Formulation of Glycerol-Based Emulsion Containing Myrica gale L. Essential Oil for Enhanced Repellent Activity

Background/Objectives: Glycerol-based emulsions containing Myrica gale L. essential oils (EOs) were developed and evaluated for stability, volatilisation behaviour and in vivo mosquito repellent activity. Preliminary GC-MS characterisation of the oils demonstrated complex chemical profiles comprised mainly of a mixture of mono- and sesquiterpenes. Methods: Emulsions containing 10% oil were prepared at two surfactant concentrations (2.5 and 5%) and differing ratios of glycerol to water as the non-polar phase. The optimal system was determined to be 10% oil and 2.5% surfactant (i.e., cetearyl alcohol and cetearyl glucoside) in 100% glycerol as assessed over a 91-day storage period by changes in particle size, polydispersity and zeta potential. This formulation approach was subsequently applied to three proven insect repellent M. gale EOs. Volatilisation behaviour was investigated using headspace GC-MS analysis of selected marker compounds (α-pinene, camphene, β-pinene, myrcene, α-phellandrene, 3-carene, p-cymene, γ-terpinene). Results:M. gale-based emulsions demonstrated significantly improved volatilisation over time, compared with the EO coarsely dispersed in a lotion base. This enhanced partitioning behaviour correlated well with increased protection times observed in an arm-in-cage repellent assay against Aedes aegypti mosquitos. It was found that all formulated M.gale EOs, F_EO15M (CPT = 40.05 min, p = 0.032), F_EO21C (CPT = 42.023 min, p = 0.01) and F_EO15C (CPT = 99.478 min, p < 0.0001), performed significantly better than their non-formulated counterparts EO15M (CPT = 11.45 min), EO21C (CPT = 13.053 min) and EO15C (CPT = 33.02 min). Conclusion: This study describes for the first time an effective, topical M. gale-loaded emulsion for the improvement of insect repellent activity.