The influence of covalently attaching hydroxymethylene to the methyl groups of methyl-tri-n-alkylphosphonium halides ([H(CH₂)_n]₃P⁺CH₃ Xˉ, where n = 10, 14 or 18, and Xˉ= Clˉ or Brˉ), [H(CH₂)_n]₃P⁺(CH₂)₂OH Xˉ, or adding methanol as a solute to the [H(CH₂)_n]₃P⁺CH₃ Xˉ salts on their solid, liquid-crystalline (smectic A₂), and isotropic phases has been investigated using a variety of experimental techniques, which include optical microscopy, thermogravimetry, differential scanning calorimetry, X-ray diffraction, and ²H-NMR spectroscopies. These structural and compositional changes are found to induce liquid crystallinity in some cases and to enhance the temperature range and lower the onset temperature of the liquid crystalline phases in some others. The results are interpreted in terms of the lengths of the three n-alkyl chains attached to the phosphorus cation, the nature of the halide anion, the influence of H-bonding interactions at the head group regions of the layered phases, and other solvent-solute interactions. The fact that at least 1 molar equivalent of methanol must be added to effect complete conversion of a solid methyl-tri-n-alkylphosphonium salt to a liquid crystal demonstrates a direct and strong association between individual methanol molecules and the phosphonium salts.

We thank the National Science Foundation for financial support.