The structures of Langmuir–Blodgett films of fatty acids and their salts

Peng, J. B., Barnes, G. T. and Gentle, I. R. (2001) The structures of Langmuir–Blodgett films of fatty acids and their salts. Advances in Colloid and Interface Science, 91 2: 163-219. doi:10.1016/S0001-8686(99)00031-7


Author Peng, J. B.
Barnes, G. T.
Gentle, I. R.
Title The structures of Langmuir–Blodgett films of fatty acids and their salts
Journal name Advances in Colloid and Interface Science   Check publisher's open access policy
ISSN 0001-8686
Publication date 2001-05
Sub-type Article (original research)
DOI 10.1016/S0001-8686(99)00031-7
Volume 91
Issue 2
Start page 163
End page 219
Total pages 57
Place of publication Netherlands
Publisher Elsevier Science
Language eng
Subject 0904 Chemical Engineering
0306 Physical Chemistry (incl. Structural)
Abstract Recent advances in several experimental techniques have enabled detailed structural information to be obtained for floating (Langmuir) monolayers and Langmuir–Blodgett films. These techniques are described briefly and their application to the study of films of fatty acids and their salts is discussed. Floating monolayers on aqueous subphases have been shown to possess a complex polymorphism with phases whose structures may be compared to those of smectic mesophases. However, only those phases that exist at high surface pressures are normally used in Langmuir–Blodgett (LB) deposition. In single LB monolayers of fatty acids and fatty acid salts the acyl chains are in the all-trans conformation with their long axes normal to the substrate. The in-plane molecular packing is hexagonal with long-range bond orientational order and short-range positional order: known as the hexatic-B structure. This structure is found irrespective of the phase of the parent floating monolayer. The structures of multilayer LB films are similar to the structures of their bulk crystals, consisting of stacked bilayer lamellae. Each lamella is formed from two monolayers of fatty acid molecules or ions arranged head to head and held together by hydrogen bonding between pairs of acids or ionic bonding through the divalent cations. With acids the acyl chains are tilted with respect to the substrate normal and have a monoclinic structure, whereas the salts with divalent cations may have the chains normal to the substrate or tilted. The in-plane structures are usually centred rectangular with the chains in the trans conformation and packed in a herringbone pattern. Multilayer films of the acids show only a single-step order–disorder transition at the melting point. This temperature tends to rise as the number of layers increases. Complex changes occur when multilayer films of the salts are heated. Disorder of the chains begins at low temperatures but the arrangement of the head groups does not alter until the melting temperature is reached. Slow heating to a temperature just below the melting temperature gives, with some salts, a radical change in phase. The lamellar structure disappears and a new phase consisting of cylindrical rods lying parallel to the substrate surface and stacked in a hexagonal pattern is formed. In each rod the cations are aligned along the central axis surrounded by the disordered acyl chains.
Keyword Fatty acids
Langmuir–Blodgett films
Structure determination
Grazing incidence
X-ray diffraction
Monolayers
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Chemistry and Molecular Biosciences
 
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Created: Fri, 14 Apr 2006, 02:32:07 EST