Using the analytical constants for sorbitan monolaurate, monopalmitate, monostearate, and monooleate given in the National Formulary (NF), calculations were carried out that indicated that these emulsifiers are esters of sorbitol mono- and dianhydrides. Contrary to the NF description, no significant amount of sorbitol ester was calculated to be present, in agreement with recent experimental findings. Further calculations were made using the NF-defined analytical constants of polysorbate 20, polysorbate 40, polysorbate 60, and polysorbate 80, which indicate that these emulsifiers are esters primarily of polyoxyethylated sorbitol monoanhydride (i.e., sorbitan), with lesser quantities of polyoxyethylated esters of sorbitol dianhydride. Since all hydroxyl groups of the polysorbates are primary, random distribution of acyl groups on the available hydroxyls can be assumed, and the likely composition of these emulsifiers can be calculated. The most abundant compounds appear to be polyoxyethylated sorbitan mono-, di-, and triesters. Although the polysorbates are stated to contain 20 moles of ethylene oxide per mole of ester, the oxyethylene contents stated in the Food Chemicals Codex reveal that somewhat more than 20 moles of ethylene oxide are combined.

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Summary A method has been devised for determining the relative esterifiability of the primary and secondary hydroxyl groups of glycerol. Contrary to the theory previously advanced by Feuge and Bailey, the primary and secondary hydroxyls are not equally esterifiable. The equilibrium constant favoring esterification of primary hydroxyl over secondary isca. 2.3 at reaction temperature (200°C.) and between 6 and 10 at room temperature. Since the equilibrium constant is substantially different at room temperature from that at reaction temperature, monoglycerides as customarily prepared are not at equilibrium at room temperature and undergo intramolecular migration of acyl groups frombeta toalpha hydroxyl positions. The rate of migration depends on the physical form of the ester and is accelerated by basic catalysts. In the vicinity of room temperature intermolecular rearrangement occurs only over very prolonged periods. The method of calculating relative esterifiability of primary and secondary hydroxyls should be applicable to other polyols.

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