Study of the Effect of Polyion Charge Density on Structural Properties of Complexes between Poly(acrylic acid) and Alkylpyridinium Surfactants

Structural properties of complexes of poly(acrylate) anion (PA) and the cationic surfactants dodecylpyridinium (DPC) and cetylpyridinium chloride (CPC) were investigated by synchrotron X-ray scattering at various degrees of neutralization α N of poly(acrylic acid) (HPA) and at a fixed surfactant:polymer molar ratio. In all scattering curves, one broad (PA−DPC complexes, all α N -values, and PA−CPC complexes, α N = 0) or several sharp peaks (PA−CPC complexes, α N > 0) were observed, pointing to a high degree of ordering in these systems. The fundamental type of organization of surfactant is in the form of polymer-induced micelles that are clustered around the polymer. The multiple reflections in the case of PA−CPC complexes point to a Pm 3 n cubic structure. The calculated values of structural parameters indicate a larger degree of ordering in complexes at low α N -values (i.e., at α N < 0.5) than that at higher ones. This suggests that the binding of the surfactant by the polyelectrolyte is influenced by factors other than the charge density of polyion. At low α N -values, the micelles are attracted by the polymer mainly through hydrophobic interactions. However, pH measurements have demonstrated that a nonnegligible contribution to attractive surfactant−polymer interactions at α N -values close to 0 comes also from a small but pronounced increase of polymer charge upon surfactant binding. The presence of surfactant gives rise to a nearly 5-fold increase in the degree of ionization of HPA at α N = 0. This increase becomes less pronounced with increasing α N and is negligible for α N ≥ 0.5. From the position of the peaks in the scattering curves, the characteristic distance ā in the complexes was derived. It increases linearly with α N -values in PA−DPC solutions from 30.2 Å at α N = 0 to 36.6 Å at α N = 1, and correspondingly, ā increases from 44.1 to 51 Å in PA−CPC solutions. The ā -value at α N = 0 almost matches one diameter of a globular DPC or CPC micelle. This suggests that the polymer in a coil conformation does not contribute significantly to the intermicellar distance in complexes. It is proposed that micelles are clustered side by side around the polymer coil. The electrostatic repulsion between likely charged micelles could possibly be overcome by the rearrangement of their counterions from the site of binding with the polymer to the solution layer separating them. The ā -value at α N = 1 exceeds that at α N = 0 by 6.4 and 7 Å for DPC and CPC, respectively. This dimension is comparable to the radius of the PA anion, which is around 5.5 Å. This finding indicates that a layer of a fully charged polymer chain at α N = 1 separates the micelles, and in this way, it contributes to the reduction in electrostatic repulsion among them. The linear variation of ā with α N is an indication of a smooth expansion of the PA chain upon neutralization.