Complex formation between polyacrylic acid and cationic/nonionic mixed micelles: effect of pH on electrostatic interaction and hydrogen bonding

The interaction of a weak polyacid, sodium polyacrylate (PAA), and cationic mixed micelles of n-hexadecyl trimethyl ammonium chloride/n-dodecyl hexaoxyethylene glycol monoether (CTAC/C12E8) was studied by turbidimetric titrations at varying pH, in order to determine the effect of polymer charge. Electrostatically-driven complex formation is controlled by three variables: the polymer linear charge density (xi), the micelle surface charge density (a), and the ionic strength (I) or the related Debye-Huckel parameter (kappa). In this system, the former two parameters depend, respectively, on alpha, the degree of ionization of the polymer carboxylic acid groups, and on Y, the micellar mole fraction of ionic surfactant. In 0.10 M salt, the critical mole fraction of CTAC corresponding to the onset of polymer-micelle complex formation, Y-c, varies inversely with pH, as expected, in the range 0.80 < alpha < 1.0. At these pH's, Y-c x alpha is constant as predicted by our previously proposed equation sigma(c) x xi x kappa(-1) = constant, suggesting that the polymer-micelle interaction is predominantly controlled by electrostatic forces. However, at pH 4.0 (alpha = 0.10), Y, is much smaller than expected. Turbidimetric titration of PAA + C12E8 with HCl shows complex formation bellow pH 4.2 even in the absence of CTAC. The concomitant pH increase indicates that the interaction at low pH is dominated by hydrogen bonding. The contribution of H-bonding to PAA-CTAC/C12E8 complex formation at moderate pH is superimposed on the electrostatically controlled polymer-micelle interaction. An additional factor in PAA-CTAC/C12E8 complexation is the role of the PAA counterion. Comparison of PAA with poly(acrylamido-2-methylpropane sulfate) in NaCl and in tetramethylammonium chloride indicates that Na+ binds to PAA at large a, reducing the effective polymer charge density. (C) 1999 Elsevier Science B.V. All rights reserved.