engleski

Solid Phase Transitions of Dodecylammonium Picrate

A novel surfactant, dodecylammonium picrate (DDAP), was crystallized from aqueous solution and its thermal properties were examined by X-ray, thermal and microscopic analysis. X-ray diffraction pattern of DDAP 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 solid crystalline (SC) phase and the other was a two-dimensionally ordered liquid crystalline (LC) phase. DDAP melted clearly into the isotropic phase and then supercooled below its melting point exhibiting a monotropic liquid crystalline behaviour. The LC and SC phase coexisted over a wide range of temperature before crystallization was completed indicating that structural ordering was kinetically controlled. In order to shed more light on the phase ordering process the transition from disordered to ordered phase was investigated by annealing. Annealing is usually used for the improvement of polymer crystallization, however up to now this process was not used for ordering of surfactant crystals. The results showed that the number, intensities and locations of thermal events depended on the thermal history of samples in contrast to the “memory effect” observed for polymer crystals. The increase of the time of annealing and number of repeated treatments decreased enthalpy of melting, temperature of isotropization, temperature and enthalpy of crystallization, and increased the lamellar basic thickness of the SC phase. DDAP crystals ordered by annealing exhibited almost the same properties as monocrystals prepared from suitable solvents.

liquid crystals; phase transitions; thermal analysis

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