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Conformational disorder in alkylsiloxane monolayers at elevated temperatures.
Jan Weber1, Thorsten Balgar1, Eckart Hasselbrink1
1Fakultät für Chemie and Center for Nanointegration (CENIDE), Universität Duisburg-Essen, D-45117 Essen, Germany.
The Journal of Chemical Physics
|January 7, 2014
Summary
Octadecylsiloxane monolayers on glass exhibit increasing disorder in alkyl chains with rising temperature, observed via vibrational spectroscopy. This disorder is reversible up to 410 K but becomes permanent at higher temperatures.
Area of Science:
- Surface Science and Spectroscopy
- Materials Chemistry
- Physical Chemistry
Background:
- Octadecylsiloxane monolayers are model systems for studying self-assembled monolayers on silica-based surfaces.
- Understanding the thermal stability and conformational behavior of these monolayers is crucial for applications in surface modification and nanotechnology.
- Previous studies often assume an ordered, all-trans configuration, which may not hold at elevated temperatures.
Purpose of the Study:
- To investigate the thermally induced conformational disorder in octadecylsiloxane monolayers on glass substrates.
- To determine the temperature range and reversibility of alkyl chain disordering.
- To assess the validity of the all-trans configuration assumption at elevated temperatures.
Main Methods:
- Vibrational sum frequency generation (VSFG) spectroscopy was employed to probe the vibrational modes of C-H bonds.
- Measurements were conducted on octadecylsiloxane monolayers on glass substrates under ambient conditions.
- The frequency range of 2850-3000 cm(-1) was monitored to analyze C-H stretching vibrations of methyl and methylene groups.
Main Results:
- A progressive increase in conformational disorder, attributed to gauche defects, was observed as temperature increased from 300 K to 510 K.
- The conformational disorder was found to be reversible within the temperature range of 300 K to approximately 410 K.
- Above 410 K, the monolayer did not fully re-establish its ordered state within the experimental timescale, indicating irreversible changes.
Conclusions:
- The assumption of an all-trans configuration for alkyl chains in octadecylsiloxane monolayers is an oversimplification, especially at elevated temperatures.
- Elevated temperatures, even below decomposition, significantly induce and can permanently alter the conformational order of the alkyl chains.
- VSFG spectroscopy is a powerful tool for characterizing the dynamic conformational changes in molecular monolayers.
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