Analysis of surfactants adsorbed onto the surface of latex particles by small-angle X-ray scattering

A comprehensive study of the process of adsorption of a nonionic surfactant C18E112 onto poly(styrene) (PS) latex particles by small-angle X-ray scattering (SAXS) is presented. The PS latexes (particle radii 35 and 71 nm) employed in this investigation bear no chemically bound surface charges. The analysis of the process of adsorption by SAXS demonstrates that the point of saturation of the surface may be determined directly from the scattering curves. Free micelles are formed beyond saturation, and no second layer of the surfactant is built up at higher concentrations of the surfactant. Any association of the micelles with the covered latex particle can be ruled out as well. Moreover, an analysis of the radial structure of the surface layer in terms the zeroth and the second moment of the electron density of the layers along the radial direction is given. Both moments can directly be obtained from the SAXS data. The zeroth moment of the excess electron density corroborates the finding that the surfactant is firmly adsorbed onto the surface of the particles. The average extension of the adsorbed layer (2-4 nm) as express through the second moment increases with the amount of adsorbed surfactant. This points to a stretching of the chains due to their mutual interaction when the point of saturation of the surfaced is approached.