Formation and stability of nano-emulsions in mixed nonionic surfactant systems

The formation of nano-emulsions has been studied in water/ mixed nonionic surfactant/oil systems using two emulsification methods. In one method, the composition was kept constant and the temperature was changed (phase-inversion temperature, PIT, method), while in the other method, water was added dropwise to a solution of the mixed surfactants in oil at constant temperature (method B). The droplet size and stability were determined as a function of surfactant mixing ratio, W₁, at 25 °C. The droplet size of nano-emulsions obtained by the PIT method is practically independent of W₁ and falls in the range 60-80 nm. In contrast, the droplet size of nano-emulsions prepared by method B, is highly dependent on W₁ and varies between 60 and 300 nm. At W₁ values where the PIT or the hydrophile—lipophile balance temperature (Thlb) of the system is close to 25 °C, the droplet sizes of the nano-emulsions are similar for both emulsification methods. There are three equilibrium phases of the latter compositions: an aqueous micellar solution or oil-in-water microemulsion (W_m), a lamellar liquid-crystalline phase and an oil phase (O) in addition, these nano-emulsions showed higher kinetic stability than those with lower W₁ values (higher Thlb) and consisting of two liquid phases (W_m + O).