At. Florence et al., Some rheological properties of nonionic surfactant vesicles and the determination of surface hydration, J PHYS CH B, 103(11), 1999, pp. 1995-2000
Rheological studies of dilute aqueous nonionic surfactant vesicle (niosome)
dispersions formed mainly from hexadecyl diglycerol ether (C(16)G(2)) or s
orbitan monostearate (Span 60) were performed by capillary viscometry. By v
ariation 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 spheri
cal vesicles above a transition temperature of 45 degrees C, while choleste
rol-rich spherical/tubular niosomes remain intact up to 80 degrees C. These
changes in niosome morphology are reflected in their rheological propertie
s. The relative viscosity (eta(rel)) Of spherical/tubular niosome dispersio
ns changes little with increase in temperature, while that of polyhedral ni
osome 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 incre
ase 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 b
etween 270 nm and 8.8 mu m reduces the viscosity of the system. interpretat
ion of the intrinsic viscosity data depends to a large extent on the estima
tion of phi, the volume fraction occupied by the vesicles, because of inter
nal hydration. Results are consistent with surface hydration in the range b
etween 2 and 2.8 g g(-1) for niosomes containing 10% Solulan C24 at 25 degr
ees C.