Crystal polymorphism of hexylammonium chloride and structural properties of its mixtures with water

The thermal stability of hexylammonium chloride (HeAC) has been investigated in situ real time by high-temperature X-ray powder diffraction. A phase transition from a low-temperature (LT) tetragonal to a high-temperature (HT) cubic polymorph has been detected at 473 K. A first sharp diffraction peak (FSDP) starts to grow at the same temperature and disappears at 493 K, just before melting starts to occur. The dependence of cell parameters from temperature has been quantified for the LT polymorph and shown to be very anisotropic. In particular, the tetragonal a parameter expands, while the c parameter contracts, and as a net result, the volume increases. The HT polymorph shows features reminiscent of that of plastic phases of molecular crystals and is characterized by a less efficient packing as compared to the LT polymorph as indicated by a volume expansion of ca. 11%. Moreover, the structural properties of HeAC/water mixtures, up to very high dilution, have been investigated by combining MD simulations and X-ray diffraction experiments. 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 for all the studied systems. The Cl- ions in the mixtures were found to form both a first and second shell of water molecules. Moreover, a complex structural behavior has been highlighted, in which a strong interaction between cations and anions survives also in conditions of very high dilution. As a consequence, cations and anions do not always possess a completely closed hydration shell of their own, but rather solvent-shared ion pairs are formed to some extent in all the investigated mixtures.