Crystal Polymorphism of Propylammonium Chloride and Structural Properties of Its Mixture with Water

The thermal behavior of propylammonium chloride (PAC) has been investigated by parallel beam X-ray powder diffraction in the 303–463 K thermal range. A polymorphic transition has been observed at 403 K, whereas the melting process starts at 438 K. The low-temperature (LT) polymorph is tetragonal, P4/n or P4/nmm, and a = 6.2429(2) Å, c = 7.3830(3) Å, and Z = 2. Thermal expansion is isotropic as the N···Cl interactions have components along the three crystallographic axes. At the transition temperature, a high-temperature (HT) polymorph was observed, with candidate space groups P23, Pm3̅, P432, P4̅3m, and Pm3̅m and cell parameter a = 11.715(3) Å, consistent with Z = 12. This phase has features reminiscent of those of plastic phases of molecular crystals. The structural properties of a PAC/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 the SPC/E and TIP5P water models. By using a Cl–water Lennard-Jones parameter previously refined for a similar system, a very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior has been highlighted, in which cations and anions do not possess a completely closed hydration shell of their own, but rather “solvent-shared ion pairs” are formed, where one or more water molecules act as a bridge between the chloride and propylammonium ions.