engleski

Thermic Phase Transitions of Alkylammonium cholates

Novel series of catanionic surfactants, n-alkylammonium cholates, was prepared by precipitation from equimolar mixtures of n-alkylammonium chlorides and sodium cholate. The influence of the alkyl chain length of cationic surfactant (the number of carbon atoms per chain being 10, 12, 14 or 16) on the structural and thermal properties of n-alkylammonium cholates was examined by means of differential scanning calorimetry, X-ray diffraction, light microscopy, and transmission and scanning electron microscopy. Microscopic observation revealed that needle-like crystals, i.e. tubular molecular assemblies are formed in all precipitates. The prepared compounds exhibited a complex polymorphism and thermotropic mesomorphism from the crystalline form to the isotropic phase. The phase transition parameters (transition temperaturs, enthalpy and entropy) changed almost linearly with the number of carbon atoms in the chain of cationic surfactant. In the heating cycle, X-ray diffraction patterns of n-alkylammonium cholates displayed a superposition of rather sharp diffraction lines on a diffuse amorphous maximum, indicating the presence of two phases ; one was a three-dimensionally ordered crystalline phase and the other was a disordered phase. It seems that the diffuse amorphous maximum corresponds to some kind of two-dimensionally ordered liquid crystalline phase. The fraction of the amorphous phase decreases with the length of cationic surfactant alkyl chain. On cooling, all compounds underwent reversibly the isotropic liquid-liquid crystalline transition. Sharpening of diffraction lines in cooled samples indicated the increase of ordering in crystal lattice, while their different shifts toward higher Bragg angles revealed the anisotropy in thermal expansion.

alkylammonium cholate; phase transition

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