EFFECT OF NONIONIC SURFACTANT ON ION ADSORPTION AND HYDRATION FORCES

Measurements have been carried out of the force between mica surfaces immersed in aqueous micellar solutions of the nonionic surfactant penta(oxyethylene) n-dodecyl ether with sodium sulfate as a background electrolyte. At surfactant concentrations below about 5 ˟ 10-3 M (=100 X cmc), adsorption to the mica surface is weak and can only be indirectly inferred from a reduction in the surface potential in dilute electrolyte solution. Further evidence for adsorption is provided by a decrease in the measured adhesion between the surfaces and an enhanced viscous drag at small (<5.0 nm) separations. In more concentrated salt solution, there is a decrease in the magnitude of the hydration force due to surfactant adsorption. Unlike the case with cationic surfactants, the adsorbed surfactants do not give rise to any steric force; they are easily pushed out as the surfaces are brought together. At high surfactant concentrations (>100 x cmc), extensive aggregation occurs between the surfaces, and this gives rise to very long-range, repulsive forces. We speculate that the reduction in surface potential may be a more general feature of nonionic adsorbates and rationalize some earlier results on the adsorption of dextran. © 1990, American Chemical Society. All rights reserved.