Abstract:
Phase transitions during heating in odd normal alkanes, characterized by orthorhombic unit cell symmetry at room temperature, were investigated by differential scanning calorimetry. The analysis of thermograms revealed the presence of a series of crystalline and rotator phases during the transition from the solid to the liquid state. The phase transformations were characterized from the perspective of heterogeneous process development based on the theory of diffuse phase transitions. A similar multi-stage mechanism of phase transitions was established for the three normal alkane homologs. For the first time, it was found that the transition from the initial crystalline phase to the rotator phase occurs in a uniform manner in heneicosane (C21), tricosane (C23), and pentacosane (C25) via two intermediate crystalline phases: $O_i\to O_{dci}\to M_{dci}\to R_V$. The subsequent transition to the liquid state in heneicosane proceeds through two rotator phases: $R_V\Rightarrow R_I\to$ liquid, whereas in tricosane and pentacosane the phase sequence was found to be more complex: $R_V\Rightarrow R_I\to R_{II}\to$ liquid.
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