(541f) Influence of Surfactant-Nanoparticle Interactions on the Stability and Rheology of Emulsions

Surfactant/nanoparticle interactions are expected to influence the surfactant/nanoparticle adsorption onto the liquid-liquid interface, which in turn would affect the structure and rheology of emulsions. We investigated the influence of non-ionic surfactant (Span 80) addition to Pickering emulsions stabilized by either partially hydrophobic (Aerosil R711) or hydrophobic (Aerosil R812S) nanoparticles under negligible shear conditions. In addition, we investigated the influence of a mixed surfactant/nanoparticle emulsifier system on the behavior of oil-in-oil (O/O) emulsions. Vegetable (soybean) oil and silicone oils (50 cSt and 100 cSt) were used to form oil-in-oil emulsions. Surfactants with varying HLB values (Span 80/Triton X-100) and hydrophobic silica nanoparticles were used as emulsifiers to form oil-in-oil emulsions.

Our results showed that the addition of a non-ionic surfactant to emulsions stabilized by hydrophobic nanoparticles decreased the droplet size of emulsions unlike emulsions stabilized by partially hydrophobic nanoparticles. Even under negligible shear conditions, the addition of a non-ionic surfactant displaced the hydrophobic nanoparticles from the oil-water interface unlike partially hydrophobic nanoparticles. Our results suggested that the displacement of hydrophobic nanoparticles by the addition of a non-ionic surfactant was not due to the change in nanoparticle wettability by surfactant adsorption onto the surface of nanoparticles. The viscosity of emulsions stabilized by partially hydrophobic nanoparticles decreased upon the addition of a non-ionic surfactant. In contrast, the viscosity of emulsions stabilized by hydrophobic nanoparticles increased upon the addition of a non-ionic surfactant. Depending on the nanoparticle wettability, the addition of a non-ionic surfactant influenced the elastic and viscous properties of Pickering emulsions.

Our results further showed that simultaneous emulsification using hydrophobic nanoparticles and a surfactant (either Span 80 or Triton X-100) resulted in a single-step formation of multiple oil-in-oil-in-oil (O/O/O) emulsions. Regardless of the initial phase to which the surfactant was added, simultaneous emulsification using surfactants and hydrophobic nanoparticles led to the formation of multiple emulsions. The addition of Span 80 to a vegetable oil-in-PDMS oil (VO/PDMS) emulsion stabilized by only hydrophobic nanoparticles did not have a significant influence on the droplet size. However, the addition of Triton X-100 to a VO/PDMS emulsion stabilized by only hydrophobic nanoparticles resulted in the formation of a multiple PDMS/VO/PDMS emulsion. Multiple O/O/O emulsions had lower viscosity compared to simple O/O emulsions. Simple O/O emulsions had a lower degree of shear thinning behavior compared to multiple O/O/O emulsions.