Adsorption of poly(styrenesulfonate) to the air surface of water by neutron reflectivity

The adsorption of the strong polyelectrolyte poly(styrenesulfonate) (PSS) t o the air surface of dilute aqueous solutions was investigated as a functio n of molecular weight and salt concentration. Detailed segment profiles of the deuterated polymer were determined by neutron reflection. Surface activ ity was also examined through surface tension measurements. In general, the segment profiles are composed of a thin layer (10-20 Angstrom thick) of hi gh concentration at the air surface, followed by a distinct second layer of much lower segment concentration that extends to larger depths into the li quid. The high segment density at the air surface is due to a strong surfac e attraction, arising from the low surface energy of the PSS backbone relat ive to the surface tension of water. At low salt concentration, the profile s tend toward a single dense layer, suggesting that the chains lie nearly f lat at the interface in that limit. The adsorbed amount increases with salt concentration, with a stronger dependence for higher molecular weight chai ns. The adsorbed amounts at the air/water interface are higher than reporte d previously for PSS adsorbed onto neutral solid surfaces, consistent with the fact that the air-liquid interface provides a stronger surface attracti on. While the trends of adsorbed amount with salt concentration and molecul ar weight are in good agreement with numerical self-consistent-field lattic e calculations, the measured bilayer profiles are rather different from the smoothly decaying theoretical profiles. The surface tensions of the PSS so lutions are significantly lowered relative to those of pure salt solutions. Combining the reflectivity and surface tension measurements, an approximat ely linear relationship is established between the surface pressure and the PSS adsorbed amount.