Rheological and interfacial properties of Pickering emulsions prepared by fumed silica suspensions pre-adsorbed poly(N-isopropylacrylamide)

Pickering emulsions were prepared by mixing silicone oil with fumed silica suspensions pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in the continuous water phase as a function of added amount of two PNIPAM samples. Characteristics of the resulting oil dispersed in water (O/W) Pickering emulsions were determined by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions pre-adsorbed PNIPAM, oil droplet size, and some rheological responses, such as hysteresis loop, stress–strain sweep curve, and dynamic viscoelastic moduli. They were compared with those of the silicone oil emulsions prepared by the fumed silica suspensions pre-adsorbed hydroxypropyl methyl cellulose (HPMC). The adsorbed amounts of the silica suspensions pre-adsorbed polymer were increased with an increase in the added amount of polymer, irrespective of the polymer. This led to (1) an increase in the volume fraction of emulsified silicone oil, (2) a decrease in the size of oil droplets, (3) an increase in the difference between the up and down curves at lower shear rates for the negative hysteresis loops, and (4) an increase in the storage moduli (G′) for the solid-like viscoelastic responses. However, the volume fraction of the emulsified oil in the emulsified phase for the emulsions in the presence of PNIPAM increased with an increase in the adsorbed amount of the silica suspension, whereas the opposite was obtained in the presence of HPMC. This was attributed to the stronger hydrophobic interactions and they caused (1) the larger difference of the hysteresis loops and (2) the larger magnitude of G′ value for the former emulsion than those for the latter one at the higher added amounts of polymers.