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

TL;DR

This study investigates the phase transition of octadecanoic acid films at the toluene-water interface, revealing a melting process driven by tail disordering with detailed structural parameters obtained via X-ray reflectometry.

Contribution

It provides the first detailed structural characterization of the solid-liquid phase transition of octadecanoic acid monolayers at the toluene-water interface using synchrotron X-ray reflectometry.

Findings

Solid monolayer on n-hexane-water interface with perpendicular tails.

Melting of the monolayer on toluene-water interface with temperature increase.

Monolayer thickness remains nearly constant during transition.

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 +/- 1) Angstrom thick, in which aliphatic tails are perpendicular to the surface and the area per molecule is A=(18 +/- 2) Angstrom^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 +/- 1) Angstrom. In the solid phase, we have A=(20+/- 2) Angstrom^2 and the angle of deviation of the molecular tails from the normal to the surface is about ~ 30 deg. The density of the liquid monolayer phase with A=(24 +/- 2) Angstrom^2 corresponds to liquid n-octadecane.