Essential oil-based nano-emulsions: Effect of different surfactants, sonication and plant species on physicochemical characteristics

Essential oils (EOs) are promising active ingredients for biopesticides, although their use under field conditions is limited by several criticisms concerning their high volatility and degradability. To overcome these negative qualities, EOs can be encapsulated inside nanostructures (i.e. nanoparticles and nano-emulsions), which can guarantee the preservation of the insecticidal properties. In the current study, oil in water (O/W) nano-emulsions of seven commercial EOs (15 % w/w of anise, artemisia, fennel, lavender, peppermint, rosemary, sage) were developed using different non-ionic surfactants and formulation processes, to identify the best possible surfactant/process to produce stable nano-formulations. The EOs were firstly examined by gas-chromatography analyses to identify their chemical constituents. Sage, rosemary, peppermint, lavender and artemisia EOs were characterized by high percentage (up to 50 % of the detected compounds) of oxygenated monoterpenes, while fennel and anise EOs were mainly constituted by phenylpropenes (e.g. anethol). Then, nano-emulsions were developed via the self-emulsifying process alone or in combination with sonication, using four surfactants with different Hydrophilic Lipophilic Balance (HLB) index (5 % w/w of Tween 20, Tween 80, Span 20 or Span 80). The physical characteristics (droplet size and surface charge) of nano-emulsions were analyzed using the dynamic light-scattering technique. Sonicated nano-formulations presented smaller and more homogeneous size of the micelles than the non-sonicated ones, resulting in more stable nano-emulsions. Furthermore, usually emulsions produced using Tween 80 as surfactant gave the best results in terms of droplet size and polydispersity index (PDI) values. Therefore, Tween 80 sonicated nano-emulsions were examined during a storage period of 28 weeks to determine their stability over time and possible alteration of their physical characteristics. Results suggest that these nano-formulations had a good stability over time, since relatively small increases in PDI and size values were recorded. Formulation stability is a key issue to consider when proposing botanical biopesticides for agricultural applications. Our study reports the first step in the introduction of EO-based nano-emulsions into practical application.