Effect of polymer charge density and ionic strength on the formation of complexes between sodium arylamido-2-methyl-1-propane-sulfonate-co-acrylamide gels and cetylpyridinium chloride

The effects of sodium chloride on the composition and structure of polyelectrolyte gel-surfactant complexes (PSCs) formed by the sodium salt of acrylamide-2-methyl-1-propane-sulfonic acid-co-acrylamide gels and cetylpyridinium chloride have been studied. At a low ionic strength of the solution, the composition of all the complexes is close to stoichiometric by charge. In the presence of 0.3 M sodium chloride, the composition of the complexes formed by the gel with 99 mol % charged groups is close to stoichiometric, while for the gel with 33 mol % charged monomer units, a nonstoichiometric complex with a high excess of the surfactant is formed. Further decrease of the charge density up to 10 mol % leads to partial or complete dissociation of the PSCs. The study of PSCs by the method of small-angle X-ray scattering (SAXS) shows that the complexes formed by the gels with high and intermediate charge densities are highly ordered. The decrease of the charge density of the swollen networks at first leads to a change in symmetry of the ordered domains in the PSCs and then to their disordering. The formation of nonstoichiometric PSCs at a high enough concentration of salt is explained by the effect of fitting, when the packing of the surfactant and polymer components in the PSCs is improved due to the inclusion of extra surfactant molecules together with their counterions in the ordered domains.