Emulsions stabilized by the interaction of silica nanoparticles and palmitic acid at the water-hexane interface

The interaction between palmitic acid (PA) and silica nanoparticles at the water/hexane has been studied in relation to the ability in the stabilization of water in oil emulsions. To this aim, emulsions formed by aqueous silica nanoparticle dispersions and PA solutions in hexane have been investigated. In contrast with other previous studies, where the interaction of nanoparticles and surfactant molecules occurs already in the bulk phases, here PA and silica nanoparticles are dissolved/dispersed in different phases and interact only at the liquid-liquid interface. As for other particle-stabilized emulsions, the investigated system can provide emulsions stable over very long time because of the formation of nanoparticle-surfactant complexes, which accumulate at the droplets interface, providing a steric hindrance of coalescence. However, the present study has pointed out that these enhanced stability conditions are achieved only when the amount of surfactant leads to the formation of single surfactant layers onto the particles surface. Whereas an excess surfactant amount interacts with the particles, the formation of multilayers would be possible, and the formed complexes can be redispersed again in the aqueous bulk phase. This recalls to a scenario where the degree of hydrophobization of the particles is critical for the emulsion stabilization. This can be easily tuned by the control of the surfactant concentration and the volume ratio between the liquid phases. Interfacial tension measurement of the same couple of liquids, has allowed us to explore in more detail such phenomena, revealing a non-monotonic evolution of the dynamic interfacial tension at large PA concentrations. Also these observations can be rationalized assuming the redispersion of the complexes mentioned above, which allows explaining the interfacial tension evolution on the basis of the succession of adsorption and desorption of the different species - nanoparticles, nanoparticle-PA complexes, PA molecules - involved.