Partitioning Effect in Formulated Products

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It is well accepted that the anti-microbial activity resides in the aqueous phase of a preserved two phase system and is therefore dependant on the equilibrium concentration of the preservative in this phase. It is expected that tea tree oil will partition between the oil and water phases present in a two component system in accordance with the partition coefficient and the relative ratio of oil and water in the system. To estimate the likely concentration in the aqueous phase of a simple two component system, the partitioning behaviour of tea tree oil between water and Crodamol GTCC (Caprylic/ Capric triglycerides, a typical oily component of cream formulations) was investigated. The effect of surfactant concentration was also noted.

Initial aqueous solutions were prepared using polyoxyl 35 castor oil. The ratio of surfactant to tea tree oil varied between 1:1 and 1:3 on a weight basis and no surfactant as a control. The solubilised mixtures were mixed with an equal volume of Crodamol GTCC and the final concentration of tea tree oil in the mixture was 0.5%. The mixtures were mixed on a vortex mixture and allowed to equilibrate over 3 days with occasional shaking. The distribution of tea tree oil in the aqueous phase was determined by GC assay of terpinen-4-ol in the aqueous phase after centrifugation. The results were as follows:

Tea tree oil concentration With surfactant Without surfactant

With no surfactant present nearly all the tea tree oil had migrated into the oil phase. In the presence of the surfactant the concentration of the tea tree oil in the aqueous phase remains at about 0.4%. The partitioning appeared to be independent of the concentration of the surfactant in the range studied. This has important implications for formulated products. It is thus essential that appropriate microbial evaluation be undertaken for all tea tree oil products. It is recommended that specific organisms be used depending on the use of the product e.g. Propionibacterium acnes and Candida albicans for acne and antifungal products respectively.

PACKAGING

Perhaps as important as microbial evaluation is the need for stability testing on the final formulation in the proposed pack for marketing. Glass, whenever possible is the most appropriate for tea tree oil products. In low-density polyethylene (LDPE) a loss of terpinen-4-ol occurs at room temperature after 3 months. At 45°C after 3 months, the terpinen-4-ol is barely detected. Even with low concentrations of tea tree oil the oil migrates through LDPE walls and the solvent attacks the external features on the container such as the label print.

Initial Final

High-density polyethylene has been reported to be satisfactory for products containing around 20-25% tea tree oil. The maximum concentration acceptable will be a function of the nature of the product and the wall thickness of the plastic. Deformation has also been reported due to reactions between tea tree oil and plasticising resins. If plastic caps are employed it is important to use impenetrable liners.

STABILITY The Pure Oil

Tea tree oil is reasonably stable at room temperature when stored in brown glass or stainless steel. It is however sensitive to heat, light and air. On storage there is a drop in terpinen-4-ol, a-terpinene and 7-terpinene and an increase in _p-cymene. High levels of this compound can indicate poor storage, old oil or bad extraction techniques. The use of antioxidants in long term storage should be considered.

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