Stabilization of highly concentrated emulsions with oversaturated dispersed phase: Effect of surfactant/particle ratio

Currently, there is considerable interest in highly concentrated emulsions (HCE) due to both the variety of rheological effects that are observed in their deformation and flow and to their practical application in the mining, pharmaceutical, cosmetics and food industries. The material investigated is highly concentrated water-in-oil emulsion with a dispersed phase volume fraction of approximately 90%. The dispersed phase is a super-cooled solution of inorganic salts. Instability of such emulsions arises either from crystallization of the dispersed phase in the system during ageing or under high shear conditions. Here, we report a new approach to stabilize this highly concentrated water-in-oil emulsion by adding colloidal particles in combination with short amphiphilic molecules. A series of five fumed silica nanoparticles, each with a different hydrophobicity index (HI) in the ranges 0.60-1.34 and >3, were mixed with sorbitan monooleate (SMO) into the oil phase prior to emulsification. The refinement time, rheological properties and stability of the resulting emulsions were investigated for varying SMO/particle ratios. At low surfactant-to-particle ratio, the silica content controls emulsion stability, but above some critical level (transitional point) SMO dominates over particles. The relationship between shear modulus and SMO-to-particle ratio shows the same transition point related to the same SMO-to-particle ratio found with the refinement time. Interestingly, for each HI, the emulsions are most stable in terms of both shelf life and under high shear when prepared with a SMO-to-particle ratio equaling exactly the value at this transitional point. Using the relationship between the zero modulus of particle dispersions in oil and the SMO-to-particle ratio, we show that the most stable emulsions are formed from dispersions having maximum particle flocculation, induced by SMO reverse micelles, in line with the SMO-to-particle ratio transitional point.