Interaction of Tetradecyltrimethylammonium Bromide with Poly(acrylic acid) and Poly(methacrylic acid). Effect of Charge Density

The effect of polymer chalge density on the interaction of tetradecyltrimethylammonium brolnide <TrAB) with poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) has been investigated usmc a surfactant-sensitive solid-state membrane electrode. The experimental results indi~te that the intenction with PAA begins at about the same equilibrium TTAB concentration wben the degree of ionization (i) of the polymer is &Teater than about 0.4. This value, according to Manning's counterion condensation theory. corresponds to the critical degree of ionization for P AA above which the effective charge density of the polymer is constant. The salt, NaBr, because of its high concentration in the system (10-2 M). essentially controls charge density of the polymer, and thj., CaD account for the absence of any measurable variation in the onset of binding above an ionization of 0.4. Surprisingly. wben i < ic( =0.4), the onset of TrAB binding to PAA and PMA ~ to lower free TrAB concentrations. The result.1 suggest that the onset of binding is significantly influenced by changes in polymer conformation and p(8ibly bydrophobicity of the backbone that occur at theae low degrees of ionization. In the plateau region of the binding isotherDl. PAA and PMA exhibit opposite behavior. The plateau binding fraction for PAA at deg:rees of ionization (i) of 0.10,0.26. and 0.50 exceed., 1.0, the expected value required for charge neutralization. In contrut PMA at i = 0.26 exhibits a plateau binding fraction of 0.5, much less than that required for charge neutralization. The low plateau value for PMA at low degree of ionization j., thougbt to result from the highly coiled nature of PMA which possibly renders a substantial fraction of the ionized carboxyl &TOUPS unavailable for interaction with the surfactant. The sharp increase beyond the plateau j., attributed to surfactant-induced opening of the coiled PMA.