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Industry expert Tony O'Lenick asks Mark Garrison of Avon to explain the difference between esters and lactones.
An ester is a molecule that contains the functional group shown in Figure 1. An ester is formed from the reaction between an alcohol and an acid. A molecule of water is eliminated and an ester linkage is formed. The reaction is reversible; i.e., a molecule of water can hydrolyze an ester, giving back the original acid and alcohol, as shown in Figure 2.
R and R’ can be the same or different. When R is H, it is still an ester. When R’ is H, it is no longer an ester; the compound is then a carboxylic acid. The structures of esters, and hence their physical properties can vary widely. For example, the starting molecule of carboxylic acid may have any number of acid groups that can react. Likewise, the starting alcohol molecule may contain any number of hydroxyl groups. Consider also that either starting material can contain any number of carbon atoms, as well as branching, rings, unsaturation, etc.—and the possibilities are fairly limitless.
There are literally thousands of esters in use in the cosmetic industry. Esters provide emolliency and contribute considerably to the overall feel of a cosmetic formula. They can help to solubilize actives or sunscreens. The feel of cosmetic esters varies from the extremely light and “dry,” to oily, heavy, unctuous and everything in between. They vary in viscosity and consistency—from thin liquids, to very viscous liquids, to solid waxes. Volatile esters find use in the flavor and fragrance industry.
Since the esterification reaction is reversible, one may wonder why esters can be used in cosmetic emulsions; in other words, why is the hydrolysis of esters not a widespread stability problem? This is because hydrolysis generally is not an issue due to the poor water solubility of most cosmetic esters and the relatively mild pH levels of most cosmetic products. However, water-soluble esters can hydrolyze in aqueous formulations, and such reactions are accelerated at extremes of pH and elevated temperatures.