Mixtures of cationic polyelectrolyte and anionic surfactant studied with small-angle neutron scattering

Small-angle neutron scattering (SANS) data for solutions containing a highl y charged cationic polyelectrolyte and an anionic surfactant are presented. The scattering data were obtained in pure D2O, emphasizing the scattering from the polyelectrolyte, and in a H2O/D2O mixture that contrast matches th e polyelectrolyte. In the absence of surfactant, a broad scattering peak du e to the mesh size of the polyelectrolyte solution is the most characterist ic feature. This peak moves to larger q-values (smaller distances) as the p olyelectrolyte concentration is increased, as expected for a semidilute pol yelectrolyte solution. Addition of a small amount of surfactant reduces and finally removes this peak. Instead a sharp diffraction peak appears at hig h q-values. This Bragg peak corresponds to a characteristic distance of 37- 39 Angstrom, and it is observed when either the polyelectrolyte or the surf actant is contrast matched by the solvent. Once this peak has appeared, its position does not change when the surfactant concentration is increased. T he intensity of the peak grows, however, until a stoichiometric polyelectro lyte-surfactant complex has been formed. The Bragg peak remains in excess s urfactant solution. These results are discussed in relation to the structur e of the polyelectrolyte-surfactant aggregates and in connection with recen t results from surface force and turbidity measurements using the same poly electrolyte-surfactant pair.