Solid-Phase Transitions of Catanionic Surfactants

Abstract

A series of catanionic surfactants were prepared by making an equimolar mixture of cationic (alkylammonium chloride) and anionic (sodium alkyl sulfate) surfactants, both containing an isomer of the same chain length, namely, C10, C12, or C14. The prepared compounds exhibited complex thermal behavior, characterized by several successive phase transitions in the solid state as proved by differential scanning calorimetry, X-ray diffraction, and optical birefringence observation using a polarizing microscope. On heating, three main phase transitions were observed for all compounds; solid crystalline–solid crystalline, solid crystalline–liquid crystalline, and liquid crystalline–isotropic liquid. On cooling, all three compounds underwent reversibly the isotropic liquid–liquid crystalline phase transition, while the liquid crystalline–solid crystalline phase transition displayed peculiar properties. Phase transition temperatures increased with an increase in hydrocarbon chain length.