Kinetic study of the protolysis of benzoic acid derivatives in a micelle of dodecylammonium chloride

Abstract

The protolysis of o-benzoic acid derivatives (OBAD), m/p-benzoic acid derivatives (BAD) and salicylic acid derivatives (SAD) in a micelle of dodecylammonium chloride (DAC) have been studied by the ultrasonic relaxation absorption method. The relaxation absorption was larger than in tetradecyltrimethylammonium bromide (TTAB) micellar solution owing to the electrostatic force of NH ₃ ⁺ in DAC. The rate constants (γ ² k _f , k _b ), apparent dissociation constant (K _a =k _b /γ ² k _f ) and volume change (ΔV) on protolysis were determined. The dependence of the rate constants on pK _a in the BAD–DAC system was the same as in the OBAD–DAC system, but differed from those obtained in aqueous solution or in a TTAB micellar solution. The pK _a of OBAD in a TTAB micellar solution exceeded that in aqueous solution but in a DAC micellar solution it was less than in aqueous solution. The results, in comparison with those in a TTAB micellar solution, are discussed, based on differences in the electrostatic force of the DAC and TTAB micelles.