Viscoelasticity enhancement induced by salts for highly concentrated oil-in-water (O/W) emulsions

Abstract

Highly concentrated viscoelastic paraffin oil-in-water emulsions were prepared by using tetradecyltrimethylammonium laurate (TTAL) as the emulsifier at a water percentage of 10%. The influences of four different inorganic salts, i.e., NaCl, NaBr, CaCl2 and MgSO4, on the structures and rheological properties of the emulsions have been investigated. It was found that the addition of salt eventually induces macroscopic phase separation of the emulsions and the influence following the order of NaCl<NaBr<CaCl2<MgSO4. At intermediate salt concentrations, the introduction of salt leads to obvious viscoelasticity enhancement of the emulsions. The ability of the salt to induce the viscoelasticity enhancement approximately follows an opposite order to that found in phase separation, which is 100 folds at 500mmolL−1 NaCl (potentially not reach the maximum), 400 folds at 300mmolL−1 NaBr, 10 folds at 50mmolL−1 CaCl2 and 6 folds at 15mmolL−1 MgSO4, respectively. It is speculated that the adsorption of the inorganic cation and anion onto the charged surfactant monolayer at the oil/water interface changes the properties of the emulsion droplets, accounting for this unusual viscoelasticity enhancement. By conductivity measurements, it is also evidenced that the dissociation and mobility of the adsorbed ions can be hardly influenced by temperature, which leads to pronounced stability of the emulsions at elevated temperatures. The salt-induced structural transitions have been monitored by optical microscopy and confocal fluorescence microscopy observations, both of which show the presence of large oil droplets surrounded by hierarchically-organized smaller ones.