Unusual Effects of Pure Nonionic and Mixed Nonionic–Cationic Micelles on the Rate of Alkaline Hydrolysis of N-Hydroxyphthalimide

Abstract

Pseudo-first-order rate constants, k_obs, for the alkaline hydrolysis of N-hydroxyphthalimide, 1, at 0.02 M NaOH and 30°C remain essentially independent of the total concentration of C₁₂E₂₃, [C₁₂E₂₃]_T, at ≤0.005 M C₁₂E₂₃. The increase in [C₁₂ E₂₃]_T from 0.005 to 0.015 M causes a nonlinear decrease in k_obs. The rate of hydrolysis becomes either too slow or the change in absorbance values becomes significantly small to allow a reliable observed data fit to a first-order kinetic equation at ≥0.020 M C₁₂E₂₃ in the absence and presence of total concentration of cetyltrimethylammonium bromide, [CTABr]_T ranging from 0.003 to 0.020 M. The values of fraction of nonionized 1, F_SH, obtained at reaction time t = 0 and 0.02 M NaOH, remain ∼0 at ≤0.010 M C₁₂E₂₃ while they increase from 0.39 to 0.89 with the increase of [C₁₂E₂₃]_T from 0.015 to 0.10 M. The values of k_obs show a nonlinear decrease of ∼5-fold with the increase of [C₁₂E₂₃]_T from 0.0 to 0.010 M in the presence of 0.02 M NaOH and [CTABr]_T range of 0.003 to 0.020 M. The values of F_SH remain ≤∼0.10 at ≤0.015 M C₁₂E₂₃ while they vary between 0.40 and 0.90 within a [C₁₂E₂₃]_T range 0.02 to 0.05 M in the presence of 0.02 M NaOH and [CTABr]_T ranging from 0.003 to 0.020 M. The values of F_SH represent the fraction of nonionized 1 trapped almost irreversibly by pure C₁₂E₂₃, and mixed C₁₂E₂₃–CTABr micelles.

Keywords:

• Alkaline hydrolysis
• kinetics
• mixed nonionic-cationic micelles
• N-hydroxyphthalimide
• nonionic micelles

The authors thank the University of Malaya for financial assistance.

Notes

^a [1 ₀] = 6.0 × 10⁻⁴ M, λ = 410 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN and  = 0.72 ± 0.04.

^b Total concentration of C₁₂E₂₃ surfactant.

^c The values of A₀ were obtained either from the relationship: A₀ = δ_app [1 ₀] + A_∞ or from extrapolation of the plots of A_ob versus t to t = 0.

^d F_SH = 1 − F_S- and F_S- = A₀/ where F_SH and F_S- represent respective fraction of nonionized (SH) and ionized (S⁻) 1 at the reaction time t = 0 with representing the average value of A₀ values, obtained within [C₁₂E₂₃]_T range 0.0 to 0.010 M. The values of F_SH and F_S- give the measure of respective fraction of 1 remained non-reactive and reactive toward product (2) formation.

^e Error limits are standard deviations.

^f Either the reaction became too slow or the value of A₀–A_∞ became significantly small to allow a reliable observed data fit to Equation (1).

^a [1 ₀] = 6.0 × 10⁻⁴ M, [NaOH] = 0.02 M, λ = 410 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN, A_∞ values remained zero within the limits of their standard deviations at ≤0.008 M C₁₂E₂₃,  = 0.57 ± 0.02 at 0.003 M CTABr and  = 0.52 ± 0.03 at 0.005 M CTABr.

^b Total concentration of C₁₂E₂₃ surfactant.

^c The values of A₀ were obtained either from the relationship: A₀ = δ_app [1 ₀] + A_∞ or from extrapolation of the plots of A_ob versus t to t = 0.

^d F_SH = 1 – F_S- and F_S- = A₀/ where F_SH and F_S- represent respective fraction of nonionized (SH) and ionized (S⁻) 1 at the reaction time t = 0 with representing the average value of A₀ values, obtained within [C₁₂E₂₃]_T range 0.0 to 0.010 M. The values of F_SH and F_S- give the measure of respective fraction of 1 remained non-reactive and reactive toward product (2) formation.

^e Error limits are standard deviations.

^f Parenthesized value represents A_∞ calculated from Equation (1).

^g Parenthesized value represents A_∞ calculated from Equation (1) where calculated values of k_obs and δ_app may not be very reliable because the rate of reaction became very slow to monitor it for a few half-lives.

^h The value of A₀–A_∞ became significantly small to allow a reliable observed data fit to Equation (1).

ⁱ Parenthesized values represents A_ob obtained at t = 24.3 hours for kinetic run at 0.02 M C₁₂E₂₃ and 0.005 M CTABr; t = 24.3 hours for kinetic run at 0.05 M C₁₂E₂₃ and 0.003 M CTABr and at t = 300 seconds for kinetic run at 0.05 M C₁₂E₂₃ and 0.005 M CTABr.

