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Open AccessArticle

Structure of Langmuir Monolayers of Perfluorinated Fatty Acids: Evidence of a New 2D Smectic C Phase

1
Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin, BP48, 91192 Gif sur Yvette CEDEX, France
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Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
3
Institut des NanoSciences de Paris, UMR 7588 CNRS, Sorbonne Université, 4 place Jussieu, 75252 Paris CEDEX, France
4
Faculté des Sciences Fondamentales et Biomédicales, Université de Paris, 45 rue des Saints-Pères 75006 Paris CEDEX, France
*
Author to whom correspondence should be addressed.
Molecules 2019, 24(19), 3590; https://doi.org/10.3390/molecules24193590
Received: 7 September 2019 / Revised: 29 September 2019 / Accepted: 30 September 2019 / Published: 5 October 2019
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. View Full-Text
Keywords: perfluorinated amphiphiles molecules; Langmuir monolayer; 2D phase diagram perfluorinated amphiphiles molecules; Langmuir monolayer; 2D phase diagram
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Fontaine, P.; Filipe, E.J.M.; Fauré, M.-C.; Rego, T.; Taßler, S.; Alves, A.C.; Silva, G.M.C.; Morgado, P.; Goldmann, M. Structure of Langmuir Monolayers of Perfluorinated Fatty Acids: Evidence of a New 2D Smectic C Phase. Molecules 2019, 24, 3590.

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