Some rheological properties of nonionic surfactant vesicles and the determination of surface hydration

Rheological studies of dilute aqueous nonionic surfactant vesicle (niosome) dispersions formed mainly from hexadecyl diglycerol ether (C(16)G(2)) or sorbitan monostearate (Span 60) were performed by capillary viscometry. By variation of the ratio of C(16)G(2), cholesterol, and a poly-24-oxyethylene cholesteryl ether (Solulan C24), vesicles with either polyhedral or mainly spherical structures can be formed. Polyhedral niosomes transform to spherical vesicles above a transition temperature of 45 degrees C, while cholesterol-rich spherical/tubular niosomes remain intact up to 80 degrees C. These changes in niosome morphology are reflected in their rheological properties. The relative viscosity (eta(rel)) Of spherical/tubular niosome dispersions changes little with increase in temperature, while that of polyhedral niosome dispersions decreases dramatically, indicating the transformation of the vesicles to a more spherical shape. As the intrinsic viscosity, [eta], of colloidal dispersions is affected not only by vesicle shape but also by surface hydration, it is possible to make estimates of hydration. The increase in viscosity with the increase in the amount of the hydrophilic Solulan C24 in the vesicle surface is a reflection of increased hydration. However, the effect of size complicates interpretation; increase in vesicle size between 270 nm and 8.8 mu m reduces the viscosity of the system. interpretation of the intrinsic viscosity data depends to a large extent on the estimation of phi, the volume fraction occupied by the vesicles, because of internal hydration. Results are consistent with surface hydration in the range between 2 and 2.8 g g(-1) for niosomes containing 10% Solulan C24 at 25 degrees C.