Interaction between poly(ethylene oxide) and monovalent dodecyl sulfates studied by neutron reflection

The compositions of the air/solution interface of aqueous mixtures of lithium dodecyl sulfate (LiDS) and cesium dodecyl sulfate (CsDS) with poly(ethylene oxide) (PEO) have been studied as a function of surfactant concentration using neutron specular reflection. Comparison of the results with those for NaDS shows that for all three ionic species the polymer is progressively displaced from the surface on the addition of surfactant until it can no longer be directly detected at the critical aggregation concentration (area per segment >80 Angstrom(2)). Comparison of the behavior of the three surfactants with each other and with measurements on the noninteracting combination dodecyltrimethylammonium bromide (C(12)TAB)/PEO shows that the anionic surfactants interact favorably with PEO at the surface. The additional free energies of binding of surfactant to the surface in the presence of PEO exactly parallel the polymer/micelle binding free energies. Of the three anionics LIDS interacts most strongly, and this is also manifested in adsorption of the PEO at concentrations above the critical micelle concentration of LiDS.