Kinetic Study of the Formation of Ruhemann's Purple in Micellar and Microemulsion Phases

Abstract

The reaction kinetics of ninhydrine (Nin) with glycine-glycine di peptide (Gly) was studied in aqueous solution. The same reac tion was studied in micellar solutions of different surfactants such as sodium dioctylsulfosuccinate (AOT) and sodium dodecyl sulfate (SDS). The reaction was also studied in the L2 micro emulsion phase of the three systems AOT/heptane/water, SDS/ pentanol/heptane/water, CTAB/pentanol/heptane/water. Spectrophotometric measurements were perfomed at 570 nm. The data show that the reaction is first-order with respect to Nin and Gly in all cases. Values of the observed rate constants (kobs) increase with increasing concentration of the surfactant in micel lar media. AOT was more promotional than SDS. The reaction rate in the presence of different micelles could be explained using a pseudo-phase kinetic model. Association constants of Nin and Gly with surfactant micelles are reported. Values of kobs for the reaction rates in the microemulsion phases are reported. AOT/heptane L2 microemulsion system has been clearly found as the most promotional among all the other studied systems. Association constants of Nin and Gly with surfactant in micro emulsion were obtained. The rate was increased with the tem perature in aqueous, micellar and microemulsion phases. The activation parameters AH*, AS* and AG* have also been obtained. The reaction rate in different microemulsion media is slightly in creased with the change of oil (heptane > hexane > pentane), while co-surfactants (butanol, pentanol and hexanol) have no effect on kobs values in AOT, CTAB or SDS microemulsion.