Catalysis of the reaction between sorbic acid and thiols by surfactants

Summary Sorbic acid reacts slowly with simple thiols at 80°C. The reaction involves nucleophilic attack on the sorbic acid molecule by the thiolate anion. Second order rate constants for the reactions of mercaptoethanol, mercaptoacetic acid, cysteine and glutathione are, respectively, 0.86, 0.68, 2.3 and 2.5 M-1 h−1 at 80°C and independent of pH in the range pH 3.7–5.7. The effects of dodecyltrimethylammonium bromide (DoTAB), sodium dodecyl sulphate (SDS), lecithin, datem ester (diacetyltartaric acid ester of C18 monoglyceride) and Tween 80 on the initial rates of these reactions at pH 5.0 are reported. The greatest catalytic effect was shown by DoTAB, giving a maximum enhancement of rate by a factor of 21.4 for the mercaptoethanol reaction and 4.6 for the glutathione reaction. SDS was much less effective. The nonionic surfactants all behaved as catalysts. The mechanism of the surfactant-catalysed reaction is discussed. Kinetically derived stoichiometries of binding of the solutes in question to DoTAB micelles depend on surfactant concentration, and for the thiols used (mercaptoacetic acid, cysteine and glutathione) it is found that each thiol molecule is associated with at least 10 surfactant molecules making up the micelles for surfactant concentrations greater than or equal to 50 mM. The implications of micellar catalysis to food additive-food component interactions are considered.