Solution behavior and solid phase transitions of quaternary ammonium surfactants with head groups decorated by...

Hydrogen bonds are strong intermolecular interactions, which are very important in molecular aggregation and new phase formation. Three long-chain quaternary ammonium surfactants, N,N-diethyl-N-(2-hydroxyethyl)-N-octadecylammonium bromide with one hydroxyl group, N-ethyl-N,N-bis (2-hydroxyethyl)-N-octadecylammonium bromide with two hydroxyl groups and N,N,N-tris (2-hydroxyethyl)-N-octadecylammonium bromide with three hydroxyl groups, abbreviated as SHQ, DHQ, and THQ, respectively, were synthesized in this work. Their solution behavior and solid phase transitions were investigated by surface tension, differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarizing optical microscopy (POM), and Infrared (IR) spectroscopy. The hydrogen bonds introduced by the substituted hydroxyl groups promoted surfactant adsorption at the air/water interface and aggregation in solution. In the crystal state, an increased number of hydroxyl groups caused a larger tilt angle of the long axis of surfactant molecules with the layer normal. Above certain temperatures, SHQ and DHQ formed highly ordered smectic T and smectic A phases while THQ only formed less ordered smectic A phase. The weakened electrostatic attractions between opposite ions and the thicker polar sublayers of mesophases caused by the enhanced number of hydrogen bonds are responsible for the mesophase formation and transition of these surfactants.