J-aggregation of a thiacarbocyanine dye in reverse microemulsions

J-Aggregation of the thiacarbocyanine dye 3,3'-di-(gamma-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine betaine pyridinium salt (DEC) has been studied in reverse microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-hexane/water and polyoxyethylene (5) nonylphenyl ether (NP5)/cyclohexane/water. In such systems, the dye occurs mainly as a trans-monomer and a J-aggregate. It has been shown that the equilibrium between these species is determined by the solubility of the trans-monomer in monolayers of surfactant molecules covering microemulsion droplets. When the total concentration of the dye in the microemulsion is less than its solubility, the equilibrium state corresponds to the presence of the dye only in the form of the trans-monomer. When the concentration is higher than the solubility, the excess dye forms J-aggregates, which gradually grow and precipitate. For the reverse microemulsions AOT/n-hexane/water, the solubility was found to be [DEC]/[AOT] = (1.45 +/- 0.34) x 10(-5).