^a [1 ₀] = 6.0 × 10⁻⁴ M, [NaOH] = 0.02 M, λ = 410 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN, A_∞ values remained zero within the limits of their standard deviations at ≤0.008 M C₁₂E₂₃,  = 0.57 ± 0.02 at 0.003 M CTABr and  = 0.52 ± 0.03 at 0.005 M CTABr.

^b Total concentration of C₁₂E₂₃ surfactant.

^c The values of A₀ were obtained either from the relationship: A₀ = δ_app [1 ₀] + A_∞ or from extrapolation of the plots of A_ob versus t to t = 0.

^d F_SH = 1 − F_S- and F_S- = A₀/ where F_SH and F_S- represent respective fraction of nonionized (SH) and ionized (S⁻) 1 at the reaction time t = 0 with representing the average value of A₀ values, obtained within [C₁₂E₂₃]_T range 0.0 to 0.010 M. The values of F_SH and F_S- give the measure of respective fraction of 1 remained non-reactive and reactive toward product (2) formation.

^e Error limits are standard deviations.

^f Parenthesized value represents A_∞ calculated from Equation (1).

^g The value of A₀–A_∞ became significantly small to allow a reliable observed data fit to Equation (1),  = 0.54 ± 0.03 at 0.007 M CTABr and  = 0.55 ± 0.03 at 0.010 M CTABr.

^h Parenthesized values represents A_ob obtained at t = 11.1 hours for kinetic run at 0.02 M C₁₂E₂₃ and 0.007 M CTABr; t = 300 seconds for kinetic run at 0.05 M C₁₂E₂₃ and 0.007 M CTABr and at t = 195 seconds for kinetic runs at 0.02 and 0.05 M C₁₂E₂₃ and 0.01 M CTABr.

^a [1 ₀] = 6.0 × 10⁻⁴ M, [NaOH] = 0.02 M, λ = 410 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN, A_∞ values remained zero within the limits of their standard deviations at ≤0.008 M C₁₂E₂₃,  = 0.57 ± 0.02 at 0.003 M CTABr and  = 0.52 ± 0.03 at 0.005 M CTABr.

^b Total concentration of C₁₂E₂₃ surfactant.

^c The values of A₀ were obtained either from the relationship: A₀ = δ_app [1 ₀] + A_∞ or from extrapolation of the plots of A_ob versus t to t = 0.

^d F_SH = 1 – F_S- and F_S- = A₀/ where F_SH and F_S- represent respective fraction of nonionized (SH) and ionized (S⁻) 1 at the reaction time t = 0 with representing the average value of A₀ values, obtained within [C₁₂E₂₃]_T range 0.0 to 0.010 M. The values of F_SH and F_S- give the measure of respective fraction of 1 remained non-reactive and reactive toward product (2) formation.

^e Error limits are standard deviations.

^f Parenthesized value represents A_∞ calculated from Equation (1).

^g The value of A₀–A_∞ became significantly small to allow a reliable observed data fit to Equation (1),  = 0.54 ± 0.04 at 0.015 M CTABr and  = 0.54 ± 0.01 at 0.020 M CTABr.

^h Parenthesized values represents A_ob obtained for kinetic runs at 0.015 M CTABr and at t = 1500 seconds, 0.015 M C₁₂E₂₃; t = 500 seconds, 0.02 M C₁₂E₂₃ and t = 135 seconds, 0.05 M C₁₂E₂₃ as well as for kinetic runs at 0.02 M CTABr and at t = 250 seconds, 0.02 M C₁₂E₂₃ and t = 300 seconds, 0.05 M C₁₂E₂₃.

^a [1 ₀] = 6.0 × 10⁻⁴ M, [₁₂E₂₃]_T 0.02 M, [CTABr]_T 0.003 M, λ = 410 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN.

^b Calculated from Equation (1) by considering A_ob values within t range 35 to 87.4 × 10³seconds and calculated parameters, 10⁴ k_obs = 2.18 ± 1.60 s⁻¹, δ_app = 236 ± 18 M⁻¹ cm⁻¹ as well as 10² A_∞ = 34.9 ± 0.8.

^c RE = 100 × (A_ob − )/A_ob.

^d Calculated from Equation (1) by considering A_ob values within t range 35 to 25.5 × 10⁵seconds and calculated parameters, 10⁶ k_obs = 3.77 ± 0.33 s⁻¹, δ_app = 805 ± 25 M⁻¹ cm⁻¹ as well as 10² A_∞ = 34.9 ± 0.8.

^e Arbitrarily assigned value of A_ob at 25.5 × 10⁵seconds.