Effect of polymer on the arrangement of mixed anionic/cationic wormlike surfactant micelles revealed by SANS

We report the results of small-angle neutron scattering (SANS) study of the effect of neutral water-soluble polymer poly(vinyl alcohol) on the structure of salt-free aqueous solutions of charged cylindrical micelles formed by the mixture of anionic (potassium oleate) and cationic (n-octyltrimethylammonium bromide (C(8)TAB)) surfactants at a fixed molar ratio [C(8)TAB]/[potassium oleate] = 0.46. The dependences of the scattering on surfactant concentration were examined both in the presence and in the absence of polymer. It was observed that at high q range, the scattering function is independent of surfactant concentration and added polymer indicating that the local cylindrical structure of the micelles remains unchanged. At intermediate q range, with growing surfactant concentration the scattering function becomes increasingly dominated by a structure peak that reveals enhanced interactions between neighboring micelles. The peak position q* scales with the surfactant concentration C-surf as C-surf(0.6) and C-surf(0.53) in the absence and in the presence of polymer, respectively. The most important observation concerns the fact that added polymer decreases significantly the average intermicellar distance, which may be due to microsegregation of polymer and surfactant components typical for microphase separation.