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.