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Freezing transitions in mixed surfactant/alkane monolayers at the air-solution interface
Katherine M Wilkinson1, Lei Qunfang1, Colin D Bain1
1Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, UKOX1 3TA. c.d.bain@durham.ac.uk.
Soft Matter
|July 11, 2020
Summary
Mixed monolayers of hexadecyltrimethylammonium bromide (CTAB) and alkanes exhibit surface freezing. This phenomenon, where alkanes form ordered structures at interfaces, is enhanced at the air-water interface.
Area of Science:
- Surface science
- Physical chemistry
- Materials science
Background:
- Cationic surfactants like CTAB form monolayers at interfaces.
- Alkanes can exhibit phase transitions, including freezing.
- Understanding interfacial behavior is crucial for various applications.
Purpose of the Study:
- To investigate the interfacial behavior of mixed monolayers of CTAB and alkanes at the air-water interface.
- To characterize the freezing transition in these mixed monolayers.
- To compare surface freezing in mixed monolayers with other interface types.
Main Methods:
- Sum-frequency spectroscopy to probe molecular structure.
- Ellipsometry to measure monolayer thickness and properties.
- Systematic variation of alkane chain length.
Main Results:
- Mixed CTAB-alkane monolayers display a first-order freezing transition with temperature.
- The low-temperature phase features upright alkane chains, distinct from the high-temperature liquid-like phase.
- Surface freezing is more favorable in mixed monolayers at the air-water interface compared to bulk alkane-water interfaces.
Conclusions:
- CTAB enhances both wetting and surface freezing of alkanes on water.
- The surfactant plays a critical role in promoting ordered alkane structures at the air-water interface.
- These findings have implications for understanding interfacial phenomena and designing new materials.
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