Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
J Phys Chem B. 2013 May 23;117(20):6323-9. doi: 10.1021/jp4021849. Epub 2013 May 8.
Crystallization and phase transition behaviors of n-hexadecane (n-C16H34, abbreviated as C16) confined in microcapsules and n-alkane/SiO2 nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C16 is enhanced in both the microcapsules and SiO2 nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid-solid phase transition is observed only in the microencapsulated C16, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C16/SiO2 composite, however, no novel rotator phase emerges during the cooling process, and C16 crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C16/SiO2 composite, leading to phase transition behaviors similar to those of bulk C16.
已通过差示扫描量热法（DSC）和温度依赖 X 射线衍射（XRD）的组合研究了十六烷（n-C16H34，缩写为 C16）在微胶囊和正烷烃/SiO2 纳米球复合材料中的结晶和相转变行为。从 DSC 测量可以明显看出，由于两种限制条件下的表面积与体积比都大大增加，C16 的表面冻结现象在微胶囊和 SiO2 纳米球复合材料中都得到了增强。从 XRD 结果可以看出，仅在微封装的 C16 中观察到新的固-固相变，该相变通过介稳态旋转相（RI）结晶为稳定的三斜相。然而，对于 C16/SiO2 复合材料，在冷却过程中不会出现新的旋转相，C16 直接从液态结晶为稳定的三斜相。表面冻结相引起的异质成核在微封装样品中占主导地位，有助于出现新的旋转相，而由外来结晶核引起的异质成核则主导 C16/SiO2 复合材料，导致与体相 C16 相似的相转变行为。
