Nanoparticle encapsulation of emulsion droplets

Abstract

The overall aim of this study is to coat emulsion droplets with nanoparticles using a simple heterocoagulation process in aqueous dispersion and determine: the adsorption behavior and interfacial layer microstructure, droplet physical stability against flocculation and coalescence, and the release profile of a model lipophilic molecule (dibutylphtalate (DBP)) from within the droplets. Polydimethylsiloxane (PDMS) droplets were used as a model emulsion due to their colloidal stability in the absence of added stabilisers. Aerosil™ type silica nanoparticles with different hydrophobicity levels were used as the model nanoparticles. The adsorption behavior of silica nanoparticles at the droplet–water interface was studied using adsorption isotherms and SEM imaging. Adsorption of hydrophilic nanoparticles is weakly influenced by pH, but significantly influenced by salt addition, whereas for hydrophobically modified nanoparticles a balance of hydrophobic and electrostatic forces controls adsorption over a wide range of pH and salt concentrations. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Adsorbed layers of hydrophilic nanoparticles introduced a barrier to coalescence of ∼1 kT and form kinetically unstable droplet networks at high salt concentrations. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics. Significant sustained release of DBP can be achieved using rigid layers of hydrophobic silica nanoparticles at the interface. Activation energies for release are in the range 580–630 kJ mol −1, 10 times higher than for barriers introduced by Pluronic ® stabilisers.