title: Double inversion of emulsions induced by salt concentration.
abstract: The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.
authors.affiliation: Key Laboratory for Colloid and Interface Chemistry of the Ministry of Education, Shandong University, Jinan, Shandong 250100, People's Republic of China.
authors.forename: Jingchun
authors.initials: J
authors.lastname: Zhang
authors.forename: Lu
authors.initials: L
authors.lastname: Li
authors.forename: Jun
authors.initials: J
authors.lastname: Wang
authors.forename: Haigang
authors.initials: H
authors.lastname: Sun
authors.forename: Jian
authors.initials: J
authors.lastname: Xu
authors.forename: Dejun
authors.initials: D
authors.lastname: Sun
mesh_terms: D004655:Emulsions; D006601:Hexoses / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D012492:Salts / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry*
chemical_list: D004655:Emulsions; D006601:Hexoses; D012492:Salts; D017640:Silicates; D013501:Surface-Active Agents; C018665:sorbitan monooleate; C524813:laponite