Autoxidation of polysorbates.
Aqueous solutions of polysorbate 20 undergo autoxidation on storage, with the peroxide number increasing and subsequently decreasing again, the acidity increasing continuously, the pH and surface tension falling and tending to level off, and the cloud point dropping sharply until turbidity begins at room temperature. The changes are accelerated by light, elevation of temperature, and a copper sulfate catalyst. At the same time, hydrolysis occurs, liberating lauric acid. Analysis of the alterations in these properties leads to the conclusion that hydrolysis has the major influence near room temperature and that oxyethylene undergoes chain shortening at temperatures above 40 degrees. However, evidence of degradation is detectable even in previously unopened commercial samples of polysorbates 20, 40, and 60, warranting attention to the stability of and standards for these surfactants as compared with the solid alkyl ether type of nonionic surfactant.