PH IN REVERSE MICELLES - THE ACID DISSOCIATION-CONSTANT OF THE 7-HYDROXY COUMARIN IN WATER AOT/ISOOCTANE WATER-IN-OIL MICROEMULSIONS/

The apparent pK(a) of the 7-hydroxycoumarin (Umbelliferone) was spectr ophotometrically measured in a Water/AOT/Isooctane microemulsion in th e presence of Tris, Ammonia, and Carbonate buffer solutions. The appar ent pK(a) increases of about 2 units, in comparison with the pK(a) mea sured in bulk water, for the smallest values of the molar ratio Water/ AOT, and decreases with the radius of the nanodroplets. The results ar e discussed taking into account the profile of the electrical potentia l drop into the water pool and the possible partition of the Umbellife rone between the aqueous core and the surfactant.