The effect of nanoparticle hydrophobicity on the rheology of highly concentrated emulsions

Abstract

A series of fumed silica nanoparticles were used as an additional emulsifier for highly concentrated (HC) water-in-oil (W/O) emulsions. These nanoparticles, with different hydrophobicity index (HI) in the 0.60– 1.34 and HI>3range, were mixed with the conventional low molecular weight surfactant, sorbitan monooleate (SMO), in theoil phase prior to the emulsification process. The rheological properties of these emulsions were measured andcompared with the properties of emulsions stabilized with SMO alone. In the mixed emulsifier system, thechanges in rheological parameters were clearly expressed as a function of HI. The mixture of silica nanoparti-cles and SMO significantly increases the yield stress and plateau modulus of fresh emulsion, compared to theSMO only system. The effect was found to be more pronounced with a decrease in the HI. This is probably relat-ed to the reduction in micelle content with the decrease in HI, owing to a concomitant increase in the amountof SMO adsorbed onto the particle surface. Then, interestingly, the Foudazi-Masalova model recently developedfor surfactant-stabilized highly concentrated emulsions (HCE) was found to describe successfully the rheologi-cal behavior of emulsions in the presence of a mixture of surfactant and fumed nanosilica