Effect of cationic surfactants on the addition-elimination type interaction between aspartic acid and ninhydrin

Cationic micelles of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) increase the pseudo-first-order rate constant for the addition-elimination type interaction between aspartic acid and ninhydrin leading to the formation of purple colored diketohydrindylidenediketohydrindamine (DYDA). The rate enhancements have been treated in terms of concentration of both the reactants at the micellar surface and the related micelle-substrate, and micelle-ninhydrin association constants (KS and KN), second-order rate constants (k′m) and activation parameters have been evaluated. The presence of inorganic salts (NaCl, NaBr, Na2SO4) does not reveal any detectable effect on the rate constants. On the other hand, the observed data with organic salts (NaBenz, NaSal, NaTos) show an increase in the rate followed by a decrease. The role of the hydrophilic side chain of aspartic acid (HOOCH2C) is also discussed.