Specific micellar catalytic effects of the protolysis of ortho-Benzoic acid derivatives studied by the ultrasonic absorption method

Abstract

Ultrasonic relaxation absorption has been observed in aqueous solutions of ortho-benzoic acid derivatives (OBAD) in the presence of a micelle of tetradecyltrimethylammonium bromide (TTAB). Relaxation absorptions were ascribed to the protolysis of acids on the surface of the micelle. The apparent dissociation constant (pK_a=k_b/γ²k_f), rate constants (γ²k_f, k_b) and volume change (ΔV) for the reactions were determined. The dependence of rate constants on pK_a for OBAD was compared with that for meta- and para-benzoic acid derivatives (BAD). It was especially noticeable that the dissociation of OBAD in the micellar solution is depressed more than that in aqueous solution. This can be explained as due to electrostatic effects, the polarity of solution and solvation in the TTAB micellar solution. The dependence of ΔV of OBAD on pK_a was also compared with that of BAD. The result was considered to be due to differences in solute–solvent interactions and the situation around OBAD and BAD in the micellar solution.