Adsorption of Weak Polyelectrolytes to an Oppositely Charged Langmuir Film: Change in the Conformation of the Adsorbed Molecules and Saturation of the Ionization Degree

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 < σ 0 to perpendicular to the surface for σ > σ 0 . A signature of this regime change also appears in the compression elastic modulus. The value of σ 0 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.