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Solvent and Substituent Effects on the Aggregation Behavior of Surface-Active Ionic Liquids with Aromatic Counterions and the Dispersion of Carbon Nanotubes in their Hexagonal Liquid Crystalline Phase

Xu, Wenwen, Yin, Qiwen, Gao, Yan’an, Yu, Li
Langmuir 2015 v.31 no.46 pp. 12644-12652
X-radiation, aromatic compounds, carbon nanotubes, cylinders, ionic liquids, light microscopy, liquid crystals, nitrates, rheology, surface tension, temperature, viscoelasticity, viscosity
The aggregation behavior of surface-active ionic liquids (SAILs) 1-dodecyl-3-methylimidazolium m- and p-hydroxybenzoate (m-C₁₂mimHB and p-C₁₂mimHB) in water and ethylammonium nitrate (EAN) was investigated. Surface tension measurements indicate that the cmc values of SAILs in EAN are much higher than those in water, resulting from the weaker solvophobic effect of EAN, and the stronger stability of SAILs/EAN complexes proven by DFT calculations. Compared to 1-dodecyl-3-methylimidazolium salicylate (C₁₂mimSal), the effect of substituent position leads to weaker interactions between aromatic counterions and headgroups. The hexagonal liquid crystal (H₁) phase formed by C₁₂mimHB in water or EAN at a higher concentration was determined by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheology techniques. Structural parameters estimated from SAXS curves suggest that the higher SAILs concentration or temperature leads to a smaller lattice parameter (a₀) and a denser arrangement of cylinders. For C₁₂mimHB, the formation of the H₁ phase in H₂O is easier than that in EAN. Furthermore, compared to C₁₂mimSal, C₁₂mimHB exists over a broad region of the hexagonal liquid crystalline (H₁) phase, which is due to the different position of the substituents on the aromatic ring of counterions. Therefore, the H₁ phase of the lypotropic liquid crystals (LLCs) formed in the C₁₂mimHB/H₂O system exhibits excellent performance in uniformly dispersing multiwalled carbon nanotubes (MWCNTs). Increasing the concentration of MWCNTs results in a larger lattice parameter (a₀) value, indicating the integration of MWCNTs within the cylinders of the H₁ phase. The rheological measurement results demonstrate that MWCNTs/LLCs composites are highly viscoelastic, and the presence of MWCNTs obviously strengthens the apparent viscosity of the H₁ phase.