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.

本研究通过原位实验室平行束X射线粉末衍射技术，探究了乙基氯化铵结构的温度依赖性。在358 K时观测到了单斜晶低温相（LT phase）向四方晶高温相（HT phase）的多型转变，该转变具有重构型特征。两种多型体的热膨胀均较小且呈现各向异性，这源于其通过各向异性相互作用网络形成的组织结构。高温相（HT phase）的可能空间群为P4/n或P4/nmm，晶胞参数a=5.05 Å，c=9.99 Å，其体积相较于低温相（LT phase）高出约11%。利用粉末衍射数据，通过直接法解析了高温多型体的晶体结构；由于缺乏明确的结构判定依据，最终在空间群P4/nmm下完成精修。本研究还通过整合X射线衍射测量与分子动力学模拟的综合手段，探究了常温常压下乙基氯化铵/水混合体系的结构特性，其中分子动力学模拟采用了SPC/E与TIP5P两种水模型。通过精修单一相互作用势，理论衍射图谱与实验衍射图谱实现了极佳的吻合，尤其在采用TIP5P水模型的模拟中效果更为显著。研究揭示了该体系的复杂结构行为：阴阳离子均未形成完全闭合的水化壳层。与之相反，体系中形成了"溶剂共享离子对"，即一个或多个水分子作为桥梁连接氯离子与乙基铵离子。此外，观测到显著的水分子间相关性，表明混合体系中的水分子倾向于聚集形成水团簇。