Interestingly, the f-potential of Tween 20 emulsions was negative (around 10 mV) (Fig. 2) even though Tween 20 is a
non-ionic surfactant. It is thought that the negative surface charge
was due to adsorption of anionic species derived from the materials
used to make the nanoemulsions, such as free fatty acids
(COO) in oils or hydroxyl ions (OH) present in sodium hydroxide
solution when adjusting the pH of emulsion