Cationic micellar effect on the kinetics of the protolysis of aromatic carboxylic acids studied by the ultrasonic absorption method

Abstract

The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the presence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution. The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid derivatives (BAD). The rate constant (γ²k_f, k_b), the apparent dissociation constant (K_a), and the volume change of the reaction (ΔV) were obtained. Different K_a dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pK_a dependences were also observed for ΔV of SAD and BAD. These dependences are larger than those in aqueous solution. This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution.