title: Phase diagram for the adsorption of weak polyelectrolytes at a soft charged surface.
abstract: 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, Gamma, at equilibrium was measured by two reflectivity techniques-ellipsometry and polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS)-as a function of the surface charge density, sigma, and of the polymer ionization degree, alpha. Different adsorption regimes were evidenced. For weakly charged surfaces, sigma < sigma sat, Gamma increases with sigma and with 1/alpha, as expected for a neutralization of the surface by the adsorbed polymers. For highly charged surfaces, sigma > sigma sat, the adsorption of polyelectrolytes saturates. The mean orientation of the adsorbed chains also depends on the value of sigma: it is parallel to the surface for sigma < sigma (< sigma sat) and orthogonal to the surface for sigma > sigma. We have measured the values of sigma sat and sigma as a function of alpha and compared the results with existing theories.
authors.affiliation: Matière et Systèmes Complexes, CNRS UMR 7057, Université Paris 7, case courrier 7056, 2 place Jussieu, 75251 Paris Cedex 05, France.
authors.forename: Laurianne
authors.initials: L
authors.lastname: Vagharchakian
authors.forename: Bernard
authors.initials: B
authors.lastname: Desbat
authors.forename: Sylvie
authors.initials: S
authors.lastname: Hénon