On the formation, physicochemical properties and antibacterial activity of colloidal systems containing tea tree (Melaleuca alternifolia) oil

Abstract In this study, we investigated the influence of tea tree oil (TTO) on the formation and physicochemical properties of colloidal systems stabilized by the nonionic surfactant polysorbate 80. These systems were prepared by spontaneous emulsification with quantities of TTO ranging from 0% to 0.5% w/w, at a fixed surfactant concentration (2% w/w). The dispersed structures usually found in microemulsions (oil-swollen micelles) and emulsions (oil droplets) were produced under these experimental conditions. The relative contribution of the oil-swollen micelles (∼11 nm) to the overall scattered intensity decreased as a function of TTO concentrations, while an opposite behavior was observed for the oil droplets (∼275 nm). Such variations led to significant increases in the z-average particle size and turbidity of the TTO containing samples. Additional investigations also revealed that the surface charge and fluidity of oil-swollen micelles and oil-droplets are very similar. Although their sizes remained practically unchanged over the storage time, the oil droplets were considered to be physically unstable since they undergo a gradual transition to oil-swollen micelles. Finally, in vitro susceptibility studies with TTO containing colloidal systems showed that the encapsulation of this essential oil with polysorbate 80 did not improve its antimicrobial activity. This effect has been attributed to the electrostatic repulsion between the dispersed structures and the bacterial outer membrane as well as a greater tendency of essential oil molecules to remain dissolved in the hydrophobic core of colloidal systems.