Xu Wenwen, Wang Tao, Cheng Ni, Hu Qiongzheng, Bi Yanhui, Gong Yanjun, Yu Li
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education , Jinan 250100, P. R. China.
Langmuir. 2015 Feb 3;31(4):1272-82. doi: 10.1021/la503884v. Epub 2015 Jan 22.
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(α)) 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' and G″) 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(-) counterion presents a stronger electronegativity than the HNC(-) 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 π-π interaction.
合成了两种带有芳香族抗衡离子的咪唑基表面活性离子液体，即1-十二烷基-3-甲基咪唑水杨酸酯（C12mimSal）和1-十二烷基-3-甲基咪唑3-羟基-2-萘酸酯（C12mimHNC），并系统地研究了它们在水溶液中的聚集行为。表面张力和电导率测量表明，与具有相同烃链长度的常见咪唑基表面活性离子液体1-十二烷基-3-甲基咪唑溴化物（C12mimBr）相比，C12mimSal和C12mimHNC均表现出优异的表面活性。该结果表明，引入芳香族抗衡离子有利于胶束的形成。由于抗衡离子的疏水性不同，C12mimHNC比C12mimSal表现出更高的表面活性。与C12mimBr相比，C12mimSal不仅能在水溶液中形成六方液晶相（H1），而且在较高浓度下还表现出较宽的立方液晶相（V2）区域。对于C12mimHNC/H2O体系，观察到层状液晶（L(α)）相。通过偏光显微镜（POM）和小角X射线散射（SAXS）对这些溶致液晶（LLC）进行了表征。由SAXS图谱计算得到的结构参数表明，表面活性离子液体的浓度越高，排列越致密，而温度升高则产生相反的效果。流变学结果表明，C12mimSal/H2O体系中形成的H1相表现出令人印象深刻的粘弹性行为，其模量（G'和G″）比C12mimBr高1个数量级。密度泛函理论（DFT）计算表明，C12mimSal与水分子的相互作用能更负，且Sal(-)抗衡离子的电负性比HNC(-)抗衡离子更强。C12mimSal/H2O和C12mimHNC/H2O体系的特定相行为可归因于咪唑阳离子与芳香族抗衡离子之间的强协同相互作用，包括静电吸引、疏水相互作用，尤其是π-π相互作用。
