Effects of Sodium Dodecyl Sulfate on the Adsorption of Poly(<i>N</i>-isopropylacrylamide) at the Air−Water Interface

Abstract

The effects of sodium dodecyl sulfate (SDS) on the adsorption behavior of poly(N-isopropylacrylamide) (PNIPAM) at the air−water interface have been studied using neutron reflectivity and surface tension measurement. PNIPAM adsorbs strongly at the air−water interface due to its partially hydrophobic nature. It is also a thermosensitive polymer; thus a variation in temperature can modify its adsorption behavior. In the presence of SDS below the critical aggregation concentration (CAC), the polymer adsorption is unaffected. Above the CAC however, the polymer is progressively displaced from the surface. This loss of polymer from the surface layer can be explained by the complexation of the polymer with the surfactant in solution and an equilibrium of the charged complex at the surface and in the bulk phase. Furthermore, the thermal response of the polymer adsorption is attenuated and shifted to higher temperatures. This effect is equivalent to the increased solubilization of the polymer chain by charged micellar aggregates in solution. The concentration profiles of the adsorbed polymer indicate the presence of micellar aggregates in the adsorbed layer, the repulsions of which decrease the monomer packing density. Thus, polymer−surfactant interaction at the surface reflects that in the bulk solution.