Dilational viscoelasticity of polyelectolyte/surfactant adsorption films at the air/water interface: Dodecyltrimethylammonium bromide and sodium poly(styrenesulfonate)

The dynamic surface elasticity of mixed aqueous poly(styrenesulfonate)/alkyltrimethylammonium bromide solutions has been measured by the oscillating barrier, oscillating drop, and capillary wave methods as a function of time and surfactant concentration. At low surfactant concentrations (<0.3 mM) the surface viscoelastic behavior is close to that of relatively concentrated pure PSS solutions. The classical Goddard et al. model cannot explain all the experimental results in this concentration range, and a modification of this model is proposed. At intermediate concentrations the real part of the dynamic surface elasticity drops abruptly by almost 1 order of magnitude. This feature can be connected with the formation of a heterogeneous surface film in accordance with recent results by Monteux et al. (Langmuir 2004, 20, 57). At high surfactant concentration (>2 mM) the modulus of the dynamic surface elasticity is low and the adsorbed film is viscoelastic.