INFLUENCE OF COUNTERION BINDING ON MICELLAR REACTION-RATES - REACTION BETWEEN PARA-NITROPHENYL ACETATE AND HYDROXIDE ION IN AQUEOUS CETYLTRIMETHYLAMMONIUM BROMIDE

The reaction between p-nitrophenyl acetate (PNPA) and hydroxide ion has been studied in aqueous solutions of cetyltrimethylammonium bromide (CTAB), in the absence of buffer substances. To account for the results, a model is presented. The fundamental factors affecting the rate are (i) the solubilization equilbrium of PNPA and (ii) the counterion binding equilibrium. The latter is treated as a Langmuir adsorption, with competition between OH- and Br- for the available sites. The experimental results show that the total degree of counterion binding might not be constant. The individual counterion binding constants are dependent on the surface potential of the micelle. Due to the preferential binding of Br-, not only the ionic strength but also the composition of the ionic solution determines this potential. The change in surface potential is accounted for in two ways: (i) changes in surface charge and (ii) changes in other surface properties. The results are compatible with the model if Br- is bound stronger than OH- by a factor of 40 ± 10 and if the second-order rate constant is about 6.5 M-1 s-1, somewhat smaller than in the aqueous solution (10.9 M-1 s-1). The catalysis is therefore in this case a mere concentration effect. © 1979 American Chemical Society.