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

In addition to common spherical non-ionic surfactant vesicles (niosomes), disc-like, tubular, and polyhedral niosomes have also been reported. The permeability and osmotic activity of niosomes are important in determining their use as controlled-release drug-delivery systems. These properties have been compared for polyhedral niosomes prepared by hydrating a mixture of a hexadecyl diglycerol ether (C16G2), a poly(24)oxyethylene cholesteryl ether (Solulan C24), 91:9 or 98:2, and conventional spherical niosomes prepared 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 counterparts and they are more permeable to the hydrophilic solute 5(6)-carboxyfluorescein. In 2 M NaCl the rate of release of carboxyfluorescein from polyhedral niosomes (both containing 9% Solulan C24) into either a hypotonic (water) or an isotonic medium (2 M NaCl) was low. This contrasted with similarly loaded spherical niosomes and polyhedral niosomes containing 2% Solulan C24, from which release was high in hypotonic media (e.g. water) but less in an isotonic medium (2 M NaCl). For both polyhedral and spherical niosomes encapsulating carboxyfluorescein (pKa = 6.4), release rates were higher at pH 8 than at pH 5. Polyhedral niosomes are thus, in general, less osmotically active than spherical niosomes because of their rigid but highly permeable membranes. The unusual polyhedral membrane impermeability to carboxyfluorescein co-entrapped 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, therefore, a property that might be manipulated in the design of a drug-delivery system.