title: Thermotropic phase behavior of choline soaps.
abstract: Choline carboxylates (ChCm with m = 12-18) are simple biocompatible anionic surfactants with very low Krafft temperatures, possessing a rich aqueous phase behavior. In the present work, we have investigated the thermotropic mesomorphism of anhydrous ChCm salts for m = 12-18. Transition temperatures and enthalpies determined by differential scanning calorimetry reveal that all investigated compounds exhibit three different phases between -20 and 95 °C. The phases were further characterized by optical polarizing microscopy, NMR spin-spin relaxation, and X-ray scattering measurements. The nature of the phases was identified with increasing temperature as crystalline, semicrystalline, and liquid-crystalline lamellar. Even long-chain choline carboxylates (m = 18) were found to melt into a lamellar liquid-crystalline phase below 100 °C. Accordingly, with choline as counterion in simple fatty acid soaps, not only the water solubility is considerably enhanced but also the melting points are substantially reduced, hence facilitating thermotropic mesomorphism at temperatures between 35 and 95 °C. Thus, simple choline soaps with m = 12-18 may be classified as ionic liquids.
authors.affiliation: Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany.
authors.forename: Regina
authors.initials: R
authors.lastname: Klein
authors.forename: Helen
authors.initials: H
authors.lastname: Dutton
authors.forename: Olivier
authors.initials: O
authors.lastname: Diat
authors.forename: Gordon J T
authors.initials: GJ
authors.lastname: Tiddy
authors.forename: Werner
authors.initials: W
authors.lastname: Kunz
mesh_terms: D002152:Calorimetry, Differential Scanning; D002794:Choline / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008859:Microscopy, Polarization; D012915:Soaps / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry; D013816:Thermodynamics; D044366:Transition Temperature; D014961:X-Ray Diffraction
chemical_list: D052578:Ionic Liquids; D012915:Soaps; D013501:Surface-Active Agents; D002794:Choline