INTERACTION OF METHYL-ORANGE WITH CATIONIC MICELLES AND ITS EFFECT ON DYE PHOTOCHEMISTRY

The interaction of the azo dye methyl orange with cationic and anionic surfactant micelles was studied by absorption and emission spectroscopy. The dye interacts strongly with the cationic micelles cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB), but does not interact with the anionic micelle sodium dodecylsulphate (SDS). The association constants of the dye to CTAC and CTAB micelles are 5.6 x 10(4) and 5.9 X 10(5) respectively. The fluorescence quenching of pyrene by methyl orange in CTAC micellar solution occurs via non-radiative energy transfer. The variation of the relative fluorescence intensity agrees well with a model in which the dye is almost completely associated with the micellar phase. The photolysis of an aqueous solution of the dye in the presence of acetone gives rise to an irreversible photoreduction of the azo dye by the ketyl radical formed during photolysis. The presence of CTAB micelles reduces the photodecomposition rate of the dye. This effect is ascribed to the micellar cage effect which inhibits the second-order disproportionation reaction of the hydrate compound formed.