Thermotropic phase transition in an adsorbed melissic acid film at the $n$-hexane-water interface

A reversible thermotropic phase transition in an adsorption melissic acid film at the interface between $n$-hexane and an aqueous solution of potassium hydroxide (pH $\approx$ 10) is investigated by X-ray reflectometry and diffuse scattering using synchrotron radiation. The experimental data indicate that the interface “freezing” transition is accompanied not only by the crystallization of the Gibbs monolayer but also by the formation of a planar smectic structure in the $\sim$300-Å-thick adsorption film; this structure is formed by $\sim$50-Å-thick layers.