Effects of a Spacer on the Phase Behavior of Gemini Surfactants in Ethanolammonium Nitrate.

The aggregation behavior of quaternary ammonium gemini surfactants (12-s-12) in a protic ionic liquid, ethanolammonium nitrate (EOAN), was investigated by small-angle X-ray scattering, freeze-fracture transmission electron microscopy, polarized optical microscopy, and rheological measurements. The rarely reported nonaqueous two phases in the ionic liquid were observed at lower 12-s-12 concentrations. The upper phase was composed of micelles, whereas only the surfactant unimers or multimers were detected in the low phase. At higher 12-s-12 concentrations, different aggregates were formed. The lamellar phase was observed in the 12-2-12/EOAN system, whereas the normal hexagonal phases in 12-s-12/EOAN (s = 3, 4, 5, 6, 8) systems and the micellar phase in the 12-10-12/EOAN system were observed. Such a dramatic phase transition induced by the spacer chain length was due to the unique solvent characteristics of EOAN compared to those of water and its counterpart ethylammonium nitrate.