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

The pseudo-first-order rate constants (k obs ) for the alkaline hydrolysis of phenyl benzoate (PB) decrease monotonically from 72.0 Â 10 À4 to 7.4 Â 10 À4 s À1 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 À ) 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 À in the micellar region of micellized PB molecules. The values of k obs for hydrolysis of PB and PS À 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 Â 10 À4 M whereas an increase in [C 12 E 23 ] T from !7 Â 10 À4 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 Â 10 À4 -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. EFFECTS OF SURFACTANTS ON HYDROLYSIS OF PHENYL SALICYLATE AND BENZOATE 671 6.41 AE 0.07 6078 AE 23 0.074 AE 0.005 a [Phenyl salicylate] 0 = 2 Â 10 À4 M, 35°C, l = 350 nm, reaction mixture for each kinetic run contained 2% (v/v) CH 3 CN. b Error limits are standard deviations. c The absorbance value (A obs = 0.012) remained unchanged with change in the reaction time (t) from $20 s to 3 h.Figure 3. Effect of [C 12 E 23 ] T on E app for hydrolysis of PB at (◊) 0.0, (*) 0.006, (~) 0.01, (&) 0.02 and (!) 0.03 M CTABr Figure 4. Effect of [C 12 E 23 ] T on k obs for hydrolysis of PS À at (*) 0.01, (~) 0.02 and (!) 0.03 M CTABr

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].…”

“…Shear rates (c) were fixed in the range 0.5-15 s À1 and 1.0-1000 s À1 for respective period of 2400 and 8000 s. Thus, the dependent shear viscosity (g) and shear stress (s) were recorded after each 80 s. 16 E 20 ] T from 0.002 to 0.17 M which was attributed to the increase in microturbidity due to increasing concentration of C 16 E 20 [12]. Thus, the nonlinear increasing effect of increasing [C 16 E 20 ] T on A 0 may be attributed to the microturbidity effect of surfactant solution [9][10][11]. Perhaps, it is noteworthy that the microturbidity, even at 0.17 M C 16 E 20 (the highest value of [C 16 E 20 ] T attained in the study), was not strong enough to cause the observed data (A obs vs. t) to deviate from the satisfactory fit to Eq.…”

“…Near irreversible micellar partitioning of the reactants (of bimolecular reactions), differing greatly in terms of hydrophobicity, has been observed at larger than a typical value of [C 12 E 23 ] T /[HO À ] T (=R t ) in C 12 E 23 micellar-mediated alkaline hydrolysis of phthalimide (2) [9], phenyl benzoate (3) [10], and phenyl salicylate (4) [10], as shown in reaction Scheme 1. The symbol [X] T (throughout the manuscript) represents total concentration of X.…”

“…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].…”