Current consumer trends favor clear products in the cosmetic market. To meet this requirement, chemists have developed two ways to make clear emulsions: by micro emulsion1 and by refractive index matching. The former has been widely explored, leading to many microemulsion-conditioning products in the market. In contrast, the latter has not been widely explored because the physical principle is not well explained and there has been no practical methodology to follow to realize many different applications.
In our exploration of refractive index matching in formulation of cosmetics, a practical method has been developed and leads to many unique formulations. Refractive index matching enables chemists to make many unique formulas that cannot be achieved by other methods. Refractive index matching should become a common technique for formulation chemists.
Optical refraction: Consider a beam of light transmitted through air and directed onto the surface of a body of water. Some of the light is reflected at the interface between the air and water; the remainder enters the water and is transmitted through it. Every transparent material has a property called optical density, which is an inverse measure of the speed of light through the material. Because water has a higher optical density than air, the speed of light is reduced as the light enters the water. The beam of light changes direction abruptly as it enters the water because of the change in speed. This bending of the light ray is called optical refraction.
Index of refraction: The ratio of the speed of light in a vacuum to its speed in a substance is called the index of refraction for that substance or the refractive index (RI). The index of refraction of a homogeneous substance is a constant quantity that is a definite physical property of the substance. Consequently, the identity of such a substance can be determined by measuring its index of refraction with an instrument known as a refractometer. Some indexes of refraction for common cosmetic ingredients are listed in Table 1.
What cosmetic chemists want to know is how to calculate the refractive index of a solution and how to design a formula with the refractive index of each ingredient in solution. Experimentally, it turns out that if one mixes several miscible ingredients together to form a clear homogeneous liquid phase, the refractive index of the mixture can be calculated from each individual component’s refractive index in the composition. The calculated value of refractive index normally is very close to the value measured instrumentally.