INTERACTION OF TETRADECYLTRIMETHYLAMMONIUM BROMIDE WITH POLY(ACRYLIC ACID) AND POLY(METHACRYLIC ACID) - EFFECT OF CHARGE-DENSITY

The effect of polymer charge density on the interaction of tetradecyltrimethylammonium bromide (TTAB) with poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) has been investigated using a surfactant-sensitive solid-state membrane electrode. The experimental results indicate that the interaction with PAA begins at about the same equilibrium TTAB concentration when the degree of ionization (i) of the polymer is greater than about 0.4. This value, according to Manning's counterion condensation theory, corresponds to the critical degree of ionization for PAA 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 this can account for the absence of any measurable variation in the onset of binding above an ionization of 0.4. Surprisingly, when i < i(c)(=0.4), the onset of TTAB binding to PAA and PMA shifts to lower free TTAB concentrations. The results suggest that the onset of binding is significantly influenced by changes in polymer conformation and possibly hydrophobicity of the backbone that occur at these low degrees of ionization. In the plateau region of the binding isotherm, PAA and PMA exhibit opposite behavior. The plateau binding fraction for PAA at degrees of ionization (i) of 0.10, 0.26, and 0.50 exceeds 1.0, the expected value required for charge neutralization. In contrast 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 is thought to result from the highly coiled nature of PMA which possibly renders a substantial fraction of the ionized carboxyl groups unavailable for interaction with the surfactant. The sharp increase beyond the plateau is attributed to surfactant-induced opening of the coiled PMA.