Crystal structures and phase transitions of long-chain n-alkylammonium bromide monohydrates

Abstract

The crystal structures of five long-chain n-alkylammonium bromide monohydrates, with carbon chain lengths ranging from 13 to 18, were determined. n-Tridecylammonium bromide monohydrate 1, n-tetradecylammonium bromide monohydrate 2, n-pentadecylammonium bromide monohydrate 3, n-hexadecylammonium bromide monohydrate 4, and n-octadecylammonium bromide monohydrate 5, all crystallise in layered structures, and the same hydrogen bonding network is observed for the odd- and even carbon chain compounds. Comparison of an anhydrous and a hydrated n-alkylammonium bromide structure indicate that the structures differ in alkyl chain tilt angle, methylene subcell packing as well as hydrogen bonding network. The thermotropic phase transitions exhibited by the hydrates were investigated via DSC and TGA, and indicate stepwise melting behaviour through a number of high temperature phases, including a liquid crystalline phase.