Water-in-Oil Nano-Emulsions Prepared by Spontaneous Emulsification: New Insights on the Formulation Process

Nano-emulsions consist of stable suspensions of nano-scaled droplets that have huge loading capacities and are formulated with safe compounds. For these reasons, a large number of studies have described the potential uses of nano-emulsions, focusing on various aspects such as formulation processes, loading capabilities, and surface modifications. These studies typically concern direct nano-emulsions (i.e., oil-in-water), whereas studies on reverse nano-emulsions (i.e., water-in-oil) remain anecdotal. However, reverse nano-emulsion technology is very promising (e.g., as an alternative to liposome technology) for the development of drug delivery systems that encapsulate hydrophilic compounds within double droplets. The spontaneous emulsification process has the added advantages of optimization of the energetic yield, potential for industrial scale-up, improved loading capabilities, and preservation of fragile compounds targeted for encapsulation. In this study, we propose a detailed investigation of the processes and formulation parameters involved in the spontaneous nano-emulsification that produces water-in-oil nano-emulsions. The following details were addressed: (i) the order of mixing of the different compounds (method A and method B), (ii) mixing rates, (iii) amount of surfactants, (iv) type and mixture of surfactants, (v) amount of dispersed phase, and (vi) influence of the nature of the oil. The results emphasized the effects of the formulation parameters (e.g., the volume fraction of the dispersed phase, nature or concentration of surfactant, or nature of the oil) on the nature and properties of the nano-emulsions formed