Lyotropic Liquid Crystalline Mesophases
Many salts can be used as a solvent to design new type of lyotropic liquid crystalline mesophases. Since the discovery in 2001, the salt-surfactant mesophases have been mostly investigated in my group. We studied the role of various salts that includes transition metal (mostly 1st row), alkali metal (lithium salts) and some alkaline earth metal (calcium and magnesium) salts. These materials can also be expanded to lanthanide and actinide salts. We also explored type of surfactants and found out that the oligo(ethylene oxide) type surfactants and pluronics are excellent surfactants for this assembly process. Interestingly, the phase becomes even more stable at high salt contents in the presence of a charged surfactant such as CTAB and SDS.
We also discovered that the salt melts in the hydrophilic domains of the mesophase and remain liquid down to -50oC and lower. The depression of the melting point is due to the soft confinement effect that also enhances the solubility of the salts in the mesophases. For instance, the solubility of lithium salts can be increased in the hydrophilic domains of the mesophases where one can dissolve more last that one can dissolve in water.
We are currently focused to understand what the fundamental concept behind these unusual phases is. (PLEASE CHECK OUR RECENT PAPER ON THESE MATTERS, some examples are listed below).
The LiNO3-H2O-C12EO10, LiCl-H2O-C12EO10, and LiClO4-H2O-C12EO10 systems form LLC mesophases over a broad range of compositions.
These mesophases have high ionic conductivity and can be used as gel electrolyte in various electrochemical systems (Highlighted as an inside cover of Chemistry A-European Journal).
The zinc nitrate salt acts as a solvent in the ZnX-C12EO10 (ZnX is [Zn(H2O)6](NO3)2 and C12EO10 is C12H25(OCH2CH2)10OH) lyotropic liquid crystalline (LLC) mesophase with a drastic dropping on the melting point of ZnX. The salt-surfactant LLC mesophase is stable down to -52oC and undergoes a phase change into a solid mesostructured salt upon cooling below -52oC; no phase separation is observed down to -190oC. The ZnX-C12EO10 mesophase displays a usual phase behaviour with an increasing concentration of the solvent (ZnX) in the media with an order of bicontinuous cubic(V1)-2D hexagonal(H1)-a mixture of 2D hexagonal and micelle cubic(H1+I)-micelle cubic(I)-micelle(L1) phases.
This is the first phase diagram of [Zn(H2O)6](NO3)2-C12EO10. The salt-surfactant mesophases are stable down to -55oC and freezes to a mesostructured solid.
The mixture of two surfactants (C12EO10-CTAB and C12EO10-SDS) forms lyotropic liquid-crystalline (LLC) mesophases with [Zn(H2O)6](NO3)2 in the presence of a minimum concentration of 1.75 H2O per C12EO10.
The metal ion/C12EO10 mole ratio can be increased up to 8.0, which is a record high metal ion density in an LLC mesophase. The metal ion concentration can be increased in the medium by increasing the CTAB/C12EO10 or SDS/C12EO10 mole ratio at the expense of the stability of the LLC mesophase. The structure and some thermal properties of the new mesophase have been investigated using XRD, POM, FTIR, and Raman techniques.
This work explains the role of a charged surfactant in the salt-surfactant mesophases.
This work shows for the first time that the Pluronics can also form salt-surfactant mesophases.
Effect of the counter anion of the salt was explored in this work.
Above work is the first paper and it was highlighted as a frontispiece of Angewandte Chemie International Edition 2001.