Effects of pure non‐ionic and mixed non‐ionic–cationic surfactants on the rates of hydrolysis of phenyl salicylate and phenyl benzoate in alkaline medium

The pseudo‐first‐order rate constants ( k obs ) for the alkaline hydrolysis of phenyl benzoate (PB) decrease monotonically from 72.0 × 10 −4 to 7.4 × 10 −4 s −1 with increase in the concentration of polyoxyethylene 23 lauryl ether ([C 12 E 23 ] T ) from 0.0 to 0.01  M at 0.01  M NaOH and 35 °C. The results fit satisfactorily to a pseudophase model of micelles. The values of k obs for the hydrolysis of ionized phenyl salicylate (PS − ) are almost independent of [C 12 E 23 ] T within the range 0.0–0.02  M at 0.01 and 0.03  M NaOH and 35 °C. The rate of alkaline hydrolysis of PB appears to be negligible in the micellar pseudophase than in the aqueous pseudophase, which is ascribed to the extremely low concentration of HO − in the micellar region of micellized PB molecules. The values of k obs for hydrolysis of PB and PS − at 0.01  M NaOH become almost zero at ≥0.02 and ≥0.03  M C 12 E 23 , respectively. The effects of mixed C 12 E 23 –CTABr on k obs for the alkaline hydrolysis of PB reveal that mixed micelles behave like pure CTABr micelles within the [C 12 E 23 ] T range 0.0–5 × 10 −4   M whereas an increase in [C 12 E 23 ] T from ≥7 × 10 −4 to 0.02  M decreases k obs at 0.006, 0.01, 0.02 and 0.03  M CTABr. However, the values of k obs are significantly higher at [CTABr] T  = 0.006, 0.01, 0.02 and 0.03  M than at [CTABr] T  = 0 in the presence of [C 12 E 23 ] T in the range >5 × 10 −4 –0.02  M . An increase in [C 12 E 23 ] T from 0.0 to 0.015  M causes nearly a twofold increase in k obs at 0.01, 0.02 and 0.03  M CTABr and 0.01  M NaOH. Copyright © 2001 John Wiley & Sons, Ltd.