Competitive adsorption of sodium dodecyl sulfate and polyethylene oxide at the air/water interface

The interfacial and bulk behavior of the aqueous solutions of sodium dodecyl sulfate (SDS) and polyethylene oxide (PEO) was investigated by surface tension measurements in the presence of excess NaBr. The Gibbs analysis demonstrated that the surfactant nearly completely displaces the polymer from the surface even below the cac. Furthermore, the adsorbed amount of SDS revealed universal features in the presence of PEO above M-PEO = 8 x 10(3), i.e. it was found to be independent of the polymer molar mass. The results were also compared with previous neutron reflection measurements on PEO/SDS system without added salt. This comparison revealed that the extent of surfactant adsorption as well as the displacement of the polymer is more pronounced at high ionic strength. This finding might be attributable to the increased adsorption driving force of SDS in 0.1 M NaBr. Above the cac the criteria of the precise determination of surfactant binding isotherms from surface tension measurements were investigated. It was shown that the usual formula used for the calculation of the bound amount of surfactant from surface tension can only be used at high ionic strength. Finally, it was suggested that a sufficient test for the plausibility of the calculated binding isotherms might be their independence on the polymer concentration in the whole surfactant concentration range and not only at the cmc as the earlier studies claimed. (c) 2007 Elsevier Inc. All rights reserved.