Osmotic behaviour of polyhedral non-ionic surfactant vesicles (niosomes)

In addition to common spherical non-ionic surfactant vesicles (niosomes), d isc-like, tubular, and polyhedral niosomes have also been reported. The per meability and osmotic activity of niosomes are important in determining the ir use as controlled-release drug-delivery systems. These properties have b een compared for polyhedral niosomes prepared by hydrating a mixture of a h exadecyl diglycerol ether (C(16)G(2)), a poly(24)oxyethylene cholesteryl et her (Solulan C24), 91:9 or 98:2, and conventional spherical niosomes prepar ed from the same surfactants but with cholesterol. When subjected to osmotic gradients, polyhedral niosomes, the membranes of which are in the gel phase, swell and shrink less than their spherical coun terparts and they are more permeable to the hydrophilic solute 5(6)-carboxy fluorescein. In 2 M NaCl the rate of release of carboxyfluorescein from pol yhedral niosomes (both containing 9% Solulan C24) into either a hypotonic ( water) or an isotonic medium (2 M NaCl) was low. This contrasted with simil arly loaded spherical niosomes and polyhedral niosomes containing 2% Solula n C24, from which release was high in hypotonic media (e.g. water) but less in an isotonic medium (2M NaCl). For both polyhedral and spherical niosome s encapsulating carboxyfluorescein (pK(a) = 6.4), release rates were higher at pH 8 than at pH 5. Polyhedral niosomes are thus, in general, less osmotically active than sphe rical niosomes because of their rigid but highly permeable membranes. The u nusual polyhedral membrane impermeability to carboxyfluorescein co-entrappe d with salt in hypotonic media is a function of Solulan C24 content, and is possibly a result of salting out of the polyoxyethylene chains; this is, t herefore, a property that might be manipulated in the design of a drug-deli very system.