Thermally induced conformational changes of Ca-arachidate Langmuir-Blodgett Films at different compression

The conformational order in Ca-arachidate Langmuir-Blodgett films on solid glass supports is investigated by means of vibrational sum-frequency generation spectroscopy (VSFG). The symmetric C-H stretching vibrations of both the terminal methyl and the methylene groups are utilized to monitor the cha...

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Bibliographic Details
Published in:The Journal of chemical physics 2014-07, Vol.141 (4), p.044912-044912
Main Authors: Weber, Jan, Beier, Andre, Hasselbrink, Eckart, Balgar, Thorsten
Format: Article
Language:English
Subjects:
Chains
Defects
Degeneration
Enthalpy
High temperature
Langmuir-Blodgett films
Molecular conformation
Packing density
Physics
Temperature dependence
Vapor phases
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Summary:The conformational order in Ca-arachidate Langmuir-Blodgett films on solid glass supports is investigated by means of vibrational sum-frequency generation spectroscopy (VSFG). The symmetric C-H stretching vibrations of both the terminal methyl and the methylene groups are utilized to monitor the chain conformation at various sample temperatures under ambient conditions. At room temperature the film is well-ordered consisting almost entirely of all-trans configured chains. Between 340 and 430 K we observe a marked increase in gauche-defects before oxidative degeneration starts at sample temperatures above 470 K. The temperature dependence of the data is well represented by apparent enthalpy changes for the formation of gauche-defects, sharply increasing with packing density from 29 to 62 kJ/mol; values, which are an order of magnitude larger than those of the gas phase molecule. These large apparent enthalpies do not prevent the formation of a high degree of conformational disorder at elevated temperatures.
ISSN:0021-9606
1089-7690
DOI:10.1063/1.4891243