Emulsification synergism in mixtures of polyelectrolyte brush-grafted nanoparticles and surfactants

Silica nanoparticles displaying densely grafted poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) polycationic brushes are highly efficient emulsifiers, stabilizing oil-in-water emulsions at concentrations as low as 0.05 to 0.1 wt%. Adding conventional surfactants is expected to alter emulsification efficiency by interacting with grafted nanoparticles in bulk and/or at the oil/water interface.

Emulsification efficiency was studied in the presence of three different surfactants, namely the anionic surfactant sodium dodecyl sulfate, the nonionic water-soluble surfactant Triton X-100 and the nonionic, oil-soluble surfactant Span 85. SDS complexation with PDMAEMA-grafted nanoparticles was monitored by electrophoretic mobility and optical density changes.

Even though SDS is a poor emulsifier, emulsification was promoted by SDS at low concentrations, where complexation with PDMAEMA-grafted particles produced a synergistic effect on emulsification efficiency. Increasing the SDS concentration did not enhance emulsification, and stable emulsions formed in the absence of SDS were broken by its addition. The enhancement of emulsification efficiency with dilute SDS correlated with low degrees of complexation with PDMAEMA such that the nanoparticle-grafted brush had a lower charge density and more of an amphiphilic nature. The hindrance of emulsification at higher SDS concentrations was attributed to more extensive complexation and the resulting brush charge reversal that would inhibit adsorption at the oil/water interface in the presence of adsorbing SDS. Synergism was also observed in the presence of Triton X-100, while Span 85 had no discernible effect on emulsification efficiency, although it is noted that only a single concentration of the latter surfactant was investigated.