Association of naphthalene-labeled poly(acrylic acid) and interaction withcationic surfactants. Fluorescence studies

Water-soluble poly(acrylic acid) has been covalently labeled with a fluores cent hydrophobic chromophore, naphthalene (Np), randomly attached onto the polymer backbone with an amount of 3 mol %. The polymer, which is a new typ e of hydrophobically modified polymer denoted PAAMeNp-34, was investigated using steady-state fluorescence spectroscopy in aqueous solutions of differ ent pH and in methanol. The fluorescence emission spectra of PAAMeNp-34 in water exhibit both Np monomer emission (with intensity I-M) and Np excimer emission (with intensity I-E) The excimer emission is mainly due to the ass ociation of Np groups, preformed in their ground electronic state as a resu lt of the hydrophobic interaction. For a PAAMeNp-34 aqueous solution, the i ntensity ratio, I-E/I-M, decreases in the pH range where the electrostatic repulsive forces overcome the hydrophobic interactions between the Np group s and the polymer chain expands because of the intrapolymer repulsion betwe en the negatively charged carboxylate groups. In methanol, the excimer emis sion is low because hydrophobic interactions are insignificant in this solv ent. The interaction between PAAMeNp-34 and cationic surfactants of differe nt alkyl chain length (dodecyl-, tetradecyl-, and hexadecyltrimethylammoniu m chloride) was also studied in dilute aqueous solutions at pH 3.0 and pH 6 .8. The addition of surfactants perturbs the Np-Np interactions because of polymer-surfactant associations. This causes a detectable change in the flu orescence;emission, which is followed with increasing surfactant concentrat ion. From the onset of the change, the force that dominates the interaction between the polymer and the surfactants at different pH can be examined. A t low pH, PAAMeNp-34 is uncharged and hydrophobic forces dominate the polym er-surfactant interaction. The photophysical properties of the system there fore show a clear dependence on the hydrophobicity (or chain length) of the surfactants. On the other hand, at pH 6.8, where the polymer is negatively charged, almost no or very little difference between the three surfactants is observed at the onset of fluorescence change, which indicates that elec trostatic forces dominate the interaction at the lowest surfactant concentr ations.