Location of pinacyanol in micellar solutions of N-alkyl trimethylammonium bromide surfactants

The interaction of pinacyanol (PIN), a cationic dye formed by monomer and d imer species, with three cationic surfactants (DTAB, TTAB, and HTAB) has be en studied spectroscopically and by acid-base equilibrium in the micellar c oncentration range. In the presence of surfactants, the absorption maximum of the two main peaks undergoes bathochromic shifts. The spectral shifts su ggest a hydrophobic environment of the chromophore. The presence of micelle s favors the monomer species; i.e., it reduces the extent of dimerization. The pK(a) of PIN in micellar medium is similar to the value in pure water. When acid-base equilibrium was considered, the changes in the interfacial p K(a) allowed to us to determine the constant dielectric for the interfacial region (epsilon = 69). This led to the conclusion that the dye must be sol ubilized between the solution and the hydrocarbon chain core, i.e., in the aqueous micellar interface. This location can be explained by a cation-pi i nteraction between the uncharged ring system of the dye and the cationic he adgroups Of the surfactants. (C) 2001 Academic Press.