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Thermotropic phase behavior of choline soaps
Regina Klein1, Helen Dutton, Olivier Diat
1Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany.
The Journal of Physical Chemistry. B
|March 25, 2011
Summary
Choline carboxylates (ChCm) are biocompatible surfactants that exhibit thermotropic mesomorphism. These simple choline soaps show enhanced water solubility and reduced melting points, classifying them as ionic liquids.
Area of Science:
- Materials Science
- Physical Chemistry
- Surfactant Chemistry
Background:
- Choline carboxylates (ChCm) are biocompatible anionic surfactants.
- They exhibit low Krafft temperatures and complex aqueous phase behavior.
- Anhydrous ChCm salts (m=12-18) were studied for their thermotropic mesomorphism.
Purpose of the Study:
- To investigate the thermotropic mesomorphism of anhydrous choline carboxylates (ChCm) for chain lengths m = 12-18.
- To characterize the phase behavior and transition temperatures of these compounds.
- To explore their potential classification as ionic liquids.
Main Methods:
- Differential scanning calorimetry (DSC) for transition temperatures and enthalpies.
- Optical polarizing microscopy for phase characterization.
- NMR spin-spin relaxation and X-ray scattering for detailed phase analysis.
Main Results:
- All investigated ChCm compounds (m=12-18) displayed three distinct phases between -20 and 95 °C.
- Phases identified with increasing temperature: crystalline, semicrystalline, and lamellar liquid-crystalline.
- Even long-chain ChCm (m=18) formed a lamellar liquid-crystalline phase below 100 °C.
Conclusions:
- Choline as a counterion enhances water solubility and significantly lowers melting points of fatty acid soaps.
- This facilitates thermotropic mesomorphism in choline soaps within a practical temperature range (35-95 °C).
- Choline soaps (m=12-18) can be classified as ionic liquids due to their properties.
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