Synchrotron X-Ray Reflectivity Study of the Adsorption Film of Octadecanamide at the Toluene-Water Interface

The structure of an adsorption octadecanamide film at the planar toluene-water interface is studied by X-ray reflectometry using synchrotron radiation with photon energy of 15 keV. The electron density (polarizability) profiles, according to which the interface structure is determined by the pH level in the water subphase, are reconstructed from experimental data with the help of a model-independent approach. For a high pH~11, the adsorption film is a crystalline octadecanamide monolayer with a thickness of about 2.6 nm, in which aliphatic tails of surfactant are extended along the normal to the surface. For low pH ~ 2, the thickness of the surface structure consisting of the crystalline monolayer directly on the toluene-water interface and a thick layer of deposited octadecanamide micelles reaches about 50 nm. In our opinion, the condensation of nonionogenic surfactant micelles for which the surface concentration of the surfactant increases significantly is caused by a change in the polarization direction upon a decrease in the pH level in the electric double layer at the interface between the water subphase and the octadecanamide monolayer. The shape of the reconstructed electron density profiles also indicates the existence of a plane of the closest approach of surfactant micelles to the interface at a distance of about 7 nm from it.