  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.
