Thermotropic and lyotropic mesophases formed by some lithium soaps.
The termotropic polymorphism of anhydrous lithium stearate (n-octadecanoate, LiCi8), lithium oleate (cis-9-octadecenoate) and an unusual semi-crystalline lithium phenylstearate (LiPS) soap mixture composed of 12 positional isomers, was studied using differential scanning calorimetry (DSC) and polarizing microscopy (plus x-ray diffraction and 7Li NMR spectroscopy for the latter). Each soap exhibited a characteristic stepwise melting behaviour. In contrast to the oleate, both LiC[18] and LiPS formed high temperature liquid crystal phases (lamellar and reversed hexagonal structures respectively) directly preceding their amorphous melts.The phase behaviour of LiCi8 in the hydrocarbon solvents n-hexadecane and squalane and that of LiPS in 1-phenylheptane was investigated using DSC and microscopic and macroscopic observations in polarized light (plus x-ray diffraction and [7]Li NMR spectroscopy for the latter). Phase diagrams were constructed. The phase behaviour of each system was critically dependent upon the thermotropic polymorphism of the solvent-free soap itself. At high temperatures two distinct mesophases were formed in the LiPS-l-phenylheptane system; a solvent-swollen reversed hexagonal mesophase at low solvent concentrations and a novel reversed micellar nematic mesophase at higher solvent concentrations. No mesophase formation was observed, however, in the LiC[18]-hydrocarbon systems.The phase behaviour of LiPS with [2]H[2]O was investigated (using the experimental techniques employed for the 1-phenylheptane system plus 2H NMR spectroscopy) and a phase diagram constructed. LiPS was shown to form a single homogeneous lamellar mesophase on addition of [2]H[2]O at room temperature with a broad composition and temperature range of stability.
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Sheffield Hallam University (United Kingdom).