Properties of non-conventional direct O/W Pickering emulsions stabilized by partially hydrophobic silica particles controlled by rotor-stator or ultrasonic emulsification

Abstract

There has been a significant increase in interest in Pickering emulsions over the last two decades due to their high stability. As the nature of emulsions is crucial for applications, Bancroft’s rule has been proposed to help in the formulation. However, several studies show that this law is far from being generic and that hydrophilic particles can also stabilize water-in-oil (W/O) emulsions. In the frame of this idea, we suggest in this study to evaluate the wetting history of particles and the emulsification process by considering the structural and the rheological properties of the emulsions. In this study, non-conventional or anti-bancroft O/W Pickering emulsions stabilized by partially hydrophobic silica particles are addressed. The latter are based on water and paraffin oil at 20 and 50 vol%. Stabilization is ensured by partially hydrophobic silica particles initially dispersed at 1 or 4 wt% in the aqueous phase. Emulsification is achieved using two mixers: a high-energy sonicator (∼109 W/m3) and a less energetic rotor-stator (∼107 W/m3). Regardless of the mixing device, silica particles adsorb at the O/W interfaces and generate stable emulsions over 2 months with a drop size that slightly decreases with increasing particle concentration. However, the sonicator leads to a finer disaggregation of the silica particles and to drops around 10 times smaller than with the rotor-stator but, surprisingly, an interface 3 orders of magnitude more concentrated in silica particles. The viscosity of the emulsions seems to be little dependent on the oil volume fraction contrary to particle content. In all cases, the emulsion produced by the sonicator are more viscous than those obtained by the rotor-stator. These results clearly indicate the importance of emulsification procedure on the characteristics of the final emulsions.