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 well as competitive adsorption of SDS with the nonionic surfactant Triton X-405 on a polystyrene latex has been examined by the use of small-angle x-ray scattering (SAXS). Since the latex particles only have a low contrast towards water whereas the electron densities of the added surfactants are rather high, their gradual adsorption can be monitored with good sensitivity. The particles covered with SDS have been investigated by the contrast variation method. A distinct core-shell structure has been observed. After maximum coverage of the surface is reached, the additional surfactant forms free micelles. The analysis presented 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 the competitive adsorption could be obtained by analyzing the structure of the coated particles and the composition of the free mixed micelles. The adsorption is shown to be an equilibrium process where adsorption of the nonionic component is found to be much stronger than of the SDS molecules.