Colloid chemical stability and interfacial properties of mixed phospholipid-non-ionic surfactant stabilised oil-in-water emulsions

Phospholipid-stabilised emulsions are known to have a poor stability when prepared in the presence of electrolytes. In order to try to circumvent this problem, submicron-sized 5% o/w emulsions were prepared in the presence of up to 500 mM KCl or up to 50 mM CaCl2 using either only steric stabilisers or a combination of steric stabilisers and phospholipids. Steric surfactants are believed to provide a steric repulsion barrier around the emulsion droplets, and as such should withstand electrolyte addition better than the mainly electrostatically-stabilised phospholipid emulsions. The emulsion stability was investigated by measuring the particle size of the emulsion droplets as a function of time by photon correlation spectroscopy. Emulsions stabilised only by 0.5% (w/v) of the steric surfactants were completely stable in the presence of electrolytes. It was also shown that the addition of even low concentrations of the steric surfactants (0.025% to 0.05%) indeed dramatically improved the stability of phospholipid-stabilised emulsions in the presence of electrolytes. I n order to investigate this behaviour more fundamentally the thickness of the polymer layer adsorbed on the emulsion droplet surface was determined by viscosity and by zeta-potential measurements. Subsequently, these data were used in quantitative models based on the DLVO theory for the electrostatic interactions and on the equation of Ottewill and Walker for the steric repulsion contribution. Keeping account of the experimentally determined adsorbed layer thickness, particle size and surface potential, which was deduced from the zeta-potential, total interaction energy diagrams of the emulsions were drawn. These models allowed us to make theoretical predications about the emulsion stability, and the conclusions regarding emulsion stability agreed with the experimental results. (C) 1999 Elsevier Science B.V. All lights reserved.