Comparatively Speaking: Refractive Index vs. Optical Rotation

The refractive index, or index of refraction of a medium, is a measure by how much the speed of light is reduced when it travels within specific solvents. The speed of light traveling through a vacuum is defined as 1.000.

Vacuum, 1.000
Water (aqua), 1.333
Ethanol, 1.361
Glycerol, 1.473

Cosmetic chemists are interested in refractive index because if the refractive index on a polar and oil phase match, the resulting oil on water seperation will look homogeneous. This observation has been used to make clear emulsions.

Optical rotation relates to stereochemistry and chiral carbon atoms. A substance which is optically active is one which is capable of rotating the plane of polarized light. In a pair of optically active enantiomers, each enantiomer will rotate the plane of polarized light in equal and opposite directions. The enantiomers can, therefore, be referred to as (+) and (-) enantiomers, depending upon the direction of the observed rotation. A racemic mixture, in which there are equal concentrations of both enantiomers, will display no net optical rotation.1 An example where the cosmetic chemist would run into optical rotation is panthenol. Panthenol comes in two enantiomers, D and L. Only D-panthenol (dexpanthenol) is biologically active; however, both forms have moisturizing properties. For cosmetic use, panthenol comes either in D form, or as a racemic mixture of D and L (DL-panthenol).2


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