MICROENVIRONMENT EFFECTS ON THE KINETICS OF ELECTRON-TRANSFER REACTIONS INVOLVING DITHIONITE IONS AND VIOLOGENS .3. COMPARISON BETWEEN MICELLES AND SOME OTHER MICROHETEROGENEOUS SYSTEMS

The rate of chemical reactions can be modified by changing the local m icroenvironment. The retardation of the rate of electron-transfer reac tions appears to be a fundamental objective in photoinduced processes, potentially leading to hydrogen production. In this work, the stopped -flow technique was used to investigate the effect of different microe nvironments on the rate of electron transfer between sodium dithionite and a series of viologens (N,N'-dialkyl-4,4'-bipyridinium dibromides) of varying hydrophobicity. The apparent rate constants for the reduct ion of the viologen species were measured in the presence of alpha- an d beta-cyclodextrins (CD), and of micelles of sodium dodecyl sulfate ( SDS), sodium tetradecyl sulfate (STS), sodium oleate and disodium stea royl glutamate (AG). The results obtained are compared with those prev iously reported in the presence of polyelectrolytes and polysoaps. The effect of CD was found to be very small with even a slightly accelera ting effect in the case of beta-CD provided that the viologen has some hydrophobicity. More surprisingly, the retardation effects measured w ith different kinds of micelles were much less important than with the previously studied polysoap, even when the chemical natures of the co lloidal particles were very close. Microemulsions obtained by the addi tion of oil and cosurfactant to SDS micelles did not significantly cha nge these observations. Different explanations will be proposed to tak e into account the results obtained.