Coating of anionic surfactants onto poly(ethylene) surfaces studied with x-ray photoelectron spectroscopy

Poly(ethylene) (PE) samples were immersed in aqueous solutions of different anionic surfactants, viz. sodium dodecane sulfate (SDS), sodium 10-undecene sulfate (S11(:)), and sodium 10-undecenoate (C11(:)). When the PE samples were removed from the solutions, dried, and analyzed with X-ray photoelectron spectroscopy (XPS), surfactant was detected at the surface only if the PE samples were wetted by the surfactant solution. Wetting was accomplished by pretreating the polymer with an argon plasma for 5 s. In this way, a relatively hydrophilic surface was obtained with advancing and receding water contact angles of 89° and 31°, respectively. In a second approach, 1 vol % of hexanol was added to the surfactant solutions, thereby decreasing the surface tension of the solutions to a value below 31 mN/m, allowing direct coating of PE with the surfactants. XPS was performed to estimate the orientation of the molecules in the surfactant layer at the surface. Surfactant molecules were randomly oriented in a homogeneous layer. The thickness of this layer varied from 10 to 60 Å and increased with increasing surfactant concentration of the immersion solution.