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The structure of an adsorption octadecanamide film at the planartoluene-water interface is studied by X-ray reflectometry using synchrotronradiation with photon energy of 15 keV. The electron density (polarizability)profiles, according to which the interface structure is determined by the pHlevel in the water subphase, are reconstructed from experimental data with thehelp of a model-independent approach. For a high pH~11, the adsorption film isa crystalline octadecanamide monolayer with a thickness of about 2.6 nm, inwhich aliphatic tails of surfactant are extended along the normal to thesurface. For low pH ~ 2, the thickness of the surface structure consisting ofthe crystalline monolayer directly on the toluene-water interface and a thicklayer of deposited octadecanamide micelles reaches about 50 nm. In our opinion,the condensation of nonionogenic surfactant micelles for which the surfaceconcentration of the surfactant increases significantly is caused by a changein the polarization direction upon a decrease in the pH level in the electricdouble layer at the interface between the water subphase and the octadecanamidemonolayer. The shape of the reconstructed electron density profiles alsoindicates the existence of a plane of the closest approach of surfactantmicelles to the interface at a distance of about 7 nm from it.

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