On the Multi-Layered Adsorption of Alkanols in the Vicinity of Liquid–Vapor Transition at the Saturated Hydrocarbon/Water Interface

Abstract

Structure of the adsorption layer of unsaturated monoatomic alcohols, 1-dodecanol and 1‑tetracosanol, at n-hexane/water and n-hexadecane/water interfaces, respectively, in the vicinity of liquid–vapor thermotropic phase transition is investigated by the method of X-ray reflectometry with a synchrotron source. Model-independent structural data obtained on the adsorption layers under investigation deviate considerably from the structural parameters that have been previously proposed within a model-based approach and discussed for the said systems. It is shown that in the low-temperature mesophase the adsorption film consists of a Gibbs monolayer, a liquid transition region with thickness of two to three monolayers ~50 Å, and an extended (wide up to 200 Å) layer of micelles. Presence of a plane of the closest approach of the micellar layer to the adsorption film at the interface is established. Transition to the high-temperature mesophase is accompanied by liquefying and partial evaporation of the alkanol film along with depletion of the micellar layer down to its complete disappearance.