Vesicle (niosome)-in-water-in-oil (v/w/o) emulsions: an in vitro study

Abstract A new vesicular formulation is described in which non-ionic surfactant vesicles (niosomes) are dispersed in an aqucous phase which is then emulsified in an non-aquceus continuous phase. The resultant vesicle-in-water-in-oil (v/w/o) system allows the delivery of vesicles in a non-aqueous vehicle. The formulation stability and characteristics of several v/w/o systems are described. The non-ionic surfactants used to prepare the vesicles (niosomes) are also employed in the emulsification step to minimize surfactant redistribution. The relation between the HLB of the non-ionic surfactant used for preparation of the v/w/o systems and the in vitro release of 5(6)-carboxyfluorescein (CF) from the vesicles in the system was investigated controls being watcr-in-oil (w/o) emulsions and a vesicle suspension. A range of v/w/o systems was prepared from niosomes made from non-ionic surfactants (sorbitan monoesters Span 20, 40, 60 and 80) in the size range 600 nm—3.4 μm dispersed in water droplets of around 5–25 μm themselves dispersed in an oil (octane hexadecane isopropyl myristate). The in vitro release rate of CF showed a decrease in the order free solution > vesicle suspension > w/o emulsion > v/w/o emulsion. The rate of release of CF entrapped in the vesicles in the v/w/o system depends on the nature of the surfactants used. The hydrophobicity of the Span surfactant for the preparation of vesicles and the v/w/o emulsion had a significant influence on the release rate. In the Span 60 formulation the release rate was the slowest because Span 60 has the highest phase transition temperature and the v/w/o formulation gelled at both 25 and 37°C. The nature of the oil phase affected release as might be expected from the partitioning behaviour of CF. With increasing temperature release rates increased in Span 40 and Span 80 systems but were unaffected in the Span 60 system due to the maintenance of the gel phase.