Application of multiple linear regression and extended principal-componentanalysis to determination of the acid dissociation constant of 7-hydroxycoumarin in water/AOT/isooctane reverse micelles

The apparent pK(a) of dyes in water-in-oil microemulsions depends on the ch arge of the acid and base forms of the buffers present in the water pool. E xtended principal-component analysis allows the precise determination of th e apparent pK(a) and of the spectra of the acid and base forms of the dye. Combination with multiple linear regression increases the precision. The pK (a) of 7-hydroxycoumarin (umbelliferone) was spectrophotometrically measure d in a water/AOT/isooctane microemulsion in the presence of a series of buf fers carrying different charges at various different water/surfactant ratio s. The spectra of the acid and base forms of the dye in the microemulsion a re very similar to those in bulk water in the presence of Tris and ammonia. The presence of carbonate changes somewhat the spectrum of the acid form. Results are discussed taking into account the profile of the electrostatic potential drop in the water pool and the possible partition of umbelliferon e between the aqueous core and the surfactant, The pK(a) values corrected f or these effects are independent of w(0) and are close to the value of the pK(a) in bulk water. (C) 2000 Academic Press.