Multi-modal stabilisation of emulsions using a combination of hydrophilic particles and an amino acid

Emulsions have been formed using non-traditional particle stabilisers: hydrophilic silica particles which themselves are stabilised in suspension by L-lysine. The particles are produced using an adapted recipe for the production of monodisperse nanoparticles (NPs) of small size (15 nm), with L-lysine used as a stabiliser and catalyst during particle synthesis - and are termed SiO2@Lysine. The resultant SiO2@Lysine suspensions (particles and residual L-lysine) are used to study the pH-dependent stabilisation of hexadecane-in-water emulsions. Our results show that it is possible to stabilise hexadecane-in-water emulsions using these 15 nm SiO2@Lysine NPs when the pH of the system is fixed either at acidic pH (e.g. pH = 2.5) or alkaline pH (e.g. pH = 9.0). At high pH, the experimental evidence indicates that L-lysine acts as the primary stabiliser. In contrast, at low pH, silica nanoparticles can stabilise an oil-in-water emulsion, apparently without the aid of the free L-lysine molecules. This multi-modal action of the silica/amino acid suspension has produced an emulsion that is stable at extreme pH values and unstable at intermediate pH values, and one that is a Pickering emulsion at low pH and a traditional emulsion at high pH. Such divergent properties and behaviour may be of relevance for food or pharmaceutical applications.