Effects of ionic surfactants and cyclodextrins on hydride-transfer reaction of l-Benzyl-l,4-dihydronicotinamide with methylene blue

The kinetics of the hydride-transfer reaction between methylene blue (MB⁺) and 1 -benzyl-1,4-dihydronictinamide (BNAH) were studied in media containing cyclodextrins (β- and γ-CD) and surfactants (sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide). Cationic surfactants decreased the apparent first-order rate constant (k _obsd) above the cmc, while SDS increased k_obsd just above the cmc and then decreased k_obsd with increasing surfactant concentration. This behavior for cationic surfactants was typical of micellar effects due to a separation of the reactants by the micelles. BNAH associated with micelles, whereas MB⁺ ions were repelled from the cationic interface of the micelles. Binding of BNAH and MB to the same SDS micelle enhanced the reaction, but dilution of reagents within the micellar interface with the increase in [SDS] caused a decrease in K_obsd. In β-CD-cationic surfactant mixtures, the results were interpreted in terms of the model which takes into account the formation of CD-BNAH, CD-MB₊, and CD-surfactant complexes and the association of BNAH with micelles. The decrease in K_obsd with increasing surfactant concentration observed in γ-CD-cationic surfactant mixtures can be explained by the decrease in the concentration of free γ-CD by the formation of 1:1 and 2:1 complexes of surfactant monomer with β-CD.