Adsorption of Hydrophobically Modified Poly(acrylic acid) Sodium Salt at the Air/Water Interface by Combined Surface Tension and X-ray Reflectivity Measurements

The adsorption at the air/water interface of hydrophobically modified poly(acrylic acid) sodium salt (HMPAANa) with various degrees of grafting and lengths of graft has been investigated using both tensiometry and X-ray reflectivity techniques. Tensiometry has provided the Gibbs adsorption isotherms and has revealed that HMPAANa associating copolymers behave like low molecular weight surfactants with surface tensions leveling off at the critical aggregate concentrations (cac) determined from viscosity measurements. However, very long times (up to 2 days) were required to achieve equilibrium. X-ray reflectivity measurements have permitted us to detect a monomer units-rich zone at the air/water interface, corresponding to the proximal zone of the adsorbed polymer layer. This zone thickens with increasing either the bulk polymer concentration or the salt concentration but remains unchanged with varying the polymer backbone molecular weight. The polymer concentration within the zone is 40% in volume fraction with a bulk concentration equal to cac. By coupling both techniques, we show that the Gibbs adsorption theory is valid for HMPAANa copolymers and that the longest polymer loops and tails extending into the sublayer do not contribute to the surface activity. As a matter of fact, a good agreement between the values of the excess surface concentration Γ is obtained using both techniques.