Interaction between a silica particle and the underside of a polymer monolayer at the air/water interface in the presence of an anionic surfactant

The properties of a monolayer of the non-ionic Pluronic F127 at the air/water interface and the effect of an anionic surfactant (sodium dodecyl sulphate, SDS) were characterized using a Langmuir film balance and the Monolayer Particle Interaction Apparatus; Pluronic F127 is a triblock copolymer of poly(ethylene oxide) (PEO groups)–poly(propylene oxide) (PO groups)–poly(ethylene oxide) (PEO groups). The surface pressure–area (Π–A) isotherms and the forces between a silica particle and the monolayer in the water phase were measured in the absence and presence of SDS molecules. The isotherm of the monolayer showed the characteristics of a pancake-to-brush transition upon compression of the monolayer. An adhesion was measured between the particle and the F127 monolayer. We attribute it to hydrogen bonds between the PEO groups and the silanol groups of the particle. The adhesion increased upon compression of the monolayer, due to the resulting increase in the number of hydrophilic PEO groups in the water phase. The adhesion decreased upon the addition of SDS, interpreted by an adsorption of SDS to the air/water interface. The presence of SDS at the interface results in an electrostatic repulsion between the negatively charged SDS headgroups and the negatively charged silica particle. The addition of SDS also decreased the stiffness of the monolayer. This increased flexibility can be explained by (1) the more fluid nature of the SDS molecules in comparison to the polymer molecules, (2) the electrostatic repulsion between the SDS groups in the monolayer, and/or (3) binding between the hydrophobic PO groups and the hydrophobic tails of the SDS molecules.