Mesophases of Alkylammonium Salts. I. First-Order Transitions

Abstract

Abstract Thermal properties of symmetric tetra-n-alkylammonium salts are interpreted based on the concept of conformational motion and disorder (condis crystals). A special effort is made to separate the well-known plastic crystalline states for the lower homologs of this series from the condis state. New transition parameters for (C2H5)4NBr, (C3H7)4NBr, (C8H17)4NBr, (C10H21)4NBr, (C12H25)4NBr, (C12H25)4NI, (C16H33)4NBr, and (C18H37)4NBr are added to data in the literature. Three groups of compounds displaying distinct behavior are found: A) Short-chain tetra-n-alkylammonium salts do not melt nor disorder conformationally, they transform to plastic crystals at a single disordering transition, followed by sublimation or decomposition. B) Long-chain tetraalkylammonium salts show conformational disorder and motion before fusion and do not exhibit a plastic crystalline phase. C) In the intermediate alkyl-chain lengths both plastic and condis crystals may exist, not all phases are, however, equilibrium phases.