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Chemical Reaction: Embracing Phase Diagrams
By: Steve Herman
Posted: December 5, 2006, from the December 2006 issue of GCI Magazine.
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Determining the structures requires tests, such as X-ray diffraction or birefringence, which appears in materials that are anisotropic, meaning directionally dependent. It can be observed in a polarizing microscope—between the crossed polarizers, a birefringent sample will appear bright against a dark (isotropic) background. X-ray diffraction can quantify the distance between layers in a lamellar region.
In its company literature(3), Gattefossé showed how microemulsions can be formulated in a systematic way, using titrations and visual inspection to create a relevant phase diagram. Microemulsions are complicated systems, needing water, oil, a surfactant and cosurfactant to achieve the desired results. This requires four components, but a triangle has three sides. The system is properly a three dimensional pyramidal structure. To make a manageable experiment, the surfactant/cosurfactant ratio is fixed, which has the effect of taking a slice out of the pyramid, thus returning to a triangle—called a pseudoternary diagram.
The surfactant/cosurfactant blend is added to the oil, as water is slowly titrated in with mixing. Some areas will be hazy, but the microemulsion areas will be clear. As the test is repeated with different surfactant blends, different patterns will result. This is a wonderful example of the power of phase diagrams to systematize formulation work.
In the 1990s, Friberg and his colleagues produced a number of papers focusing on fragrances in emulsions. A number of aroma chemicals were included in these studies—including phenylethyl alcohol, benzaldehyde, limonene and lavender oil. They demonstrated how fragrance partition is based on polarity and solubility, showcasing the systems’ change during evaporation. (For a summary of this work, see Bud Brewster’s account4 in the January 2006 issue of Cosmetics &Toiletries magazine.)
Once past three variables, the situation immediately turns ugly. For the curious, a generic quaternary phase diagram, drawn from metallurgy. These more complex diagrams don’t yield their secrets easily, but do show the incredible range of possibilities that arise by combining materials in different proportions.