Fine-tuning the wetting behavior of polyelectrolyte films with sodium dodecyl sulfate

Charge-alternating polyelectrolyte multilayers containing the anionic surfactant sodium dodecyl sulfate (SDS) were constructed by adsorption from aqueous solution. The polyelectrolyte films contained the polycation PEI (poly(ethylenimine)) and the polyanion PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)-benzenesulfonamido]-1,2-ethanediyl, sodium salt]). SDS adsorption on the PEI/PAZO/PEI trilayer was studied using dynamic tensiometry and single wavelength ellipsometry. Advancing contact angle measurements of SDS adsorption onto PEI surfaces indicate that surfactant adsorption is complete within 1 min. The contact angle increases with surfactant concentration, from 30degrees (0 mM SDS) and plateaus to a constant value of 80degrees beyond 0.01 mM SDS. Constructing films from varying concentrations of PEI allowed the degree of PAZO interpenetration to be qualitatively measured. Contact angle and ellipsometric thickness studies indicate that PAZO interpenetrates 32 Angstrom into the terminal PEI layer, above which the surface is composed entirely of PEI. Maximum SDS adsorption is observed above the interpenetrated region, consistent with maximum electrostatic interaction between the terminal PEI layer and SDS.