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

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 Angstrom) 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<plus/minus>2 Angstrom thickness and surfactant volume fraction phi (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 absorption of the surfactant on PMMA is reversible. On the more hydrophobic PS substrate an adsorption layer of 11 +/-2 Angstrom thickness and surfactant volume fraction phi (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 Angstrom at 1.20 c.m.c., with a nearly constant surfactant volume fraction in the layer (phi (s) = 0.75 +/-0.03) above a concentration of 0.2 c.m.c. Experiments above the c.m.c. reveal that a surfactant layer of 18 +/-2 Angstrom thickness is adsorbed irreversibly to the PS surface.