Interaction of tri-iodide ion with anionic surfactant micelles in aqueous solution

The color of an aqueous tri-iodide ion (I 3 − ) solution fades in the presence of anionic surfactants, such as sodium dodecyl sulfate (SDS), sodium di-2-ethylhexyl sulfosuccinate (Aerosol OT) and sodium 4-sulfobutyl laurate (11C-4), when the added concentration of anionic surfactant is above the c.m.c. These results can be explained by the acceleration of hydrolysis of I 3 − in the presence of negatively charged micelles. The hydrogen ions are sequestered by anionic micelles from the aqueous solution, resulting in a change in the bulk solution property due to hydroxyl ions remaining after the fixation of protons at the micelles, and leading to an increase in the rate of hydrolysis of I 3 − . Further, the fading of I 3 − in aqueous solution depends on the type of surfactant and the concentration of sodium chloride. Above the surfactant c.m.c. the fading of the color of the I 3 − solution becomes slower with increasing concentration of sodium chloride, eventually reaching zero, except in the case of SDS. This salt effect can be explained by the exchange between hydrogen and sodium ion at the surface of the anionic micelle. The magnitude of hydrolysis of I 3 − in the various anionic surfactants in the presence of sodium chloride occurs in the following order: AOT>11C-4>SDS.