The Determination of the Formation Constant of Triiodide Ion in Micellar Solution of Dodecyltrimethylammonium Chloride

Spectrophotometric measurements were undertaken at 360 nm in order to estimate the formation constant of triiodide ion in micellar solution of dodecyltrimethylammonium chloride. The absorption spectra of the solutions suggested that both iodine molecule and triiodide ion are solubilized in a hydrophilic surface region of the surfactant micelle. The apparent formation constant of triiodide ion, Ka, was found to be much larger in micellar solution than in water. It was also shown that the value of Ka decreases regularly with increasing surfactant concentration. A tentative estimation based on a lamellar micelle model led to the intrinsic formation constant of triiodide ion of 650 mol−1 dm3 or less at 30 °C, which is comparable with that in water. The thermodynamic consideration revealed that the formation reaction of triiodide ion from iodine and iodide ion in micellar solution is accompanied by an anomalous entropy decrease.