COMPETITIVE ADSORPTION OF AN ANIONIC AND A NONIONIC SURFACTANT ON POLYSTYRENE LATEX-PARTICLES AS MONITORED BY SMALL X-RAY-SCATTERING

Adsorption of the anionic surfactant sodium dodecylsulfate (SDS) as we ll as competitive adsorption of SDS with the nonionic surfactant Trito n X-405 on a polystyrene latex has been examined by the use of small-a ngle x-ray scattering (SAXS). Since the latex particles only have a lo w contrast towards water whereas the electron densities of the added s urfactants are rather high, their gradual adsorption can be monitored with good sensitivity. The particles covered with SDS have been invest igated by the contrast variation method. A distinct core-shell structu re has been observed. After maximum coverage of the surface is reached , the additional surfactant forms free micelles. The analysis presente d here allows the simultaneous investigation of the covered particles and the free micelles. This is due to their respective characteristic scattering contributions in different angular ranges. Information on t he competitive adsorption could be obtained by analyzing the structure of the coated particles and the composition of the free mixed micelle s. The adsorption is shown to be an equilibrium process where adsorpti on of the nonionic component is found to be much stronger than of the SDS molecules.