Boundary layers of aqueous surfactant and block copolymer solutions against hydrophobicand hydrophilic solid surfaces

Abstract

The boundary layer of aqueous surfactants and amphiphilic triblock copolymers against flatsolid surfaces of different degrees of hydrophobicity was investigated by neutronreflectometry (NR), grazing incidence small angle neutron scattering (GISANS) and atomicforce microscopy (AFM). Solid substrates of different hydrophobicities were prepared byappropriate surface treatment or by coating silicon wafers with polymer films of differentchemical natures. For substrates coated with thin films (20–30 nm) of deuteratedpoly(styrene) (water contact angle ), neutron reflectivity measurements on the polymer/water interface revealed a waterdepleted liquid boundary layer of 2–3 nm thickness and a density about 90% of the bulkwater density. No pronounced depletion layer was found at the interface of water against aless hydrophobic polyelectrolyte coating (). It is believed that the observed depletion layer at the hydrophobic polymer/waterinterface is a precursor of the nanobubbles which have been observed by AFM at thisinterface.Decoration of the polymer coatings by adsorbed layers of nonionicCmEn surfactants improves their wettability by the aqueous phase at surfactantconcentrations well below the critical micellar concentration (CMC) ofthe surfactant. Here, GISANS experiments conducted on the systemSiO2/C8E4/D2O reveal that there is no preferred lateral organization of theC8E4 adsorption layers.For amphiphilic triblock copolymers (PEO–PPO–PEO) it is found that under equilibriumconditions they form solvent-swollen brushes both at the air/water and the solid/waterinterface. In the latter case, the brushes transform to uniform, dense layers after extensiverinsing with water and subsequent solvent evaporation. The primary adsorption layersmaintain properties of the precursor brushes. In particular, their thickness scales withthe number of ethylene oxide units (EO) of the block copolymer. In the caseof dip-coating without subsequent rinsing, surface patterns of the presumablycrystalline polymer on top of the primary adsorption layer develop upon drying undercontrolled conditions. The morphology depends mainly on the nominal surfacecoverage with the triblock copolymer. Similar morphologies are found on bare andpolystyrene-coated silicon substrates, indicating that the surface patterning is mainlydriven by segregation forces within the polymer layers and not by interactions with thesubstrate.