Langmuir Films of Perfluorinated Fatty Alcohols: Evidence of Spontaneous Formation of Solid Aggregates at Zero Surface Pressure and Very Low Surface Density

Abstract

In this work, Langmuir films of two highly fluorinated fatty alcohols, CF3(CF2)12CH2OH (F14OH) and CF3(CF2)16CH2OH (F18OH), were studied. Atomic Force Microscopy (AFM) images of the films transferred at zero surface pressure and low surface density onto the surface of silicon wafers by the Langmuir-Blodgett technique revealed, for the first time, the existence of solid-like domains with well-defined mostly hexagonal (starry) shapes in the case of F18OH, and with an entangled structure of threads in the case of F14OH. A (20:80) molar mixture of the two alcohols displayed a surprising combination of the two patterns: hexagonal domains surrounded by zigzagging threads, clearly demonstrating that the two alcohols segregate during the 2D crystallization process. Grazing Incidence X-ray Diffraction (GIXD) measurements confirmed that the molecules of both alcohols organize in 2D hexagonal lattices. Atomistic Molecular Dynamics (MD) simulations provide a visualization of the structure of the domains and allow a molecular-level interpretation of the experimental observations. The simulation results clearly showed that perfluorinated alcohols have an intrinsic tendency to aggregate, even at very low surface density. The formed domains are highly organized compared to those of hydrogenated alcohols with similar chain length. Very probably, this tendency is a consequence of the characteristic stiffness of the perfluorinated chains. The diffraction spectrum calculated from the simulation trajectories compares favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The present results highlight for the first time an inherent tendency of perfluorinated chains to aggregate, even at very low surface density, forming highly organized 2D structures. We believe these findings are important to fully understand related phenomena, such as the formation of hemi-micelles of semifluorinated alkanes at the surface of water and the 2D segregation in mixed Langmuir films of hydrogenated and fluorinated fatty acids.