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)

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 (C 12TAB)/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 mixtures 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 Å 2 without NaCl, and 50-60 Å 2 for C 12TAB 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 Å 2 for SDS and 17-20 Å 2 for C 12TAB). 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 PS S, a bilayer complex PS′ S, which can only adsorb on a preformed PS S layer, and a bulk solution complex PS M. PS S is adsorbed at very low concentrations of surfactant, possibly even before any PS M is formed in the bulk solution. At high concentrations, there are two effects. There may be adsorption of PS′ S complexes to the layer of PS S already at the surface. However, the formation of PS′ S is in competition with the formation of PS M. If the latter is dominant, there is no secondary adsorption of PS′ S, as is the case for PVPmCl/SDS in the absence of electrolyte, and the surface tension may increase very sharply with surfactant concentration at the point where the formation of PS M in the bulk solution is complete. If there is secondary adsorption of PS M or PS S, as for NaPSS/C 12TAB with or without electrolyte and PVPmCl/SDS with electrolyte, the surface tension should show a more modest increase at this concentration