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9(10),16-Dihydroxypalmitic acid (diHPA) is a particularly interesting polyhydroxylated fatty acid (1) because it is the main monomer of cutin, the most abundant biopolyester in nature, and (2) because the presence of a terminal and a secondary hydroxyl group in midchain positions provides an excellent model to study their intermolecular interactions in a confined phase such as self-assembled layers. In this study we have combined atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy, as well as molecular dynamics (MD) simulations to conclude that the self-assembling of diHPA molecules on mica is a layer by layer process following a Brunauer-Emmett-Teller (BET) type isotherm and with the first layer growing much faster than the rest. Interactions between secondary hydroxyls reinforce the cohesive energy of the monolayer, while the presence of the terminal hydroxyl group is necessary to trigger the multilayered growth. Besides, XPS and ATR-FT-IR spectroscopies clearly indicate that spontaneous self-esterification occurs upon self-assembling. The esterification reaction is a prerequisite to propose a self-assembly route for the biosynthesis of cutin in nature. Molecular dynamics simulations have shown that internal molecular reorganization within the self-assembled layers provides the appropriate intermolecular orientation to facilitate the nucleophilic attack and the release of a water molecule required by the esterification reaction.

Original publication

DOI

10.1021/la9001412

Type

Journal article

Journal

Langmuir

Publication Date

16/06/2009

Volume

25

Pages

6869 - 6874

Keywords

Aluminum Silicates, Esterification, Microscopy, Atomic Force, Palmitic Acids, Surface Properties