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Solid Phase Transitions of Symmetric and Asymmetric Catanionic Surfactants

Solid phase transitions of two series of symmetric (alkylammonium alkyl sulfate, with the same number of carbon atoms per alkyl chain, namely 10, 12, or 14) and asymmetric (hexadecyltrimethylammonium alkyl sulfate, the number of carbon atoms per chain of alkyl sulfate being 10, 12, or 14) catanionic surfactants were investigated by means of polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. All prepared compounds crystallized in such a way that molecules are arranged in layers with saturated portions of chains mostly in the trans configuration with a high degree of molecular parallelism. The lamellar phase comprises bimolecular leaflets of hydrocarbon chains covered on both sides by the polar head groups. The basic lamellar thickness increased linearly with the increase of the number of carbon atoms per alkyl chains. On heating, three endothermic phase transitions were observed for symmetric catanionic surfactants: (1) a polymorphic transition between two solid crystalline (SC) forms, (2) solid crystalline-liquid crystalline (SC-LC), and (3) liquid crystalline-isotropic liquid (LC-IL). The polymorphic transition (SC-SC) was connected to a relatively large endothermic effect, which varies with the chain length. High enthalpy and entropy values corresponding to SC-LC transitions may imply the disordering of the hydrocarbon chains to the liquid crystalline organization. Polarizing microscopy revealed a characteristic texture of the smectic phase. On cooling, all compounds underwent reversibly the IL-LC phase transition, while LC-SC phase transition displayed peculiar properties. Asymmetric catanionic surfactants exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the isotropic phase. The hydrophobic moiety formed by two chains of different lengths caused the existence of more polymorphs than in the case of symmetric chains. The number of polymorphs depended on the asymmetry between the lengths of surfactant tails. The difference of thermal behavior between symmetric and asymmetric catanionic surfactants arose from the hydrophobic region, i.e., the difference of thermal behavior depended on molecular packing constraints. The difference in the hydrophobic tail length of surfactants influenced a poorer hydrophobic match in the bilayer than in symmetric systems. The reverse of crystallization always took place from the smectic state. An undercooled state was observed in the cooling process; time recovery to the original state depended on the chain length.

catanionic surfactants; liquid crystals; phase transitions; thermal analysis

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