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  Steitz R. ; Schemmel S. ; Shi H. ; Findenegg G.H.: Boundary layers of aqueous surfactant and block copolymer solutions against hydrophobic and hydrophilic solid surfaces. Journal of Physics: Condensed Matter 17 (2005), p. S665-S683

Abstract:
The boundary layer of aqueous surfactants and amphiphilic triblock copolymers against flat solid surfaces of different degree of hydrophobicity was investigated by neutron reflectometry (NR), grazing incidence small angle neutron scattering (GISANS) and atomic force microscopy (AFM). Solid substrates of different hydrophobicity were prepared by appropriate surface treatment or by coating silicon wafers with polymer films of different chemical nature. For substrates coated with thin films (20-30 nm) of deuterated poly(styrene) (water contact angle ≈ 90°), neutron reflectivity measurements on the polymer/water interface revealed a water depleted liquid boundary layer of 2-3 nm thickness and a density ca. 90% of the bulk water density. No pronounced depletion layer was found at the interface of water against a less hydrophobic polyelectrolyte coating ( ≈ 63°). It is believed that the observed depletion layer at the hydrophobic polymer/water interface is a precursor of the nanobubbles which have been observed by AFM at this interface. Decoration of the polymer coatings by adsorbed layers of nonionic CmEn surfactants improves their wettability by the aqueous phase at surfactant concentrations well below the critical micellar concentration (CMC) of the surfactant. Here, GISANS experiments conducted on the system SiO2/C8E4/D2O reveal that there is no preferred lateral organization of the C8E4 adsorption layers. For amphiphilic triblock copolymers (PEO-PPO-PEO) it is found that under equilibrium conditions they form solvent-swollen brushes both at the air/water and the solid/water interface. In the latter case, the brushes transform to uniform, dense layers after extensive rinsing with water and subsequent solvent evaporation. These so-called primary adsorption layers maintain properties of the precursor brushes. In particular, their thickness scales with the number of ethylene oxide units (EO) of the block copolymer. In the case of dip-coating without subsequent rinsing, surface patters of the presumably crystalline polymer on top of the primary adsorption layer develop upon drying under controlled conditions. These patterns depend mainly on the nominal surface coverage with the triblock copolymer. Similar patterns are found on bare and polystyrene-coated silicon substrates, indicating that the surface patterning is mainly driven by segregation forces within the polymer layers and not by interactions with the substrate.