Lyotropic liquid crystalline phases formed in ternary mixtures of N-alkyl-N-methylpyrrolidinium bromide/1-decanol/water

Abstract

The phase behavior of ternary systems involving long-chain pyrrolidinium ionic liquids, N-alkyl-N-methylpyrrolidinium bromide (C_nMPB, n = 12, 14, 16), water, and 1-decanol was investigated at 25 °C. Polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheological measurements were employed to investigate the lyotropic liquid crystalline (LC) phases. Lamellar phase (L_α) and hexagonal phase (H₁) were found to exist in all the three systems, but the isotropic cubic phase only appears in the C₁₂MPB and C₁₄MPB systems. Greater surfactant content leads to a denser aggregation of the cylindrical units in the H₁ phase, and the structural parameters of L_α phase depends on water content. The alkyl chain length of C_nMPB also has interesting effect on the structural parameters and rheological properties of the LC phase. Compared with a similar ternary system of 1-hexadecyl-3-methylimidazolium chloride ([C₁₆mim]Cl) and traditional cationic surfactants cetyltrimethylammonium bromide (C₁₆TAB), the effect of the cationic group on the phase behavior was also investigated.