Adsorption of Poly(acrylic acid) at an Oppositely Charged Langmuir Film: Surface-Tension, Ellipsometry, and Elasticity Measurements

We have studied the adsorption of a weak polyelectrolyte at the free surface of water covered with an oppositely charged Langmuir film as a function of the surface density σ of the Langmuir film. The adsorption is slow (a few hours). The equilibrium surface pressure π of the mixed film and its compression elastic modulus ε are dominated by the contribution of the polyelectrolytes in a large range of σ. In the entire range of σ studied (0.20−1.1 molecule/nm 2 ) π scales as σ 2 . On the contrary, both ε and the ellipticity of the film (which is a measure of the adsorbed quantity) show a regime change at σ ≈ 0.45 molecule/nm 2, presumably from polymers lying flat at the surface to polymers forming a “carpet”, as was predicted by theory.