An Empirical Kinetic Approach for the Determination of the Ion Exchange Constants for Counterions of Cationic Micelles: The Rate of pH-Independent Hydrolysis of Phthalimide as the Kinetic Probe: Effects of (CH₃)₄N⁺ on (X=Br⁻ and Y=Cl⁻)

Abstract

Pseudo-first-order rate constants (k _obs ) for pH-independent hydrolysis of phthalimide (1), obtained at constant total concentration of cetyltrimethylammonium bromide (CTABr]_T), 2.0 × 10⁻⁴ M 1, 0.02 M KOH and varying concentration of inert salt MX (=(CH₃)₄NCl and (CH₃)₄NBr), follow a kinetic equation derived in terms of pseudophase micellar (PM) model coupled with an empirical equation. This derived equation gives empirical constants. F _X/S and K _X/S where S represents anionic 1. The magnitude of F _X/S is the measure of the fraction of micellized anionic 1 transferred to the aqueous phase by the limiting concentration of X ⁻ and the magnitude of K _X/S is the measure of the ability of the ion X ⁻, to expel another counterion ion S ⁻ from the cationic micellar surface to the aqueous phase through ion exchange process X ⁻/S ⁻. The values of the empirical constants, F _X/S and K _X/S , are used to determine the usual ion exchange constant . The values of F _X/S are ∼1 for MBr and in the range 0.97–0.78 for MCl within [CTABr]_T range 0.006 −≤0.016 M. The mean value of is 2.4 ± 0.1 and it is comparable with those obtained by other kinetic probes (with M⁺ = Na⁺) as well as physicochemical techniques.

Keywords:

• An empirical kinetic approach
• cationic surfactant
• determination of ion exchange constant at cationic micellar surface
• hydrolysis at 0.02 M KOH
• kinetics
• phthalimide
• pseudophase micellar model
• tetramethylammonium chloride and bromide

Acknowledgments

The authors thank the University of Malaya for financial assistance (Grant no. RG022/09AFR).

Notes

^a [1 ₀] = 2.0 × 10⁻⁴ M, λ = 300 nm, represents mean value of δ _ap values obtained within [(CH₃)₄NCl] range as shown in the Table 1 and aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN.

^b Total concentration of CTABr surfactant.

^c Error limits are standard deviations.

^d Calculated from Equation (2) and kinetic parameters listed in Table 4.

^a [1 ₀] = 2.0 × 10⁻⁴ M, λ = 300 nm, represents mean value of δ _ap values obtained within [(CH₃)₄NBr] range as shown in the Table 2 and aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN.

^b Total concentration of CTABr surfactant.

^c Error limits are standard deviations.

^d Calculated from Equation (2) and kinetic parameters listed in Table 4.

^a [1 ₀] = 2.0 × 10⁻⁴ M, λ = 300 nm, aqueous reaction mixture for each kinetic run contained 2% v/v CH₃CN, and A_∞ values remained zero within the limits of their standard deviations.

^b Error limits are standard deviations.

^a k ₀ = k _obs at [MX] = 0 as well as the values of k ₀ at 0.006, 0.010 and 0.016 M CTABr represent the mean values, obtained by carrying out ≥3 different kinetic runs, with the respective standard deviations of ±0.10, ±0.02 and ±0.08.

^c Error limits are standard deviations.

^d K _X/S  = K ^X/S  × .^{[ 6 ]}

^e

^f These parenthesized mean values of K ^X/S calculated from Equation (3) with F _X/S  = 1.0 as described in the text.