Investigating sterically-stabilised pH-responsive Pickering emulsifiers as microcapsule membranes

In this study, polystyrene latex particles sterically stabilised using a diblock copolymer; poly(methyl methacrylate – block - poly(2-dimethylaminoethyl methacrylate) (pMMA-b-pDMAEMA) are investigated for use as emulsifiers, their adsorption to an oil-water interface, their effect on droplet size control via rotary membrane emulsification and fabrication of smart colloidosome microcapsules. The particles are synthesized using emulsion polymerisation and the particle size can be controlled by changing the polymer block length and reaction temperature.
Emulsion studies using these latex particles show that both the pH and electrolyte concentrations affect emulsion stability to oil coalescence. At high pH’s, stable emulsions are formed due to the affinity of the particles to the interface. At low pH, protonation of the amine groups reduces the affinity and thus coalescence is observed. Increasing the electrolyte concentration improves emulsion stability but causes an increase in droplet size due to adsorption of flocculated/aggregated particles.
The solid-stabilised emulsions are used as a template to produce colloidosome microcapsules. The pDMAEMA chains on the particle surface are cross-linked producing a robust capsule shell. The oil core is removed and it is demonstrated that the membrane shell, expands and contracts in response to changes in environmental pH. Furthermore it is shown that the microcapsules can be used for the retention and release of ‘model’ dextran molecules.
Rotational membrane emulsification was employed to produce emulsion droplets with controlled sizes, stabilised by silica and pH-responsive latex particles. It was found that changes in the droplet size due to variations in the processing parameters could be related back to the kinetics involved in particle adsorption. Tensiometry experiments were performed to probe adsorption kinetics by measuring changes in dynamic interfacial tension. It was found that that bare particles do not change the tension, whereas with core-shell particles the change is pH dependent.