Transmission electron microscopy and light scattering studies on the interaction of a nonionic/anionic surfactant...

The interaction of an equimolecular mixture of nonylphenol polyethoxylated [NP(EO)10] and sodium dodecyl sulfate (SDS) surfactants with phosphatidylcholine (PC) liposomes was studied by means of transmission electron microscopy (TEM) and changes in the mean particle size (quasielastic light scattering; QELS) and in the static light scattering (SLS) of the system during liposome solubilization. A good correlation was found between the TEM diameter of particles and the mean hydrodynamic diameter (HD) determined by QELS. The aggregates resulting in this interaction were dependent on the surfactant concentration in the system. Thus, an initial vesicle growth occurred when the surfactant concentration was 15.98 mol%, together with the formation of a very small percentage of smaller particles. Additional surfactant amounts (28.32 mol%) led first to the formation of largest vesicles (HD 418 nm) and second to a fall in the vesicle diameter and in the SLS of the system. Thus, for 38.27 mol%, the TEM picture still showed the presence of vesicles, albeit with signs of disintegration. When additional amounts of surfactant were added to the system, the size curve started to show a bimodal distribution. Thus, for 51.81 mol% surfactant concentration, a sharp curve appeared at 51 nm, corresponding to the formation of small particles and TEM pictures clearly showed vesicle disintegration with formation of tubular structures. It is noteworthy that additional surfactant amounts (from 52 to 60 mol%) led to the formation of unclosed multilayered structures together with smaller aggregates. The gradual increase in the proportion of these smaller aggregates (mixed micelles) led to the complete solubilization of liposomes.