Acid-Base and Aggregation Processes of Acridine Orange Base in n-Heptane/AOT/Water Reverse Micelles

R. DARÍO FALCONE; N. MARIANO CORREA; M. ALICIA BIASUTTI; JUANA J. SILBER

LANGMUIR

Año: 2002 vol. 18 p. 2039 - 2047

0743-7463

The properties of the base acridine orange (AOB) in n-heptane/AOT/water reverse micelles were investigated by using absorption, fluorescence, and single photon counting techniques. For comparison, studies in homogeneous media (water and n-heptane) were also performed. The absorption spectra of AOB in water at pH < 10 in the range of [AOB] = 10-6-10-4 M show two bands at 467 and 492 nm which were attributed to the dimer ((AOBH)22+) and monomer (AOBH+) species, respectively. At pH > 10 in the same [AOB] range, only the basic form, AOB, was detected with a band in the visible at lmax ) 435 nm, which obeys Lambert-Beer’s law. This species was also the only one detected in n-heptane at 417 nm. These resultsshowthat only the protonated base dimerizes as confirmed by fluorescence techniques.The absorption spectra of AOB in the micellar media at various W0 (W0 = [H2O]/[AOT]) and pH = 4, working above the operational critical micellar concentration and below [AOT] = 3 x 10-3 M, show the disappearance of the band originally present in n-heptane and the appearance of the bands attributable to the protonated base mostly as the dimer, (AOBH)22+. Above this concentration, the absorption band corresponding to AOBH+ is the predominant one. There are two isosbestic points, at 426 and 475 nm. Three processes can account for the behavior of AOB in the reverse micelles: (1) distribution of the dye between the organic phase and the micellar interface followed by AOB protonation to give AOBH+, (2) dimerization of AOBH+ at the interface at low [AOT], and (3) the conversion of (AOBH)22+ to AOBH+ by the micelle at [AOT] > 3 x 10-3 M. The spectral changes allow us to estimate the equilibrium constants for the processes at different W0; the dimerization process is favorable at low water content. The fluorescence spectra of these species show at 650 nm the band for (AOBH)22+ and at 550 nm the one for AOBH+. By varying the experimental conditions, the fluorescence decay times for AOB (in n-heptane and water) as well as for AOBH+ and (AOBH)22+ in water and in AOT reverse micelles were determined. The results were used to explain the micellar influence on AOB’s protonation and aggregation processes. The fluorescence decay times at low W0 are higher than that obtained in bulk water. On increasing W0, the fluorescence decay times tend to the value obtained in water at the same pH.