Coadsorption of sodium dodecyl sulfate and a polyanion onto poly(ethylenimine) in multilayered thin films

Mixed surfactant-polyelectrolyte multilayer films were fabricated by both ionic self-assembly and spin assembly. A polycation [PEI = poly(ethylenimine)] was deposited from a dilute solution, while a polyanion (PAZO = poly [l-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamide]-l,2-ethanediyl, sodium salt]) was deposited from a mixture containing a fixed concentration of polyanion and various concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). Coadsorption of SDS and PAZO onto PEI layers was observed using both deposition methods and attributed to strong PEI-SDS interactions and entropic factors. Increasing the concentration of SDS resulted in films containing progressively less adsorbed PAZO. No further reduction in the amount of adsorbed PAZO was observed above the SDS critical micelle concentration. We attribute the film growth behavior to a fast adsorption of SDS onto PEI, followed by a slower adsorption of PAZO onto the remaining unoccupied binding sites. We observe that SDS interpenetrates throughout the PAZO and PEI layers, increasing the surface hydrophobicity of both. We observed similar behavior for both ionically self-assembled and spin-assembled systems.