Micellar and salt effects on the rate of the condensation between ninhydrin and [Cr(his)(H2O)(3)](2+)

The rates of reaction between ninhydrin and [Cr(his)(H2O)(3)](2+) (his, his tidine) have been determined in water and in micelles formed by the surfact ants cetyltrimethylammonium bromide (CTAB; cationic), sodium dodecyl sulpha te (SDS; anionic) and octylphenoxy polyethoxy ethanol (Triton X-100, TX-100 ; non-ionic) in the presence and absence of various salts at 80 degreesC an d pH 5.0. SDS micelles show no effect on the reaction rate but increase in the total concentration of CTAB from 0.0 to 20.0 x 10(-3) mol dm(-3) result s in an increase in the pseudo-first-order rate constant (k(Psi)) by a fact or of ca. 3. The micellar catalysis is explained in terms of the pseudophas e model of the micelles. CTAB micelles decrease the activation enthalpy and make the activation entropy less negative. At constant concentration of CT AB, the rate constant decreases with increase in concentration of added ani ons (bromide, chloride, sulphate and nitrate). The kinetic data have been u sed to calculate the micellar binding constants K-S for [Cr(his)(H2O)(3)](2 +) and K-N for ninhydrin and the respective values are 28 and 73 mol(-1) dm (3) at 80 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.