Characterisation of microemulsions containing orange oil with water and propylene glycol as hydrophilic components

The current study aims to investigate the effect of incorporation of orange oil, mainly consisting of the cyclic mono-terpene linolene, a known skin penetration enhancer, as oil component on microemulsion formation both in water and propylene glycol containing systems. Phase diagrams of pseudoternary mixtures containing orange oil, ethyloleate or a 1:1 mixture (w/w) of orange oil and ethyloleate as oil components, a 6:4 (w/w) mixture of polyoxyethylene 20 sorbitan monooleate and sorbitan monolaurate as surfactant components and water or propylene glycol as hydrophilic components were investigated. Smaller microemulsion regions were observed when orange oil was used as a substitute for ethyloleate in both water and propylene glycol containing systems. Polarising light microscopy, viscosity measurements, electrical conductivity measurements and cryo-field emission scanning electron microscopy were used to identify structural features of the microemulsions. Solution-type, w/o droplet-type microemulsions and microemulsion areas containing liquid crystals were found in varying areas in the phase diagrams of water containing systems. Liquid crystals formation occurs when the water concentration reaches 20%-22.5% (w/w). Only solution-type microemulsions were observed in propylene glycol containing systems. The dimension of solution-type microemulsion areas in the phase diagrams is likely to depend on the miscibility of components and larger microemulsion areas were found when ethyloleate was used instead of orange oil and propylene glycol was used instead of water. W/o droplet-type microemulsions of systems containing orange oil and ethyloleate as oil components appear in different areas of the phase diagrams. Incorporation of orange oil as a penetration enhancer into a topical microemulsion affects its physical characteristics. This in turn may lead to instability of the microemulsion and/or can influence the release patterns of drugs from these microemulsions when applied as topical formulations.