On the shear stability of water-in-water Pickering emulsions stabilized with silica nanoparticles

Water-in-water (w/w) emulsions are known for their low interfacial tensions (IFT) which makes their stability to shear questionable. This is because of low particle attachment energies, which can be just a few kT. Therefore, emulsions stabilized with larger particles should display greater stability to shear because of larger attachment energies (10-100 or more kT). This is typically not an issue with traditional oil-in-water Pickering emulsions because particle attachment energies are much larger due to higher interfacial tensions, even when very small particles are used.

Silica nanoparticles were silanized with 2-(methoxy(polyethyleneoxy)6-9propyl)trimethoxysilane (PEG-silane) to aid in emulsion stabilization. The phase behavior of an aqueous, two-phase system consisting of 20,000 g mol^-1 polyethylene glycol (PEG) and magnesium sulfate (MgSO₄) was characterized. Optical microscopy was used to characterize the static properties of the particle stabilized emulsions and shear rheology was used to study the stability of emulsions stabilized with 6 nm and 50 nm PEG-silane functionalized particles.

We demonstrated that silica nanoparticles silanized with PEG-silane can stabilize MgSO₄ drops to produce MgSO₄-in-PEG emulsions. We found emulsions stabilized with 6 wt% particles, regardless of particle size (6 nm or 50 nm), had similar viscosities, emulsion drop size, and were statically stable for one week. Emulsion drops stabilized with 6 wt% 50 nm particles doubled in size after 80 min of shear at 10 s^-1 whereas those stabilized with 6 wt% 6 nm particles required only 25 min to double in size. We attribute these differences in doubling time to the larger particle attachment energies associated with the 50 nm particles.