Effect of pH and electrolytes on the colloidal stability of stearic acid–based lipid nanoparticles

Stearic acid–based solid lipid nanoparticles, stabilised with Tween® 20, were synthesised using a microwave-assisted microemulsion technique and their stability was tested in simulated blood plasma, several liquids mimicking gastrointestinal fluids and solutions containing different electrolyte compositions and pH levels. It was discovered that simulated blood plasma had a stabilising effect on the particles, as did simulated saliva and intestinal fluid, which was due to hydration forces, facilitated by the presence of hydrated cations. It was determined that the hydration stabilisation was pH dependent, with highly acidic conditions depriving the solid lipid nanoparticles of the negative charge required for the cations to hydrate the surface. In environments lacking hydrated cations, the particles were predominately reliant on steric stabilisation and were particularly susceptible to pH extremes due to hydrolysis/oxidation of the Tween® 20 layer. The results suggest that the SLNs have potential as a systemic drug carrier, as the physicochemical conditions of blood provide a stabilising environment that inhibits particle growth.