Phase diagram for the adsorption of weak polyelectrolytes at a soft charged surface
Résumé
We have experimentally studied the adsorption of polyelectrolytes at oppositely charged surfaces. A weak flexible polyelectrolyte, poly(acrylic acid), was adsorbed from dilute solutions on a Langmuir film of a cationic amphiphile, dimethyldioctadecylammonium bromide. The polymer surface coverage, Γ, at equilibrium was measured by two reflectivity techniquesellipsometry and polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS)as a function of the surface charge density, σ, and of the polymer ionization degree, α. Different adsorption regimes were evidenced. For weakly charged surfaces, σ < σsat, Γ increases with σ and with 1/α, as expected for a neutralization of the surface by the adsorbed polymers. For highly charged surfaces, σ > σsat, the adsorption of polyelectrolytes saturates. The mean orientation of the adsorbed chains also depends on the value of σ: it is parallel to the surface for σ < σ* (<σsat) and orthogonal to the surface for σ > σ*. We have measured the values of σsat and σ* as a function of α and compared the results with existing theories.
