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Structure of langmuir monolayers of perfluorinated fatty acids: Evidence of a new 2D smectic C phase

DOI: 10.3390/molecules24193590 DOI Help

Authors: Philippe Fontaine (Synchrotron SOLEIL) , Eduardo J. M. Filipe (Universidade de Lisboa) , Marie-claude Fauré (Institut des NanoSciences de Paris, UMR 7588 CNRS, Sorbonne Université) , Tomas Rego (Institut des NanoSciences de Paris, UMR 7588 CNRS, Sorbonne Université) , Stephanie Tassler (Synchrotron SOLEIL) , Ana Carolina Alves (Universidade de Lisboa; Institut des NanoSciences de Paris, UMR 7588 CNRS, Sorbonne Université) , Gonçalo M. C. Silva (Universidade de Lisboa) , Pedro Morgado (Universidade de Lisboa) , Michel Goldmann (Synchrotron SOLEIL; Institut des NanoSciences de Paris, UMR 7588 CNRS, Sorbonne Université; Université de Paris)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Molecules , VOL 24

State: Published (Approved)
Published: October 2019
Diamond Proposal Number(s): 17042

Open Access Open Access

Abstract: Due to the characteristic chain rigidity and weak intermolecular interactions of perfluorinated substances, the phase diagram of Langmuir monolayer formed by perfluorinated molecules has been interpreted so far as displaying only two phases, a 2D gas (G) and a liquid condensed (LC). However, in this work, we presented Grazing Incidence X-ray Diffraction measurements, which exhibit two diffraction peaks on the transition plateau: One is the signature of the hexagonal structure of the LC phase, the second one is associated to the low-density fluid phase and is thus more ordered than expected for a 2D gas or a typical fluid phase. Atomistic molecular dynamics simulations, performed on the transition plateau, revealed the existence of clusters in which domains of vertical molecules organized in a hexagonal lattice coexist with domains of parallel lines formed by tilted molecules, a new structure that could be described as a “2D smectic C” phase. Moreover, the diffraction spectrum calculated from the simulation trajectories compared favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The results were also in agreement with the thermodynamic analysis of the fluid phase and X-ray Reflectivity experiments performed before and after the transition between these two phases.

Journal Keywords: perfluorinated amphiphiles molecules; Langmuir monolayer; 2D phase diagram

Subject Areas: Chemistry

Instruments: I07-Surface & interface diffraction

Other Facilities: SOLEIL