Biosurfactants for formulating nanoemulsions

Oil-in-water (O/W) nanoemulsions are an interesting way of formulating hydrophobic compounds as means to make them more easily available in an aqueous environment. In particular, the small size of their droplets (diameters of 50-400 nm), which is naturally coupled to a large interfacial area, makes active compounds contained in the oil phase easily accessible. However, for their stabilisation typically a surfactant is employed and for many applications of interest (pharmacy, cosmetics, food, agriculture, etc.) strict restrictions apply to the type of stabilising surfactant allowed. This naturally leads to exploring biosurfactants. Correspondingly the investigation of nanoemulsions formed with biosurfactants has received comprehensive attention in recent years. This review will summarise these research activities, grouping them largely along the lines of the type of surfactant employed, in which way the nanoemulsion was formed and what droplet sizes were obtained. In addition, we will in detail describe the type of emulsified oil, including further ingredients, which, on the one hand, is directly coupled to the intended application for this nanoemulsion, and on the other hand in many cases affects the interfacial structure and consequently the nanoemulsion properties. In summary, one can see that the multi-fold investigations in this area have already led to a comprehensively enhanced understanding, of the requirements to be met on the surfactant side in order to produce small, well-defined nanoemulsion droplets of sufficient kinetic stability, which is the prerequisite for developing nanoemulsions that are tailor-made for a given application.