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 hexaox yethylene glycol monoether (CTAC/C12E8) was studied by turbidimetric titrat ions at varying pH, in order to determine the effect of polymer charge. Ele ctrostatically-driven complex formation is controlled by three variables: t he 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 critic al mole fraction of CTAC corresponding to the onset of polymer-micelle comp lex formation, Y-c, varies inversely with pH, as expected, in the range 0.8 0 < 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, suggest ing that the polymer-micelle interaction is predominantly controlled by ele ctrostatic forces. However, at pH 4.0 (alpha = 0.10), Y, is much smaller th an expected. Turbidimetric titration of PAA + C12E8 with HCl shows complex formation bellow pH 4.2 even in the absence of CTAC. The concomitant pH inc rease indicates that the interaction at low pH is dominated by hydrogen bon ding. The contribution of H-bonding to PAA-CTAC/C12E8 complex formation at moderate pH is superimposed on the electrostatically controlled polymer-mic elle interaction. An additional factor in PAA-CTAC/C12E8 complexation is th e role of the PAA counterion. Comparison of PAA with poly(acrylamido-2-meth ylpropane sulfate) in NaCl and in tetramethylammonium chloride indicates th at Na+ binds to PAA at large a, reducing the effective polymer charge densi ty. (C) 1999 Elsevier Science B.V. All rights reserved.