Nanoparticle stabilized oil in water emulsions: A critical review

It is concluded that although there are some clues in the literature addressing the emulsion stability due to mixing of surfactant with NPs of similar charge, the most stable oil in water emulsions can be obtained when the system contains oppositely-charged surfactants and NPs i.e., anionic NPs along with cationic surfactants or cationic NPs along with anionic surfactants. Therefore, it seems that the most influential factor is having two emulsifiers with different charges and the system chemistry is of less importance. In these cases, at specific surfactant concentrations the electrostatic interactions between NPs and surfactant molecules result in the least stable and most flocculated NPs which is reported as the most suitable condition (corresponding to the most favorable state for NPs hydrophobicity) to stabilize oil in water emulsions. Furthermore, adding a large amount of energy into the system by ultrasonication decreases the size of oil droplets which in turn, increases their surface area on which NPs have a higher chance to adsorb.