Fe 3 O 4 NPs provide greater stability for o/w emulsions prepared with nonpolar or weakly polar oils. Silane-coated Fe 3 O 4 NPs are more useful in stabilizing o/w emulsions, where the oil phase is strongly polar. The chain length of the modifying agent plays an important role on the stability of emulsions. Modifying agents with long alkyl chains result in greater hydrophobicity that adversely affect the stability of such Pickering emulsions. Other factors governing the stability of these emulsions include the pH of the aqueous phase, oil polarity, concentration/size of particles, ion concentration as well as oil/water volume ratio. A key parameter used for determining the effectiveness of particle stabilization of Pickering emulsions is the contact angle at the o/w interface. Contact angle (θ) measurements provide a measure of the particle hydrophobicity, while θ values provide a measure of colloidal stabilization of such particles. The aim of this review is to provide an overview of the effectiveness of Fe 3 O 4 NPs as stabilizing agents for Pickering emulsions and to bridge the knowledge gaps related to this topic in the last 5 years. This review describes the utility of iron oxide NPs as emulsion stabilizers by outlining aspects of the structure and the physicochemical properties of such systems. The use of surface modified Fe 3 O 4 NPs as stabilizing agents is anticipated to grow, where the results of this overview will catalyze further studies directed at advanced research and development of such systems for diverse applications in materials science, colloid chemistry and medicine.