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J. Phys. Chem. B 2007, 111, 11708-11713
Effects of Alcohols and Counterions on the Phase Behavior of 1-Octyl-3-methylimidazolium Chloride Aqueous Solution Guodong Zhang,† Xiao Chen,*,† Yurong Zhao,† Yizhou Xie,† and Huayu Qiu‡ Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, Shandong, 250100, People’s Republic of China, and Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Teachers College, Ministry of Education, Hangzhou, Zhejiang, 310012, People’s Republic of China ReceiVed: June 25, 2007; In Final Form: August 4, 2007
Phase behavior of ternary systems involving 1-octyl-3-methylimidazolium chloride ([C8mim]Cl), water, and different alcohols (1-hexanol, 1-octanol, 1-decanol, and 1-dodecanol) is investigated at 25 °C. With the use of polarized optical microscopy and small-angle X-ray scattering techniques, lyotropic lamellar phases (LR) are identified in all systems, the formation of which is considered as a synergetic result of the hydrophobic force and the hydrogen-bonded network comprising an imidazolium ring, Cl-, water, and alcohols. In these LR phases, alcohol molecules play important roles not only because they could partly penetrate into the palisade layer with their hydroxyl groups extruded to form the network at the aggregate interface, but also because they could partly locate themselves in the interior of the hydrophobic bilayers to twist with the alkyl chains of [C8mim]Cl. Influencing factors such as the carbon chain length and content of different alcohols on these LR phases are discussed. With comparison to analogous ternary systems of [C8mim]BF4 and [C8mim]PF6, it is found that the strong interaction between counterions and alcohols favors the appearance of ordered assemblies.
Introduction 1-Alkyl-3-methylimidazolium salts ([Cnmim]A, where n is the carbon number of the alkyl chain and A is the anionic counterion) have been known as important room-temperature ionic liquids. They have attracted considerable attention in the areas of organic synthesis,1,2 catalysis,2-5 electrochemistry,6,7 liquid/liquid extraction,8-10 and preparation of novel materials.11,12 Recently, the application of [Cnmim]A salts in amphiphilic self-organization has become a novel and active topic for their diversity by proper design on cationic or anionic parts.13,14 For example, [C4mim]PF6 has been reported to be solubilized into the lyotropic liquid crystal (LLC) formed by C12EO4 and H2O without changing the dimensions of the amphiphile layer.15 It has also been shown that both hydrophobic [C4mim]PF6 and hydrophilic [C4mim]BF4 can be solubilized by the hexagonal LLC of the Brij97-H2O binary system, being located in different areas.16 We have investigated the role of [C4mim]PF6 as a novel and benign solvent to form LLC with a triblock copolymer, Pluronic P123, and believed that the longrang ordered structures are resulted from the synergetic effects of hydrogen bonds, solvophobic forces, and other interactions.17 Besides, various amphiphilic association structures like micelles,18-20 vesicles,21 and microemulsions,22-24 have also been constructed in these [Cnmim]A salts. Except for their roles as solvents, [Cnmim]A salts with long alkyl chains also often act as cationic surfactants to form LLC phases with water to exhibit potential applications in many fields. Firestone and co-workers have found that LLC gels can * To whom correspondence should be addressed. Phone: +86-531-88365420. Fax: +86-531-8856-4464. E-mail:
[email protected]. † Shandong University. ‡ Hangzhou Teachers College.
be formed in [C10mim]Br-H2O and [C10mim]NO3-H2O binary systems because of the H-bonded network, and the ionogel template formed via self-assembly of [C10mim]Cl in H2O can be used to synthesize anisotropic gold nanoparticles.25-27 Several LLC phases in the [C16mim]Cl-H2O binary system have been clarified by Smarsly and co-workers to explain the templating mechanism of [C16mim]Cl in the hydrothermal synthesis of mesoporous silica.28,29 Very recently, Goodchild et al. have performed 2H NMR spectroscopy measurements to confirm the formation of lyotropic mesophase in the [C8mim]Cl-H2O system.30 The lamellar and hexagonal phases in the [C12mim]Br-H2O system have also been presented by Inoue et al.31 In addition, reports concerning the aggregation and micelle formation of [Cnmim]A salts in dilute aqueous solution could be found elsewhere.32-35 It should be noted, however, that much attention has been paid to the binary systems of [Cnmim]A aqueous solution in most of such studies. To enrich the ordered phase structures and extend the application fields, the ternary or quasiternary systems of [Cnmim]A should be considered. The use of the third component to modify the ordered molecular assemblies is of great importance to expand the use of [Cnmim]A salts in catalysis and material synthesis. In the present paper, the focus will be on effects of the third component (alcohols) on the phase behavior of [Cnmim]A aqueous solution. To exhibit the prominent function of alcohols, an imidazolium salt with a relatively short alkyl chain ([C8mim]Cl) was used, which is a liquid at room temperature. Its aqueous solution did not show any obvious lyotropic aggregates because of the poor amphiphilic nature. (Usually, [Cnmim]A exhibits as a surfactant with n g 10.) We investigated the phase behavior of ternary systems composed of [C8mim]Cl, water, and different alcohols, including 1-hexanol (C6H13OH), 1-octanol (C8H17OH), 1-decanol (C10H21OH), and
10.1021/jp074945f CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
Phase Behavior of [C8mim]Cl-Alcohol-H2O Systems
J. Phys. Chem. B, Vol. 111, No. 40, 2007 11709
1-dodecanol (C12H25OH). The formation mechanism of lyotropic lamellar phases (LR) in above ternary systems and effects of different alcohols are discussed. Ternary systems of [C8mim]BF4-C10H21OH-H2O and [C8mim]PF6-C10H21OH-H2O were also investigated to compare the effect of counterions on the phase behavior. This work should further our knowledge on the properties and aggregation behavior of such imidazolium salts. Experimental Section Materials. [C8mim]Cl was purchased from Hebei Lanshen Technology Co., LTD with the purity of 98.7% as given by chromatogram. [C8mim]BF4 and [C8mim]PF6 were purchased from Hangzhou Chemer Chemical Co., LTD (purity >97%, water content
