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A low temperature phase transition in Langmuir-Blodgett films.
MedLine Citation:
PMID:  18855435     Owner:  NLM     Status:  PubMed-not-MEDLINE    
Abstract/OtherAbstract:
The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of approximately 8 cm (-1) in the r+, r- and rfermi modes of the terminal methyl groups at approximately 150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile's headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems.
Authors:
Thomas P Johansson; Gary W Leach
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Publication Detail:
Type:  Journal Article     Date:  2008-10-15
Journal Detail:
Title:  The journal of physical chemistry. B     Volume:  112     ISSN:  1520-6106     ISO Abbreviation:  J Phys Chem B     Publication Date:  2008 Nov 
Date Detail:
Created Date:  2008-10-30     Completed Date:  2008-12-17     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101157530     Medline TA:  J Phys Chem B     Country:  United States    
Other Details:
Languages:  eng     Pagination:  13823-33     Citation Subset:  -    
Affiliation:
Laboratory for Advanced Spectroscopy and Imaging Research, Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada.
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