Experimental and DFT Studies on the Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Aromatic Counterions in Aqueous Solution

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• 作者：Wenwen Xu ; Tao Wang ; Ni Cheng ; Qiongzheng Hu ; Yanhui Bi ; Yanjun Gong ; Li Yu
• 刊名：Langmuir
• 出版年：2015
• 出版时间：February 3, 2015
• 年：2015
• 卷：31
• 期：4
• 页码：1272-1282
• 全文大小：631K
• ISSN：1520-5827

文摘

Two imidazolium-based surface-active ionic liquids with aromatic counterions, namely, 1-dodecyl-3-methylimidazolium salicylate (C₁₂mimSal) and 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate (C₁₂mimHNC), were synthesized, and their aggregate behavior in aqueous solutions was systematically explored. Surface tension and conductivity measurements indicate that both C₁₂mimSal and C₁₂mimHNC show superior surface activity compared to the common imidazolium-based SAIL with the same hydrocarbon chain length, 1-dodecyl-3-methylimidazolium bromide (C₁₂mimBr). This result demonstrates that the incorporation of aromatic counterions favors the formation of micelles. C₁₂mimHNC displays a higher surface activity than C₁₂mimSal, resulting from the different hydrophobicities of the counterions. In comparison with C₁₂mimBr, C₁₂mimSal not only can form hexagonal liquid-crystalline phase (H₁) in aqueous solution, but also exhibits a broad region of cubic liquid-crystalline phase (V₂) at higher concentration. As for the C₁₂mimHNC/H₂O system, a lamellar liquid-crystalline (L_伪) phase was observed. These lyotropic liquid crystals (LLCs) were characterized by polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS). Structural parameters calculated from SAXS patterns suggest that a higher concentration of the SAIL leads to a denser arrangement whereas a higher temperature results in the opposite effect. The rheological results manifest that the formed H₁ phase in the C₁₂mimSal/H₂O system exhibits an impressive viscoelastic behavior, indicated by a modulus (G鈥?and G鈥? that is 1 order of magnitude higher than that of C₁₂mimBr. Density functional theory (DFT) calculations reveal that C₁₂mimSal has a more negative interaction energy with a water molecule and the Sal^{鈥?/sup> counterion presents a stronger electronegativity than the HNC鈥?/sup> counterion. The specific phase behavior of the C₁₂mimSal/H₂O and C₁₂mimHNC/H₂O systems can be attributed to the strong synergic interaction between the imidazolium cation and the aromatic counterion, including electrostatic attraction, hydrophobic interaction, and especially 蟺鈥撓€ interaction.}