MICELLAR EFFECTS ON ABSORPTION-SPECTRA AND PROTOLYTIC EQUILIBRIA OF PHENYL-PYRIDYL-KETONES

The absorption spectra of the three isomeric phenyl-pyridyl-ketones were studied in anionic (SLS), cationic (CTAB) and non-ionic (Triton X-100) micellar solutions. The spectral changes of the n, pi* transition in micelles compared to net water provided information on the location of the ketone molecules in the micelle and the partitioning of ketones between micelles and water. The apparent pK(a) values of the three pyridyl ketones were determined in anionic, cationic and non-ionic micelles and compared to those in aqueous solution. The results indicate that the ketones reside within the interfacial head group region of the micelle. This location affects the protolytic equilibria: a decrease in nitrogen basicity in Triton X-100 (DELTA-pK(a) congruent-to -0.6) and CTAB (DELTA-pK(a) - 0.5) and an increase in SLS (DELTA-pK(a) + 1.5) are attributed to a reduced polarity at the micelle surface and to an electrostatic surface potential.