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    X-Ray Reflectometry of the Adsorption Octadecanamide Film at the Toluene-Water Interface

    Year: 2019

    Journal: J. Exp. Theor. Phys., Volume 129, SEP, page 368–374

    Authors: Tikhonov, A. M.; Volkov, Yu. O.

    Organizations: Department of Energy (USA)United States Department of Energy (DOE) [DE-AC02-98GH10886]; ChemPatCARS funds of the University of Chicago; State University of New York in Stony Brook; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00108]; University of Illinois in Chicago

    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 approximate to 11, the adsorption film is a crystalline octadecanamide monolayer with a thickness of about 26 angstrom, in which aliphatic tails of surfactant are extended along the normal to the surface. For low pH approximate to 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 500 angstrom. 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 70 angstrom from it.
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