Adsorption of polyelectrolytes at a soft charged surface
Adsorption de polyélectrolytes par une surface liquide chargée
Résumé
We have studied the adsorption of weak polyelectrolytes by a soft surface (Langmuir film) of opposite charge, as a function of the surface charge density Sigma and of the polymers charge fraction f. Different regimes have been evidenced (ellipsometry, IR spectroscopy). For small values of Sigma, the adsorbed polyelectrolytes neutralize the surface : Gamma=Sigma/f (number of adsorbed monomers per unit surface). For high values of Sigma, it costs less to continue the surface neutralization with counter-ions : Gamma~SigmaION/f with SigmaION~f^0.35. For weak f (~0.12), Gamma is significant and the mixed film properties (amphiphile+polyelectrolytes) are dominated by polymers. The surface pressure due to the polymers is proportional to Sigma^2. A regime change was observed in elasticity and overcompensation of the surface charge. It corresponds, according to recent theories, to a change in the polymer conformation, from flat to 3D. For higher f, the film properties are governed by the amphiphiles.
L'étude porte sur l'adsorption de polyélectrolytes faibles par une surface liquide (film de Langmuir) de charge opposée. Nous avons quantifié l'adsorption des polyélectrolytes et étudié les propriétés de la couche adsorbée en fonction de f le taux charge des polyélectrolytes et de Sigma le nombre de charges portées par la surface par unité de surface. Différents régimes d'adsorption ont été mis en évidence (ellipsométrie, spectrométrie IR). Aux faibles densités surfaciques s