Emulsions are well known to the cosmetic chemist. They are most commonly milky white compositions that contain oil, water and surfactant. The chemist has developed a number of technologies that allow for more cosmetically elegant emulsions, including clear emulsions and chromatic emulsions, which are not the same.
US Patent 6,468,512 to Carmody titled Gel Compositions, issued Oct. 22, 2002, describes clear w/s emulsions prepared by a matching refractive index. The gel composition preferably exhibits clarity at five nephelomedric turbidity units (NTU)–30 NTU and more preferably ~10–20 NTU. The gel composition also exhibits an index of refraction at ~1.39– 1.41, and preferably 1.399– 1.405.
Refractive index modifiers can be added to either or both of the oil or water phases to modify the index of refraction. Preferably, the oil and water phases of the gel composition exhibit indices of refraction within 0.0004 of each other to provide a preferred level of overall clarity to the composition.
US Patent 7,276,553 to Garrison et al., titled Aesthetic, Stable Chromatic Emulsions, issued Oct. 2, 2007, explains that chromatic emulsions are emulsions that appear colored due to the refractive effects of the internal and external phases. They are often referred to as having structural color since the color does not not result from dye or pigment.
US Patent 5,290,555 to Guthauser et al., titled Cosmetic Compositions with Structural Color, issued March 1, 1994, gives the following explanation for this effect:
When two transparent, immiscible liquids are mixed, the combination is often cloudy. If, however, the liquids have the same refractive index , the mixture will be substantially transparent to the human eye and appear to be homogenous. The appearance of "structural color" in such a mixture requires not only that the refractive indices (at a given wavelength of visible light) are the same, but also that the variation of the indices as a function of visible wavelength differ for the two liquids. That is, the "dispersive power" of the two phases must be different.