Aqueous phase behavior of ionic liquid-related gemini surfactant revealed by differential scanning calorimetry and polarized optical microscopy

An aqueous phase behavior was investigated for ionic liquid-related cationic gemini surfactant, [C(12)im-4-C(12)im][Br-2], in which two dodecylimidazolium parts are linked by a spacer composed of four methylene units. Differential scanning calorimetry was used to detect phase boundaries among various phases, from which a temperature-composition (T-X) phase diagram was constructed. Lyotropic liquid-crystalline phases appearing in this mixture were characterized by means of polarized optical microscopy. It was revealed that (1) two types of liquid-crystalline phases, lamellar (L-alpha) and hexagonal (H-1), are formed; (2) H-1 phase coexists with micellar solution (L-1) over a wide concentration range from a few wt.% to ca. 80 wt.% of the surfactant; and (3) the hydrated solid with the composition of [C(12)im-4-C(12)im][Br-2]center dot 2H(2)O exhibits incongruent melting point at which the hydrated solid decomposes into H-1 phase and unhydrated surfactant solid.