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

Abstract

AbstractThe pseudo‐first‐order rate constants (kobs) 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 ([C12E23]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 kobs for the hydrolysis of ionized phenyl salicylate (PS−) are almost independent of [C12E23]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 kobs for hydrolysis of PB and PS− at 0.01 M NaOH become almost zero at ≥0.02 and ≥0.03 M C12E23, respectively. The effects of mixed C12E23–CTABr on kobs for the alkaline hydrolysis of PB reveal that mixed micelles behave like pure CTABr micelles within the [C12E23]T range 0.0–5 × 10−4 M whereas an increase in [C12E23]T from ≥7 × 10−4 to 0.02 M decreases kobs at 0.006, 0.01, 0.02 and 0.03 M CTABr. However, the values of kobs 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 [C12E23]T in the range >5 × 10−4 –0.02 M. An increase in [C12E23]T from 0.0 to 0.015 M causes nearly a twofold increase in kobs at 0.01, 0.02 and 0.03 M CTABr and 0.01 M NaOH. Copyright © 2001 John Wiley & Sons, Ltd.