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 tetradecylt rimethylammonium bromide (TTAB) with poly(acrylic acid) (PAA) and poly (methacrylic acid) (PMA) has been investigated using a surfactant-sens itive solid-state membrane electrode. The experimental results indicat e that the interaction with PAA begins at about the same equilibrium T TAB 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 con stant. The salt, NaBr, because of its high concentration in the system (10(-2) M), essentially controls charge density of the polymer, and t his can account for the absence of any measurable variation in the ons et 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 T TAB concentrations. The results suggest that the onset of binding is s ignificantly influenced by changes in polymer conformation and possibl y hydrophobicity of the backbone that occur at these low degrees of io nization. In the plateau region of the binding isotherm, PAA and PMA e xhibit opposite behavior. The plateau binding fraction for PAA at degr ees of ionization (i) of 0.10, 0.26, and 0.50 exceeds 1.0, the expecte d value required for charge neutralization. In contrast PMA at i = 0.2 6 exhibits a plateau binding fraction of 0.5, much less than that requ ired for charge neutralization. The low plateau value for PMA at low d egree of ionization is thought to result from the highly coiled nature of PMA which possibly renders a substantial fraction of the ionized c arboxyl groups unavailable for interaction with the surfactant. The sh arp increase beyond the plateau is attributed to surfactant-induced op ening of the coiled PMA.