Kinetics of hydrolysis of polyoxyethylene (20) sorbitan fatty acid ester surfactants

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Abstract
The kinetics of the hydrolysis of the oleate ester of polyoxyethylene (20) sorbitan (Tween 80) in aqueous buffers were studied at an initial concentration of 0ṁ020% (w/v) and over the pH range of 1ṁ10 to 10ṁ28. The hydrolysis appears to be specific acid-catalysed at pH values below 3 and specific base-catalysed at pH values greater than 7ṁ6. The pseudo first-order rate constants for hydrogen and hydroxyl ion catalysis were determined, and the temperature and ionic strength dependence of the acid-catalysed reaction was studied. Both the initial, acid- and base-catalysed hydrolysis of Tween 80 exhibited an unusual initial, micellar surfactant concentration-rate dependence, opposite to that previously reported for the hydrolysis of anionic surfactants of the n-alkyl sulphate-type. Specifically, as the initial concentration of Tween 80 was increased above its reported critical micellar concentration, there was a progressive, marked decrease in the rate of the reaction, with the rate eventually reaching a plateau value between 0ṁ100 and 1ṁ00% (w/v). It is suggested that this behaviour is due to alterations in the micellar state of the surfactant as its concentration is increased. The influence of the chemical structure of the Tween surfactant on the acid-catalysed hydrolysis reaction at 80° was also examined using the oléate (Tween 80), stéarate (Tween 60), and palmitate (Tween 40) esters of polyoxyethylene (20) sorbitan.