The Adsorption of Oppositely Charged Polyelectrolyte/Surfactant Mixtures at the Air/Water Interface: Neutron Reflection from Dodecyl Trimethylammonium Bromide/Sodium Poly(styrene sulfonate) and Sodium Dodecyl Sulfate/Poly(vinyl pyridinium chloride)

Abstract

The interactions between oppositely charged surfactant/polymer mixtures have been studied using neutron reflectometry with supplementary surface tension measurements. The cationic surfactant dodecyl trimethylammonium bromide (C12TAB)/anionic polyelectrolyte sodium poly(styrene sulfonate) (NaPSS) system is compared with a system containing anionic sodium dodecyl sulfate (SDS) and cationic poly(vinyl pyridinium chloride) (PVPmCl). The PVPmCl/SDS mixture has been studied both with and without added electrolyte. Neutron reflection shows that for both systems, the surface consists of a mixture of polyelectrolyte and surfactant over a range of surfactant concentrations from above the critical micelle concentration (CMC) to CMC/100 for polymer concentrations between 10 and 140 ppm. In the lower surfactant concentration range, the amount of surfactant adsorbed approximately corresponds to a surfactant monolayer (area per molecule ∼ 35−45 Å 2 for SDS in the presence of 0.1 M NaCl, 35−60 Å2 without NaCl, and 50−60 Å2 for C12TAB with 0.1 M NaBr). However, at higher concentrations and in the presence of electrolyte, this increases to an amount approximately corresponding to three adsorbed layers (area per molecule = 12 Å2 for SDS and 17−20 Å2 for C12TAB). This increase is not observed for PVPmCl/SDS in the absence of 0.1 M NaCl. The structure of the higher concentration layer is a sandwich structure with an outer surfactant layer and a submerged polymer/micellar (spheres or rods) or polymer/defective bilayer. The surface tension and neutron results can be interpreted qualitatively in terms of three species in the system, a surface active complex PSS, a bilayer complex , which can only adsorb on a preformed PSS layer, and a bulk solution complex PSM. PSS is adsorbed at very low concentrations of surfactant, possibly even before any PSM is formed in the bulk solution. At high concentrations, there are two effects. There may be adsorption of complexes to the layer of PSS already at the surface. However, the formation of is in competition with the formation of PSM. If the latter is dominant, there is no secondary adsorption of , as is the case for PVPmCl/SDS in the absence of electrolyte, and the surface tension may increase very sharply with surfactant concentation at the point where the formation of PSM in the bulk solution is complete. If there is secondary adsorption of PSM or PSS, as for NaPSS/C12TAB with or without electrolyte and PVPmCl/SDS with electrolyte, the surface tension should show a more modest increase at this concentration.