Surface pressure measurements in particle interaction and stability studies of poly(lactic acid) nanoparticles

Stability of nanoparticle dispersion in different environments is one key issue in determining the performance and safety of the drug delivery system in question. In this study, aggregation tendency and particle–particle interactions of poly(lactic acid) nanoparticles were evaluated by their interfacial behavior upon compression. Surface pressure versus trough area (π vs. A) isotherms of the nanoparticles were registered on different subphases (pH, electrolyte concentration). The compressed particle populations were transferred to silica plates by Langmuir–Schaefer deposition and analyzed with scanning electron microscope. Aggregation of the electrostatically stabilized surfactant-free nanoparticles due to subphase alterations was clearly detected from the isotherms even though zeta potential value of the nanoparticles (−35 mV) suggested a stable system. When steric stabilization, provided by a surfactant (Poloxamer 188) in this study, was involved besides the electrostatic stabilization, the nanoparticles remained non-aggregated over a wider range of conditions. Steric stabilization together with electrostatic stabilization extended the repulsion to a longer distance.