ACID-BASE BEHAVIOR OF NEUTRAL RED IN COMPARTMENTALIZED LIQUIDS (MICELLES AND MICROEMULSIONS)

The acid-base behavior of the dye neutral red has been studied in aqueous micellar solution of Aerosol-OT (AOT) and water-in-oil microemulsions (w/o), viz., water-AOT-oil, water-AOT-butanol-oil and water-AOT-hexanol-oil. The dye has been observed to change into the acid form by the presence of AOT micelles as well as with increased [water]/[surfactant] mol ratio, ω, in the microemulsions. The compartmentalized liquids hardly affect the spectral maxima of both the acid and the base forms but enhance their absorbance. The pK of the dye is, however, affected by the media. High concentration of either aqueous AOT or AOT in microemulsion without cosurfactant has increased the pK of the dye, whereas the presence of cosurfactant has decreased it. It is considered that the dye molecules populate in the interfacial region between oil and water in combination with AOT and the cosurfactants. Characteristic features of the media on the dissociation behavior of the dye are discussed. © 1993 by Academic Press, Inc.