*3.2.2 Double emulsification method*

Multiple emulsion systems are capable of encapsulating both hydrophilic and lipophilic molecules simultaneously and can be obtained through the double emulsification method [20, 30, 31]. Depending on the established phase sequence, double emulsions can be water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). The preparation of nanocapsules using double emulsification involves a two-step emulsification process and the use of two stabilizers or surfactants. In fact, the key to ensuring good interphase stability and improving drug encapsulation and particle size is the correct selection and concentration of surfactants. As an example, based on w/o/w, a low hydrophilic-lipophilic balance (HLB) surfactant is needed to stabilize the w/o interface. In contrast, to stabilize the oil-in-water interface, a high HLB surfactant is required. Finally, particle hardening, or in other words, polymer shell formation, has been reported to be achieved by polymer precipitation, solvent diffusion, coacervation, or a combination of these strategies [5, 32].
