Adsorbed surfactant layers at polymer/liquid interfaces. A neutron reflectivity study

Abstract

The adsorption of the nonionic surfactant C₁₀E₄ from solutions in D₂O to a polymethylmethacrylate (PMMA) and a polystyrene (PS) surface is studied by neutron reflectometry in the concentration range from 0.1 to 4.5 c.m.c. at 289.15 K, corresponding to a temperature ca. 1 K below the lower critical solution temperature of the C₁₀E₄ + D₂O system. Polymer films of uniform thickness (140 to 300 Å) were obtained by spin-coating of silicon wafers. While non-tempered polymer films show some swelling (<5%) in D₂O, annealed films are non-swelling and stable in the aqueous surfactant solutions. Adsorption of C₁₀E₄ on the PMMA surface leads to a layer of 21 ± 2 Å thickness and surfactant volume fraction ϕ_s = 0.63 ± 0.08 over the entire concentration range (0.33–4.50 c.m.c.), and experiments above the c.m.c. show that the adsorption of the surfactant on PMMA is reversible. On the more hydrophobic PS substrate an adsorption layer of 11 ± 2 Å thickness and surfactant volume fraction ϕ_s = 0.53 ± 0.08 is found at the lowest experimental concentration (0.1 c.m.c.). As the surfactant concentration is gradually increased, the thickness of the adsorbed layer grows to 40 ± 3 Å at 1.20 c.m.c., with a nearly constant surfactant volume fraction in the layer (ϕ_s = 0.75 ± 0.03) above a concentration of 0.3 c.m.c. Experiments above the c.m.c. reveal that a surfactant layer of 18 ± 2 Å thickness is adsorbed irreversibly to the PS surface.