Excited-state proton transfer in complexes of poly(methacrylic acid) with dodecyltrimethylammonium chloride.

Proton-transfer reactions in aqueous solutions of poly(methacrylic acid) (PMA) were studied using a fluorescent probe and Fourier transform infrared (FTIR) spectroscopy. Protolytic photodissociation of 1-hydroxypyrene (HP) in water was found to be very slow. The PMA polyanion appeared to be very inefficient as a proton acceptor in the excited-state reaction with HP. However, a drastic increase in the deprotonation efficiency was observed in PMA solutions with the same pH values close to neutral when dodecyltrimethylammonium chloride (DTAC) was added. The protonated form of HP, as well as its anion, was shown to be solubilized in polyion-covered micelles. Time-resolved fluorescence data suggested at least two localization sites with different reactivities toward PMA. FTIR spectroscopy was used to quantify the degree of ionization of PMA in PMA-DTAC mixtures. The IR data indicated that protolytic dissociation of PMA could be well described by the Henderson-Hasselbach equation with an apparent pK of 6.6. In contrast, the fluorescent data revealed cooperative protonation of the PMA groups interacting with HP localized within surfactant assemblies. This selective protonation at a pH close to neutral may be associated with a conformational transition in the polymer-surfactant complex.