POLARITY DEPENDENT POSITIONAL SHIFT OF PROBE IN A MICELLAR ENVIRONMENT

Spectroscopic properties and acid-base equilibrium of hydroxyanthraqui none dyes have been studied in nonionic, cationic, and anionic micella r media as well as in other solvents, and it has been shown that the r elative position of dye in micelle depends on the polarity of dye and nature of micelle. The shift of pK(a) value in nonionic micelle, pK(a) (i), compared to that in water, pK(a)(W), has been explained consideri ng the shift of the relative position of the dye during deprotonation, associated with medium effect. Surface potentials, psi, for charged m icelles at different bulk ionic strengths are calculated for different dyes, and it is noted that the influence of the bulk ionic strength o n acid-base equilibrium of charged micelle bound dye depends on the re lative position of the dye in the micelles. A simplified form of the p seudophase ion exchange model has been found suitable for quantitative interpretation of the change in apparent pK(a) values of the dyes wit h the change in the bulk ionic strength. This model has been utilized to calculate the intrinsic micellar pK(a), pK(a)(m), for different dye s. Comparison of these pK(a)(m) values with pK(a)(i) gives an idea abo ut the average polarity of location of the dyes in micelles.