Effects of non‐ionic and mixed non‐ionic–cationic micelles on the rate of aqueous cleavages of phenyl benzoate and phenyl salicylate in alkaline medium

Pseudo‐first‐order rate constants ( k obs ) for the hydrolysis of phenyl salicylate (PSH) and phenyl benzoate (PB) in the alkaline medium show a monotonic decrease with the increase in [C 16 E 20 ] T (total concentration of Brij 58) at constant [CH 3 CN] and [NaOH]. This micellar effect is explained in terms of the pseudophase model of micelles. These results show a characteristic difference between the effects of [C 16 E 20 ] T and previously published effects of [C 12 E 23 ] T (total concentration of Brij 35) on the rates of aqueous cleavage of PSH and PB at [C n E m ] T /[NaOH]≥∼3. The values of k obs , obtained at different [C 16 E 20 ] T in the presence of a constant amount of CTABr, follow the empirical relationship k obs =( k 0 +θK[C 16 E 20 ] T )/(1+ K [C 16 E 20 ] T ), where θ and K are empirical parameters. The values of θ are not affected whereas the values of K decrease with increase in [CTABr] T in a mixed C 16 E 20 –CTABr micellar system. The values of θ at different [CTABr] T show that θ> k 0 for hydrolysis of PSH and θ< k 0 for hydrolysis of PB in the presence of 0.01  M NaOH. The effects of [CTABr] T on K and θ are explained in terms of the pseudophase model of micelles coupled with the empirical relationship K S = K S 0/(1+ K C16E20/S [C 16 E 20 ] T ). Copyright © 2004 John Wiley & Sons, Ltd.