Phase Transitions and Molecular Reorientations in Bilayered n-Heptadecylammonium Chloride

Abstract

An investigation of phase transitions and molecular motions in polycrystalline n-heptadecylam- monium chloride (C₁₇H₃₅NH₃Cl), employing differential scanning calorimetry, x-ray powder diffraction and nuclear magnetic resonance techniques, is reported. This compound can occur in two virgin polymorphs at room temperature, one interdigitated and one noninterdigitated. The temperature at which crystallisation occurs determines the polymorph that forms. If these polymorphs are heated transitions to noninterdigitated γ, β and α phases occur. Cooling to room temperature shows the same phase transitions, but the virgin phase is not formed. Instead, a noninterdigitated .epsi; phase is formed. Defect motions of chain-ends play a significant role in the spin-lattice relaxation rates in all the phases. In the α phase a degree of chain melting is present. The molecular dynamics of chains differs during heating and cooling cycles in the .epsi; phase. If the temperature is kept constant during a heating cycle in this phase, fourfold motions of chains are frozen over a period of several hours, but the sample remains noninterdigitated. It seems that the interdigitation process is hampered by the population of defect orientations of chain-ends. The methyl group executes classical threefold reorientations and the NH₃ group jumps in an asymmetric threefold potential well.