Adsorption of Pentaethylene Glycol Monododecyl Ether at the Planar Polymer/Water Interface Studied by Specular Neutron Reflection

Abstract

The adsorption of pentaethylene glycol monododecyl ether (C12E5) onto the surface of poly(methyl methacrylate) (PMMA) has been studied by specular neutron reflection. The polymeric surface was formed by dip-coating a thin layer of PMMA onto a freshly polished silicon oxide block. The structure of the coated thin film was characterized by performing neutron reflectivity measurements under D2O and a mixture of H2O and D2O. The simultaneous model fitting to the measured neutron reflectivity profiles produced a thickness of 52 ± 3 Å for the polymer film. The PMMA density within the coated film was found to be identical to that of the corresponding bulk solid, indicating no appreciable water penetration into the polymeric film. That the structure of the polymer film can be modeled by uniform layer distribution suggests that the outer surface of the thin film was smooth within the experimental resolution. The surfactant adsorption isotherm was determined by making neutron reflectivity measurements in D2O using fully hydrogenated surfactant, hC12hE5. The amount of surfactant adsorbed was found to reach a plateau well below the critical micellar concentration of the surfactant. The limiting area per molecule at the cmc was 51± 3 Å2 and the layer thickness tended to 20 ± 3 Å, suggesting monolayer adsorption. Further detailed structural information of the adsorbed surfactant layer was obtained by using a combination of measurements with chain deuterated and fully hydrogenated surfactants and different water contrasts. The thickness of the dodecyl chain layer was found to be 4 ± 2 Å only, as compared with some 16 ± 3 Å for the ethoxylate headgroup layer, suggesting that the alkyl chains lay flat on the surface of the PMMA while the ethoxylate groups are extended into the aqueous solution. These results, together with our previous work on the adsorption of nonionic surfactant at the hydrophilic silicon oxide/water and hydrophobed silica/water interfaces, show that the structural conformation of the alkyl chains within the adsorbed surfactant layer is heavily affected by the nature of the solid substrate.