Interaction between poly(ethylene oxide) and sodium dodecyl sulfate studied by neutron reflection

The composition of the air/solution interface of aqueous mixtures of sodiumdodecyl sulfate (NaDS) and poly(ethylene oxide) (PEO) has been studied as a function of NaDS concentration and at a fixed concentration of 0.1 wt % PEO using neutron specular reflection. With increasing surfactant concentration, the polymer is progressively displaced from the surface until, at the critical aggregation concentration (CAC), it can no longer be detected (area per segment greater than about 80 Angstrom(2). The adsorption of surfactant increases steadily with concentration and shows no sign of a discontinuity at the CAC (4.5 mM at 35 degrees C), which is consistent with the break in the surface tension curve being caused only by the onset of surfactant/polymer aggregation in the bulk solution. At all concentrations the adsorption of NaDS at the air/solution interface is less in the presence of polymer than that in the corresponding solutions without polymer. Nevertheless, it is shown that there is some cooperativity in the adsorption of polymer and surfactant at the interface.