Thermal and Structural Properties of Ethylammonium Chloride and Its Mixture with Water

The temperature dependence of ethylammonium chloride structure has been investigated by in situ laboratory parallel-beam X-ray powder diffraction. A polymorphic transition from a monoclinic LT phase to a tetragonal HT phase has been observed at 358 K. Such transformation has a reconstructive character. The thermal expansion of both polymorphs is small and anisotropic as a consequence of their organization through an anisotropic interaction network. The high temperature (HT) phase (possible space group P4/n or P4/nmm, a = 5.05 Å, c = 9.99 Å) has an excess volume of ∼11% as compared with the low temperature (LT) one. The HT polymorph’s structure has been solved by direct methods using powder diffraction data. In the absence of clear indications, it has been refined in P4/nmm. The structural properties of an ethylammonium chloride/water mixture at ambient conditions were also studied by using an integrated approach, which combines X-ray diffraction measurements and molecular dynamics simulations carried out with both the SPC/E and TIP5P water models. By refining a single interaction potential, very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior in which cations and anions do not possess a completely closed hydration shell of their own has been highlighted. Conversely, “solvent-shared ion pairs” are formed, in which one or more water molecules act as a bridge between the chloride and ethylammonium ions. Moreover, a strong water−water correlation is found, indicating that the water molecules in the mixture tend to aggregate and form water clusters.

本研究采用原位实验室平行束X射线粉末衍射（in situ laboratory parallel-beam X-ray powder diffraction），探究了氯化乙铵（ethylammonium chloride）的结构随温度的依赖关系。在358开尔文温度下，观测到其从单斜低温（LT）相向四方高温（HT）相的多晶型转变，该转变具有重构型特征。两种晶型的热膨胀均较小且呈现各向异性，这源于它们通过各向异性相互作用网络进行组织排布。高温（HT）相的空间群可能为P4/n或P4/nmm，晶胞参数为a=5.05埃、c=9.99埃，相较于低温（LT）相，其超额体积约为11%。该高温晶型的结构通过粉末衍射数据采用直接法解析，在无明确结构指示的情况下，最终以P4/nmm空间群对其进行精修。本研究还采用结合X射线衍射测量与分子动力学模拟（molecular dynamics simulations）的集成研究方法，分别采用SPC/E和TIP5P水模型开展模拟，对常温常压条件下的氯化乙铵/水混合物的结构特性展开了探究。通过优化单一相互作用势，理论衍射图谱与实验衍射图谱之间实现了极佳的匹配度，尤其在采用TIP5P水模型的模拟中效果更为突出。研究揭示了一种复杂的结构行为：体系中的阳离子与阴离子并不具备完全闭合的自身水合壳层，反而形成了"溶剂共享离子对（solvent-shared ion pairs）"，即一个或多个水分子作为桥梁连接氯离子与乙铵阳离子。此外，研究还观测到较强的水分子间关联作用，表明混合物中的水分子倾向于聚集形成水团簇。