Solid–Liquid Phase Transition in the Octadecanoic Acid Film Adsorbed on the Toluene–Water Interface

Abstract

The structure of the soluble protonated (pH=2) octadecanoic acid film adsorbed on the saturated hydrocarbon (n-hexane)water and aromatic hydrocarbon (toluene)-water interfaces is studied by X-ray reflectometry using synchrotron radiation. The experimental data demonstrate that a solid phase of a Gibbs monolayer 26 ± 1Å thick, in which aliphatic tails are perpendicular to the surface and the area per molecule is A = 18 ± 2Å 2 , forms in the film at the n-hexane -water interface. The solid monolayer on the toluene -water interface in the adsorbed film melts when temperature increases, and this transition is caused by disordering the hydrocarbon tails of the acid. During the solid -liquid transition, the Gibbs monolayer thickness remains almost the same, 22 ± 1Å. In the solid phase, we have A = 20 ± 2Å 2 and the angle of deviation of the molecular tails from the normal to the surface is about ≈ 30 • . The density of the liquid monolayer phase with A = 24 ± 2Å 2 corresponds to liquid n-octadecane.