Effect of the Spacer Length of Acid-base Complexes Forming Gemini-like Surfactant on the Phase Behavior

The surface chemical properties and phase behaviors of the three acid-base 2:1 complexes were systematically studied. It was composed of tertiary amine as the main chain and three dicarboxylic acids as the spacer and functioned as a gemini-type surfactant. N-methyl-N-(2,3-dioxypropyl)hexadecylamine (C16amine) was used as the main chain, and octanoic acid (C8), decanoic acid (C10), and dodecanoic acid (C12) were combined to form complexes. In dilute solutions, the surface tension and luminescence behaviors of solubilized pyrene were consistent across all three complexes, indicating similar micellar characteristics at lower concentrations. However, the concentrated complexes clearly showed different behaviors. In the case of complexes composed of C16amine and C8 or C16amine and C10, as the concentration increased, hexagonal, bicontinuous, and lamellar phases were observed. For the complex composed of C16amine and C12, the hexagonal phase was not observed. This deviation in phase behavior is assumed to arise from the folding structure of the alkyl chain in the dicarboxylic acid. The folding structure may influence the packing structure of the liquid crystal phases, thereby altering the phase formation.