Oil-in-water emulsions with triacylglycerides are interesting for many applications, e.g. in cosmetics, pharmacy or foods. However, the formulation of emulsions of sufficient stability can be a major challenge and even more so when using triacylglyceride. One way to address this challenge are nanoemulsions, which have diameters of 50-400 nm and are typically kinetically rather stable. Their high surface/volume ratio gives the additional advantage of fast and controlled release of active ingredients contained in their hydrophobic core.
In this work, we studied a wider range of bioamphiphiles, such as oleate, cocoyl taurate, saponin, rhamnolipid, lecithin and Tween80 with respect to their ability to function as stabilisers for nanoemulsions. We determined the interfacial tension (IFT), as important parameter for describing solubilisation properties, against caprylic/capric triglyceride and paraffine oil as a function of surfactant concentration and for surfactant mixtures, where marked synergistic effects in IFT reduction were observed, especially for nonionic and ionic surfactant mixtures. Different preparation methods were compared, such as vortexing, ultra turrax and sonication, where sonication was found to be best for forming small and long-time stable nanoemulsions. The nanoemulsions were characterised with respect to their size and ageing behaviour by means of light scattering experiments and complementary small-angle neutron scattering (SANS). The size of stable droplets was in the range of 100 to 150 nm, little depending on surfactant type and total concentration, and basically determined by the preparation method. The main difference between the different surfactants is the surfactant/oil ratio required for forming long-time stable nanoemulsions, where in particular cocoyltaurate and rhamnolipid performed well. In summary, this means that droplet size and stability of such nanoemulsions can be tailored by appropriate choice of the preparation method and type of stabilising biosurfactant, which then leads to long-time stable nanoemulsions that could be employed in a variety of applications.
