当前, 高昂的生产和使用成本限制了离子液体规模化应用, 如何回收再利用离子液体受到极大关注。 离子液体结晶过程对研发新型回收技术至关重要, 而降温速率对结晶过程有重要影响。 基于此, 以1-十二烷基-3-甲基咪唑四氟硼酸盐(［C12mim］［BF4］)为研究对象, 在60~0 ℃范围内对其进行快速(30 ℃·min-1)和慢速(1 ℃·min-1)降温, 采用偏光(POM)、 小角X射线散射(SAXS)和拉曼(Raman)等技术研究了该离子液体在降温过程中的相态转变和结构变化, 揭示了降温速率对其熔体结晶过程及产物影响规律。 POM研究表明: 在快速降温过程中, ［C12mim］［BF4］经历了“液态-液晶态-晶态Ⅰ”转变, 得到了晶粒数量少但尺寸较大的“球状”晶体; 在慢速降温过程中, ［C12mim］［BF4］经历了“液态-液晶态-晶态Ⅱ”转变, 得到了晶粒数量多但尺寸较小的“针状”晶体。 SAXS研究表明: ［C12mim］［BF4］存在两种晶体结构, 分别为“正交双层相”和“三斜双层相”, 且同时出现在“晶态Ⅰ”中, 而在“晶态Ⅱ”中只有“三斜双层相”一种晶体结构。 由此可知, “晶态Ⅰ”是混合相晶体, 而“晶态Ⅱ”是单一相晶体。 此外, 由Raman结果可知: “正交双层相”和“三斜双层相”中［C12mim］+构象存在较大差异, 分别为“G构象”和“A构象”。 综上所述, 快速降温时, ［C12mim］［BF4］经历了“液态-液晶态-晶态Ⅰ”转变过程, 得到了由“正交双层相”和“三斜双层相”组成的混合相晶体, 该晶体同时含有［C12mim］+的“G构象”和“A构象”。 慢速降温时, ［C12mim］［BF4］经历了“液态-液晶态-晶态Ⅱ”转变过程, 得到了由“三斜双层相”组成的单一相晶体, 该晶体只含有［C12mim］+的“A构象”。 因此, 降温速率对［C12mim］［BF4］熔体结晶过程及产物具有重要影响。 该研究结果为［C12mim］［BF4］回收技术开发提供了重要实验数据, 同时对研究其他同类型离子液体的相态转变和结构变化具有指导意义。

At present, ionic liquids’ high production and use costs are limiting their large-scale applications, so how to recycle them has attracted great attention. Crystallization processes are extremely important for developing new recycling technologies for ionic liquids, and cooling rates have important effects on the crystallization processes. Based on these, in this paper, polarizing microscopy, small angle X-ray scattering and Raman spectroscopy were employed to research the phase transitions and the structural changes of 1-dodecyl-3-methylimidazolium tetrafluoroborate (［C12mim］［BF4］) from 60 to 0 ℃ at the cooling rates of 30 and 1 ℃·min-1, in order to reveal the effect of cooling rates on the crystallization process and product. POM results show that ［C12mim］［BF4］ experienced the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅰ during the rapid cooling process, and that ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅱ during the slow cooling process. The crystal state Ⅰ consisted of many “ball-like” crystals with large sizes, while the crystal state Ⅱ was composed of a lot of “needle-like” crystals with small sizes. In addition, SAXS results show that ［C12mim］［BF4］ has two crystal structures, including perpendicular and triclinic bilayer phases. Both were found simultaneously in the crystal state Ⅰ, but only the triclinic bilayer phase appeared in the crystal state Ⅱ. Therefore, crystal state Ⅰ is a mixed phase crystal, while crystal state Ⅱ is a single phase crystal. Furthermore, it can be concluded from the Raman results of ［C12mim］［BF4］ that the ［C12mim］+ in the perpendicular bilayer phase is the G conformation, and that the ［C12mim］+ in the triclinic bilayer phase is the A conformation. In conclusion, ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅰ at a rapid cooling process, and a mixed phase crystal composed of the perpendicular bilayer phase and the triclinic bilayer phase was obtained. However, ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅱ at a slow cooling, and a single phase crystal consisting of the triclinic bilayer phase was obtained. What is more, the mixed phase crystal includes the G conformation and the A conformation of ［C12mim］+, while the single phase crystal only contains the A conformation. So, the cooling rate has an important effect on the crystallization process and product of ［C12mim］［BF4］. These results provide important experimental data for enhancing the recovery technology of ［C12mim］［BF4］, and are also helpful in investigating the phase transition and structure change of similar ionic liquids.