Nonionic Surfactant Vesicles: A Study of Vesicle Formation, Characterization, and Stability

Abstract

The aggregation state of monoalkyl polyoxyethylene ether surfactants was investigated. This study mainly focused on the aggregation of surfactant molecules into vesicular structures, which can be used as drug delivery systems. The physicochemical parameters that play a role in vesicle formation were determined by varying both the hydrophobic and the hydrophilic moiety of the amphiphilic molecules systematically. A wide range of these nonionic surfactants was studied. The alkyl chain length was varied between 10 and 18 carbon atoms, and the length of the polyoxyethylene head group was varied from 3 to 7 ethyleneoxide units. It appeared that stable vesicle suspensions could only be obtained using surfactants that form liquid state bilayers, and contain a relatively small head group compared to the alkyl chain. These surfactants exhibit a lamellar phase in the water rich regions of the water/surfactant phase diagram. If the head group were too large, micelles instead of vesicles were formed. However, after addition of 40 mol% cholesterol vesicle formation was possible for all surfactants used. It appeared that the size of the vesicles is dependent on the preparation method, but can also be controlled by shaking the suspension vigorously. The nonionic surfactant vesicles were stable at room temperature for a period of at least two months. Electrolytes such as NaCl and phosphate appeared to have an effect on the bilayer stacking of the vesicles.