Adsorption of a weak polyelectrolyte at an oppositely charged Langmuir film: change in the conformation of the adsorbed molecules, and saturation of the ionization degree

Laurianne Vagharchakian (1, 2) , Bernard Desbat (3) , Sylvie Hénon (2, 1)

1 LBHPC - Laboratoire de biorhéologie et d'hydrodynamique physico-chimique
2 MSC - Matière et Systèmes Complexes
3 LPCM - Laboratoire de Physico-Chimie Moléculaire

We have studied the adsorption of a weak flexible polyelectrolyte, poly(acrylic acid), at an oppositely charged surface, the surface of water covered with a Langmuir film of dimethyldioctadecylammonium bromide. The surface charge density and the polymer ionization degree α were conveniently varied by varying respectively the density σ of the Langmuir film and the pH of the polyelectrolyte solution. By infrared spectroscopy we evidenced a change in the mean orientation of the adsorbed chains when increasing the surface charge density, from parallel to the surface for σ < σ0 to perpendicular to the surface for σ > σ0. A signature of this regime change also appears in the compression elastic modulus. The value of σ0 scales as α at low α, as predicted by theory, but saturates when increasing α, presumably because of a saturation of the polyelectrolyte charge fraction.

Domaines: Biomécanique [physics.med-ph]

Laurianne Vagharchakian, Bernard Desbat, Sylvie Hénon. Adsorption of a weak polyelectrolyte at an oppositely charged Langmuir film: change in the conformation of the adsorbed molecules, and saturation of the ionization degree. Macromolecules, 2004, 37 (23), pp.8715-8720. ⟨10.1021/ma048540u⟩. ⟨hal-00091265⟩