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      Lyotropic liquid crystal to soft mesocrystal transformation in hydrated salt-surfactant mixtures

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      Author(s)
      Albayrak, C.
      Barım, G.
      Dag, Ö.
      Date
      2013
      Source Title
      Chemistry - A European Journal
      Print ISSN
      0947-6539
      Electronic ISSN
      1521-3765
      Publisher
      Wiley
      Volume
      19
      Issue
      44
      Pages
      15026 - 15035
      Language
      English
      Type
      Article
      Item Usage Stats
      140
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      117
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      Abstract
      Hydrated CaCl2, LiI, and MgCl2 salts induce self-assembly in nonionic surfactants (such as C12H 25(OCH2CH2)10OH) to form lyotropic liquid-crystalline (LLC) mesophases that undergo a phase transition to a new type of soft mesocrystal (SMC) under ambient conditions. The SMC samples can be obtained by aging the LLC samples, which were prepared as thin films by spin-coating, dip-coating, or drop-casting of a clear homogenized solution of water, salt, and surfactant over a substrate surface. The LLC mesophase exists up to a salt/surfactant mole ratio of 8, 10, and 4 (corresponding to 59, 68, and 40wt % salt/surfactant) in the CaCl2, LiI, and MgCl2 mesophases, respectively. The SMC phase can transform back to a LLC mesophase at a higher relative humidity. The phase transformations have been monitored using powder X-ray diffraction (PXRD), polarized optical microscopy (POM), and FTIR techniques. The LLC mesophases only diffract at small angles, but the SMCs diffract at both small and wide angles. The broad surfactant features in the FTIR spectra of the LLC mesophases become sharp and well resolved upon SMC formation. The unit cell of the mesophases expands upon SMC transformation, in which the expansion is largest in the MgCl2 and smallest in the CaCl2 systems. The POM images of the SMCs display birefringent textures with well-defined edges, similar to crystals. However, the surface of the crystals is highly patterned, like buckling patterns, which indicates that these crystals are quite soft. This unusual phase behavior could be beneficial in designing new soft materials in the fields of phase-changing materials and mesostructured materials, and it demonstrates the richness of the phase behavior in the salt-surfactant mesophases.
      Keywords
      Alkali metals
      Liquid crystals
      Mesophases
      Salts
      Surfactants
      Permalink
      http://hdl.handle.net/11693/20749
      Published Version (Please cite this version)
      http://dx.doi.org/10.1002/chem.201301662
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