Structure and stabilization mechanism of diesel oil-in-water emulsions stabilized solely by either positively or negatively charged nanoparticles

Abstract

Two diesel oil-in-water emulsions were formed by using positively charged alumina nanoparticles solely and negatively charged silica nanoparticls solely as emulsifier, respectively. The formal systems are typical Pickering emulsions in which alumina nanoparticles adsorb at oil/water interface endowing droplets very good stability, whereas the later systems are non-Pickering emulsions in which silica nanoparticles do not adsorb at oil/water interface but distribute in aqueous phase surrounding the droplets endowing droplets temporary stability. Trace amounts of anionic surface-active materials in the diesel oil are responsible for the formation of two different emulsions. In the Pickering emulsions they adsorb at alumina particle surface to endow particle surface activity via in situ hydrophobization, whereas in the non-Pickering emulsions they adsorb at oil/water interface constituting a novel emulsion co-stabilized by like charged surfactants and nanoparticles at very low concentrations. By addition of trace amount of extra anionic surfactant, such as 0.1 mM sodium dodecyl benzene sulfate (SDBS), the novel diesel oil-in-water emulsions with good temporary stability can be formed by using silica nanoparticles at a concentration as low as 0.01 wt%. The emulsions can be demulsified easily by addition of trace amount (0.06∼0.1 mM) of a cationic surfactant such as cetyltrimethylammonium bromide (CTAB), or a low concentration (0.6∼3.0 mM) of divalent electrolyte such as CaCl2, and therefore have potential applications in oil transportation. On the other hand the Pickering diesel oil-in-water emulsions are relatively difficult to demulsify, and suitable demulsification protocols need to be developed for their applications in oil transportation.