Kinetic studies of the base equilibrium of secondary amines on the surface of sodium dodecyl sulfate micelle by means of the ultrasonic absorption method.

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The ultrasonic relaxation absorption has been measured in aqueous solution of dipentylamine, dihexylamine, dioctylamine, N-methylphenethylamine, and N-ethylbenzylamine in the presence of the sodium dodecyl sulfate(SDS) micelle. The relaxation absorption has been ascribed to the base equilibrium of the amines on the surface of the micelle: RR′NH2++OH−\oversetkf\undersetkb\rightleftharpoonsRR′NH+H2O. The rate constants γ±2kf, kb, the apparent base dissociation constant Kb (=kb⁄γ±2kf), and the volume change in the base equilibrium ΔV were obtained, where γ± is the mean activity coefficient for RR′NH2+ and OH−. The dependence of the rate constants on Kb revealed that, in the detailed mechanism of the base equilibrium of the amines, RR′\underset(I)NH2++OH−\oversetk12\undersetk21\rightleftharpoons(RR′NH\underset(II)2+···OH−)\oversetk23\undersetk32\rightleftharpoonsRR′N\underset(III)H+H2O, the rate-determining step is the proton-transfer process, (II) \rightleftharpoons (III). The pKb values of the secondary amines on the surface of the SDS micelle were 0.94–1.26 smaller than the corresponding values in the aqueous solution, and the micellar effect on pKb was smaller than that of the primary amines. From the standpoint of the micellar effect, the structures of the amines solubilized in the micelle were discussed.
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