Effects of ions on the liquid crystalline mesophase of transition-metal salt:surfactant (CnEOm)

The transition-metal aqua complex salts [M(H2O)(x)]Y-2 (where M is some of the first- and second-row transition-metal ions and Y is Cl-, NO3-, and ClO4- counteranions) form liquid crystalline (LC) mesophases with oligo(ethylene oxide) nonionic surfactants (CnH2n+1(CH2CH2O)(m)OH, denoted as CnEOm). The structure of the [M(H2O)(x)]Y-2:CnEOm mesophase is usually 2D hexagonal in nitrate systems, cubic in perchlorate systems, and absent in the chloride systems. The solubility of the metal aqua complex salt follows the Hofmeister series in a [M(H2O)(x)]Y-2:CnEOm mesophase. However, the nitrate ion interacts with the metal center as a bidentate and/or unidentate ligand, therefore reducing the ion density (and/or ionic strength) of the LC medium and further enhancing the solubility of nitrate salt in the LC systems. The cobalt chloride salt is the only soluble chloride salt that undergoes ligand-exchange reactions in the [Co(H2O)(6)]Cl-2:CnEOm system. In an LC mesophase, anions have a greater influence on the hydrophilicity of nonionic surfactants than do cations. The structure and stability of the LC mesophase can be controlled by controlling either the hydrophilicity of the nonionic surfactant (by choosing the right anion type) or the ion density of the medium (by either influencing the equilibrium between the free and coordinated anions or balancing between the coordinating and noncoordinating anions in the medium).