Essential oil-in-water microemulsions for topical application: structural study, cytotoxic effect and insect repelling activity

Essential oils (EOs) extracted from aromatic plants have been well recognized for their insect repellent prop-erties. Although EOs are effective when freshly applied, their protective effect usually dissipates quickly due to low chemical stability and rapid volatilization of their ingredients. To address this issue, encapsulation in pro-tective carrier materials at the nanoscale is proposed. In this study, oil-in-water (O/W) microemulsions con-taining geraniol or Citrepel (R) 75 (citrepel) as the oil phase, a mixture of water and diethylene glycol monoethyl ether (DEGEE) as the aqueous phase and caprylocaproyl polyoxyl-8 glycerides as surfactant were formulated and structurally characterized. DLS analysis showed that the emulsified oil droplets had low polydispersity (PdI < 0.3) and average hydrodynamic diameter ranging from 13.7 +/- 0.7-35.2 +/- 2.7 nm, depending on composition. Mean viscosity of all prepared microemulsions was from 15.7 +/- 0.3 cP to 23.8 +/- 0.2 cP indicating the suitability of all preparations for topical application. EPR spectroscopy using spin probes gave information about the rigidity of the surfactants' monolayer. In all cases an outer compact configuration of the interfacial layer and a more flexible inner one was observed. SAXS measurements revealed significant changes in the low q values related to the impact of geraniol and citrepel in microemulsion formulations. Looking at the high q region of the spectra, the curves were affected by the presence of the compounds in a concentration depended manner. Cytotoxicity studies on normal human epidermal keratinocytes (NHKs) confirmed that the sub-toxic dosage for geraniol-in-water and citrepel-in-water microemulsions is 56.25 mu g active ingredient per cm2 of cell monolayer. Finally, diluted geraniol-in-water microemulsions were evaluated for repellence action against the Asian tiger mosquito (Aedes albopictus) showing activity for 3 h at a dose of 190 mu g/cm2 active ingredient. Gaining a comprehensive un-derstanding from all the results, promising candidates for topical application of geraniol as insect repellent based on safe microemulsions were proposed.