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

Abstract

Pseudo‐first‐order rate constants (kobs) for the hydrolysis of phenyl salicylate (PSH) and phenyl benzoate (PB) in the alkaline medium show a monotonic decrease with the increase in [C16E20]T (total concentration of Brij 58) at constant [CH3CN] 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 [C16E20]T and previously published effects of [C12E23]T (total concentration of Brij 35) on the rates of aqueous cleavage of PSH and PB at [CnEm]T/[NaOH]≥∼3. The values of kobs, obtained at different [C16E20]T in the presence of a constant amount of CTABr, follow the empirical relationship kobs=(k0+θK[C16E20]T)/(1+K[C16E20]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 C16E20–CTABr micellar system. The values of θ at different [CTABr]T show that θ>k0 for hydrolysis of PSH and θ<k0 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 KS=KS0/(1+KC16E20/S[C16E20]T). Copyright © 2004 John Wiley & Sons, Ltd.

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1): Kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

“…It has been observed that an increase in the total concentration of cetyltrimethylammonium bromide ([CTABr] T ) from 0.0 to 0.03 M has no effect on the apparent pK a of the conjugate acid of 4 in the presence of 0.02 M C 12 E 23 [15], which is considered to be unusual in view of our understanding (through pseudophase ion exchange model of micelle) of the occurrence of ion exchange of HO À /Br À at CTABr micellar surface [16][17][18]. The observed kinetic data (k obs vs. [C m E n ] T ), reported in earlier studies [9][10][11][12][13][14][15], failed to obey PM model at greater than the typical values of R t (=[C m E n ] T /[HO À ] T ). The plausible reason(s) for such observations remained unknown.…”

“…But the values of R t were 88 and $5 at respective 1.0 and 0.76 mM NaOH for C 16 E 10 as well as 140 at 1.0 mM NaOH for C 16 E 20 [12]. In all these studies [9][10][11][12], the observed data, obtained at R < R t , have been explained in terms of PM model. The unusual near irreversible trapping of moderately hydrophobic reactants by C m E n micelles at R P R t cannot be attributed to the usual shielding effect of micelles [12].…”

“…The symbol [X] T (throughout the manuscript) represents total concentration of X. But such unusual observations were not obtained in the presence of C 16 E 20 micelles under essentially similar conditions [9,11]. Recently, similar near irreversible micellar trapping of N-benzylphthalimide (5) was obtained in the alkaline hydrolysis of 5 in the presence of C 16 E 10 and C 16 E 20 micelles [12].…”

“…But the presence of C 12 E 23 micelles caused significant near irreversible trapping of 6 at R P 5.0 and 0.02 M NaOH [14]. The origin of near irreversible micellar trapping of one of the reactants under the typical reaction conditions was not explored in all these studies [9][10][11][12][13][14][15]. It has been observed that an increase in the total concentration of cetyltrimethylammonium bromide ([CTABr] T ) from 0.0 to 0.03 M has no effect on the apparent pK a of the conjugate acid of 4 in the presence of 0.02 M C 12 E 23 [15], which is considered to be unusual in view of our understanding (through pseudophase ion exchange model of micelle) of the occurrence of ion exchange of HO À /Br À at CTABr micellar surface [16][17][18].…”

The effect of mixed surfactants of sodium dodecyl sulfate and Triton X-100 on the base hydrolysis of Malachite green

“…Recently, the effects of non-ionic-cationic micelles on the aqueous cleavages of phenyl benzoate and phenyl salicylate in alkaline solution have been studied and the micellar effect was explained in terms of a pseudophase model of micelles coupled with changes in micellar environment of the system [8].…”