Preferential solubilization behaviours and stability of some phenolic-bearing essential oils formulated in different microemulsion systems.

The solubilization behaviour of a number of essential oils (EOs) containing volatile phenolic constituents was investigated in five different micellar solutions. These oils include clove bud (Eugenia caryophyllata), thyme (Thymus serpyllum) and oregano (Thymus capitatus). Ternary and pseudo-ternary phase diagrams were constructed to assess the ability for microemulsion formation and dilutability of each system using non-ionic surfactants. Results showed that Tween 20 (T20) was more suitable to solubilize these oils compared with Tween 80 (T80). Clove EO was found to be easily microemulsifiable compared with the other EOs, whereas oregano showed the least tendency to form a microemulsion. Particle sizes measured at different dilution lines ranged between 5.9 and 16.9 nm. The chemical composition of each EO was revealed by gas chromatography and was correlated with the observed solubilization behaviour. The presence of solubilization enhancers like poly-ols and short-chain alcohols improved solubilization of all EOs; however, establishment of new dilution lines was controlled by EO type. Physical stability assessment showed that all microemulsions were stable against alternate freeze/thaw cycles which extended for 1 week. On the contrary, each system showed different temperature sensitivity in the thermal stress assessment. The results of this investigation can be useful in fabrication of thermodynamically stable aqueous system carrying aromatic and bioactive phenolics for different applications in personal hygiene, cosmetic, fragrance and pharmaceutical products.