Moisturizing substances entrapped in W/O/W emulsions: Analytical methodology for formulation, stability and release studies

The purpose of this paper was to prove the efficiency of suitable tracer incorporation in the aqueous inner phase for multiple emulsion studies. The use of dihydralazine as tracer allowed the evaluation of both the yield of preparation and the stability of eight multiple emulsions. In these, concentrations of several components were optimized i.e. oil, lipophilic and hydrophilic emulsifiers, coemulsifier and viscosity increasing agent. From entrapment data and droplet breakdown curves, calculated from the dihydralazine chromatographic data, one emulsion was selected to study the release of some encapsulated moisturizing solutes such as urea, chitosan and polyethylene glycol. Dihydralazine proved of value to discriminate between the two mechanisms responsible for the solute release i.e. the breakdown of the multiple droplets and the diffusion of the solute through the oil layer. Diffusion curves were obtained from the levels of moisturizing substance in the outer aqueous phase, corrected by the amounts liberated by the droplet breakdown calculated through dihydralazine levels. The lipophilicity of the solutes measured under conditions similar to those partitionning in the emulsions correlated with the diffusional route of solute transfer. Thus, the partition coefficient was an efficient parameter to predict the importance and the rate of the diffusion.