Association of Napthalene-Labeled Poly(acrylic acid) and Interaction with Cationic Surfactants. Fluorescence Studies
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Water-soluble poly(acrylic acid) has been covalently labeled with a fluorescent hydrophobic chromophore, naphthalene (Np), randomly attached onto the polymer backbone with an amount of 3 mol %. The polymer, which is a new type of hydrophobically modified polymer denoted PAAMeNp-34, was investigated using steady-state fluorescence spectroscopy in aqueous solutions of different pH and in methanol. The fluorescence emission spectra of PAAMeNp-34 in water exhibit both Np monomer emission (with intensity IM) and Np excimer emission (with intensity IE). The excimer emission is mainly due to the association of Np groups, preformed in their ground electronic state as a result of the hydrophobic interaction. For a PAAMeNp-34 aqueous solution, the intensity ratio, IE/IM, decreases in the pH range where the electrostatic repulsive forces overcome the hydrophobic interactions between the Np groups and the polymer chain expands because of the intrapolymer repulsion between the negatively charged carboxylate groups. In methanol, the excimer emission is low because hydrophobic interactions are insignificant in this solvent. The interaction between PAAMeNp-34 and cationic surfactants of different alkyl chain length (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium 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 fluorescence emission, which is followed with increasing surfactant concentration. From the onset of the change, the force that dominates the interaction between the polymer and the surfactants at different pH can be examined. At low pH, PAAMeNp-34 is uncharged and hydrophobic forces dominate the polymer−surfactant interaction. The photophysical properties of the system therefore 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 electrostatic forces dominate the interaction at the lowest surfactant concentrations.