Energetics and structures of the tilted phases of fatty acid Langmuir monolayers

Toledano, Óscar, Rubio Álvarez, Miguel Ángel y Gálvez, Óscar . (2020) Energetics and structures of the tilted phases of fatty acid Langmuir monolayers. Physical Chemistry Chemical Physics

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Título Energetics and structures of the tilted phases of fatty acid Langmuir monolayers
Autor(es) Toledano, Óscar
Rubio Álvarez, Miguel Ángel
Gálvez, Óscar
Materia(s) Física
Abstract Langmuir monolayers are monomolecular deep films composed of amphiphilic molecules which are typically confined to a water/air interface in a bi-dimensional structure. Due to the important applications in many research areas, they have been studied for many years. Their phase diagrams present several condensed phases, showing untilted or tilted structures at low values of surface pressure. In this paper, we present a novel density functional study on tilted phases of different fatty acid Langmuir monolayers. By means of this study, a further understanding of the physical chemistry properties and the nature of the formation of tilted monolayers can be achieved. Our calculations reveal that, regardless of the number of carbon atoms which form the apolar chain, the transversal (or conventional in the case of untilted phases) unit cell shows similar dimensions, ca. 4.9 × 6.8 Å, which is in fair agreement with the range of the observed data. The energy variation of the unit cell as a function of the inclination of the molecules, reveals an abrupt increase in values larger than 45° and 36° for NN- and NNN-tilt, respectively, in fair agreement with the experimental observation of L2h (NN) and L2′ (NNN) phases of fatty acids. All of the fatty acids explored (from 10 to 19 carbon atoms) yield similar results. Finally, the energetics and structural changes of the monolayer along the variation of the area per molecule, obtained by enlarging in a-, b- or both axes of the untilted unit cell, have been explored. This study reveals that the untilted phases are energetically more stable at low values of area per molecule (high surface concentration), as it is expected. When the area per molecule values are increased, tilted phases (along NN or NNN-direction) with b/a ratio typical of herringbone (HB) or pseudo-herringbone (PHB) structures are found in the lowest energy configurations, which depend on how the distortion of the untilted unit cell is performed. For example, HB structures are the most stable when the molecules tilt along the enlarged axis of the untilted unit cell (a or b), meanwhile unit cell structures characteristic of PHB configurations occur in the opposite cases and at larger values of the area per molecule (low surface concentrations). All these predictions are in good agreement with the GIXD observations of the different phases of the phase diagram of fatty acid Langmuir monolayers.
Palabras clave Langmuir monolayer
fatty acids
dihydrogen interactions
tilted phases
DFT calculations
Editor(es) Royal Society of Chemistry
Fecha 2020
Formato application/pdf
Identificador bibliuned:DptoFF-FCIE-Articulos-Marubio-0004
http://e-spacio.uned.es/fez/view/bibliuned:DptoFF-FCIE-Articulos-Marubio-0004
DOI - identifier 10.1039/D0CP01361G
ISSN - identifier 1463-9084
Nombre de la revista Physical Chemistry Chemical Physics
Número de Volumen 22
Página inicial 12092
Página final 12103
Publicado en la Revista Physical Chemistry Chemical Physics
Idioma eng
Versión de la publicación acceptedVersion
Tipo de recurso Article
Derechos de acceso y licencia http://creativecommons.org/licenses/by-nc-nd/4.0
info:eu-repo/semantics/openAccess
Tipo de acceso Acceso abierto
Notas adicionales La versión registrada de este artículo, publicado por primera vez en Physical Chemistry Chemical Physics, está disponible en línea en el sitio web del editor: Royal Society of Chemistry https://doi.org/10.1039/D0CP01361G
Notas adicionales The registered version of this article, first published in Physical Chemistry Chemical Physics, is available online at the publisher's website: Royal Society of Chemistry https://doi.org/10.1039/D0CP01361G

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Creado: Fri, 09 Feb 2024, 04:31:11 CET