A combined theoretical and experimental study of solid octyl and decylammonium chlorides and of their aqueous solutions

The analysis of the thermal behavior of octyl-(OAC) and decylammonium chloride (DAC) has provided relevant new information. OAC shows a polymorphic phase transition that starts at 308 K and is completed at 313 K, between a monoclinic and a tetragonal structure, which belongs to a space group different from what was observed for other alkylammonium chlorides. At the melting temperature (463 K), the growing of a strong first sharp diffraction peak (FSDP) occurring at a d-spacing of Ca. 21.0 angstrom was observed. DAC is characterized by two phase transitions at 318 (T-d1) and 323 K (T-d2). At T-d1, the LT polymorph coexists with a new-formed phase. At T-d2 the LT polymorph is replaced by a further polymorphic modification of DAC that becomes the only one at 328 K. This second phase transition signals the occurrence of a phase, closely related to the LT polymorph of HeAC, that crystallizes in the tetragonal crystal system a = ca. 5.00 angstrom and c = ca. 28.5 angstrom, P4/nmm space group. DAC melting starts at 458 K and is accompanied, as in the case of OAC, by the growing of a strong FSDP at a d-spacing of ca. 24.8 angstrom. A parallel DSC study confirmed the above-mentioned transitions. In addition, the structural properties of OAC/water and DAC/water mixtures were studied using an integrated approach, which combines X-ray diffraction and molecular dynamics (MD) techniques. A very good agreement between theoretical and experimental diffraction patterns has been obtained for both investigated mixtures. A thorough analysis of the MD trajectories shows that strong anion-water interactions are present in the mixtures, where Cl- forms a rather unstructured first hydration shell of water molecules bound to the anion in a linear Cl center dot center dot center dot H-O configuration. Moreover, cations and anions were found to interact with each other, and to form "solvent-shared ion pairs", in which one or more water molecules are shared between Cl- and the alkylammonium cation.