Properties and stability of nanoemulsions: How relevant is the type of surfactant?

Nanoemulsions represent one of the most promising colloidal systems for the vehiculation and delivery of hydrophobic molecules, exploitable in several fields such as pharmaceutical, cosmetic, food and agricultural. In this study, we investigated the influence of the formulation parameters, in particular the type of surfactant, on the physico-chemical properties of oil in water nanoemulsions. Six different surfactants were selected based on the similar HLB value (around 15) but different chemical features, such as linear and steric hindrance molecules, PEGylated and PEG-free compounds or the presence of saturated or unsaturated fatty tails. All the surfactants were preliminary characterized in term of surface and interface tension and cytotoxicity. Nanoemulsions were then obtained through a high-pressure homogenization process and evaluated in terms of physico-chemical properties and stability (180 days). The results showed no remarkable differences in the mean diameter of nanoemulsions prepared with the six tested surfactants, suggesting that the size of the oil droplets is mainly dependent by the action of the disruptive forces generated during the homogenization process. However, the nanoemulsions stability was strongly affected by the used surfactants, varying from few weeks up to more than six months. Specifically, the polyglycerol and glucoside derived amphiphiles were the less effective while polysorbate-80 and oleth-20 were able to give optimal nanosystems in terms of both physico-chemical properties and thermodynamic stability. Interestingly, despite polysorbate 80 and oleth 20 showed comparable efficacy, their safety profiles were markedly different. Overall, it has been observed that the surfactant ability to reduce the tension at the O/W interface is closely related to the stability of the nanoemulsions, allowing the prediction of their medium or long-term stability.