Thermal behavior of long-chain alkanoylcholine soaps

Abstract

Long-chain alkanoylcholines prepared from fatty acids (nACh) are fully sustainable cationic surfactants that are known for their biological and medicinal properties. In the present work the thermal behavior of the homologous series of alkanoylcholine iodides with n = 12, 14, 16 and 18, has been examined within the 25–200 °C range of temperatures. Up to three thermotropic phases have been identified, and the thermal transitions implied in their interconversion have been characterized by DSC and simultaneous WAXS and SAXS analysis carried out in real-time. All three phases consist of a bilayered structure with alkanoyl chains confined in the space between the head group layers and interdigitated to a greater or lesser extent. Melting–crystallization of either the polymethylene segments or the choline iodide groups is involved in such transitions. Additionally, a crystal phase consisting also of a bilayered structure but excluding side chain interdigitation was observed upon crystallization from solution and its structure was elucidated by single-crystal X-ray diffraction direct methods. The close correlation existing between thermal properties, phase structure and n has been brought into evidence.