Ordering by collapse: Formation of bilayer and trilayer crystals by folding Langmuir monolayers

TitleOrdering by collapse: Formation of bilayer and trilayer crystals by folding Langmuir monolayers
Publication TypeJournal Article
Year of Publication2007
AuthorsVaknin D, Bu W, Satija SK, Travesset A
Journal TitleLangmuir
Date PublishedFeb
Type of ArticleArticle
ISBN Number0743-7463
Accession NumberISI:000243990200042
KeywordsABSORPTION SPECTROSCOPY, ACID, air-water-interface, charged interfaces, FATTY-ACID MONOLAYERS, films, METAL-IONS, reflection, STEARIC, surface, x-ray-diffraction

Neutron and synchrotron X-ray studies of arachidic-acid monolayers compressed to the collapse region, beyond their densely packed molecular area, reveal that the resulting structures exhibit a surprising degree of reproducibility and of order. The structure of the collapsed monolayers differs for films that are spread on pure water or on CaCl2 solutions. On pure water, the collapsed monolayer forms a stable crystalline trilayer structure, with acyl-chain in-plane packing practically identical to the three-dimensional (3D) crystal structure of fatty acids. For monolayers spread on Ca2+ solutions, the collapsed film consists of a bi- and trilayer mixture with a ratio that changes by the collapse protocol. Our analysis suggests that the bilayer structure is inverted, i.e., with the hydrophobic tails in contact with the water surface and the calcium ions bridging the polar heads. The inverted bilayer structure possesses a well-ordered crystalline slab of calcium oxalate monohydrate intercalated between two acyl chains. We provide theoretical arguments rationalizing that the observed structures have lower free energies compared with other possible structures and contend that the collapsed structures may, under certain circumstances, form spontaneously.

Alternate JournalLangmuir