Breakdown mechanisms of oil-in-water emulsions stabilised with Pluronic F127 and co-surfactants

Pluronic F127 is an amphiphilic, water-soluble triblock copolymer with structure EO100PO69EO100. Oil-in-water (1:1 by volume) emulsions were stabilised with 3.5 mM F127 together with 10 mM C12E4 or 50 mM CTAB or 60 mM SDS. Emulsion stability was evaluated as a function of oven ageing at 60 °C. This was done by tracking changes in the droplet size distribution and in the shear viscosity as a function of time. The particle size distributions started out as bimodal. All the emulsions featured very high initial viscosities and strong shear thinning behaviour. The emulsion prepared with 10 mM C12E4 as cosurfactant was stable. The particle size distribution did not change even after 48 days oven ageing at 60 °C. The other two emulsion broke down over time due to coalescence. The droplet size distributions shifted to larger diameters. The shift was gradual for the SDS-based emulsion suggesting droplet coalescence via binary droplet interactions. In contrast, on ageing the CTAB emulsion developed an additional population of significantly larger droplets. This is consistent with a coalescence mechanism entailing local break down of large droplet aggregates to yield much larger droplets. The observed viscosity vs. shear rate trends and the effect of ageing on the emulsion viscosity support this interpretation.