Structural studies of semifluorinated hydrocarbon monolayers at the air/water interface

Abstract

We report the results of experimental studies of the equation of state and structure of water-supported monolayers of the semifluorinated hydrocarbons F(CF₂)₈(CH₂)₁₂H, F(CF₂)₁₀(CH₂)₁₁H and F(CF₂)₁₂(CH₂)₁₈H at temperatures from 4 to 30 °C. We find that monolayers of these molecules are metastable to stable, and that the stability improves as the length of the fluorinated block increases. Grazing-incidence X-ray diffraction measurements of the F(CF₂)₁₂(CH₂)₁₈H monolayer show that it is capable of forming ordered structures defined by hexagonal close packing of the fluorinated blocks of the adsorbed molecules. This monolayer undergoes slow relaxation, which is accompanied by increasing order and increasing macroscopic inhomogeneity. X-Ray reflectivity studies of the F(CF₂)₁₂(CH₂)₁₈H monolayer were carried out on a flat area of the surface; an analysis of the reflectivity data reveals a hydrocarbon-down, fluorocarbon-up orientation for the F(CF₂)₁₂(CH₂)₁₈H molecule.