Article pubs.acs.org/Langmuir
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 Wenwen Xu,†,‡ Qiwen Yin,§ Yan’an Gao,∥ and Li Yu*,†,‡ †
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China § China Research Institute of Daily Chemical Industry, Taiyuan 030001, PR China ∥ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China ‡
S Supporting Information *
ABSTRACT: The aggregation behavior of surface-active ionic liquids (SAILs) 1-dodecyl-3-methylimidazolium m- and p-hydroxybenzoate (mC12mimHB 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 Xray 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.
1. INTRODUCTION Ionic liquids (ILs) have been widely investigated in academia1,2 and industry3−5 due to the easy manipulation of their physicochemical properties, negligible vapor pressure, high thermal stability, and so forth.6−8 Among them, ILs bearing long alkyl chains are regarded as a novel category of amphiphiles. Owing to their inherent molecular nature, consisting of a hydrophilic headgroup and a hydrophobic tail, they are called surface-active ionic liquids (SAILs). SAILs can form aggregates with specific structures, shapes, and properties. To date, their aggregation behavior has been extensively investigated in colloid and interfacial chemistry.9−18 Lyotropic liquid crystals (LLCs), including hexagonal (H1), cubic (V2), and lamellar (Lα) phases, are an important type of selfassembled aggregates of SAILs and have attracted considerable interest for their potential applications in biomaterials and © 2015 American Chemical Society
electrooptics. For instance, LLCs formed by 1-alkyl-3methylimidazolium bromides (CnmimBr, n = 10, 12),19−21 1hexadecyl-3-methylimidazolium chloride (C16mimCl),22 and Nalkyl-N-methylpiperidinium bromides (CnPDB, n = 12, 14, 16)23 in aqueous solution were thoroughly studied. ILs can also be employed as solvents. Several recent investigations have reported the solvent effect on the formation of LLCs. As a green IL solvent that has the excellent ability to build up H-bonding networks in different systems,24−26 ethylammonium nitrate (EAN) is becoming a promising candidate for studying the effect of solvent composition on the phase behavior of amphiphiles. Zhao et al. investigated the Received: September 28, 2015 Revised: October 30, 2015 Published: November 9, 2015 12644
DOI: 10.1021/acs.langmuir.5b03586 Langmuir 2015, 31, 12644−12652
Article
Langmuir aggregation behavior of N-hexadecyl-N-methylpyrrolidinium bromide (C16MPB) in EAN and mapped the phase diagram. With an increasing concentration of C16MPB, H1 and V2 phases were presented in sequence.27 Our group reported that 1-butyl3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]) acted as an efficient low-molecular-weight gelator (LMWG) in EAN.14 Because of their unique structural, mechanical, and electronic properties, carbon nanotubes (CNTs) have shown great potential applications in physics, chemistry, and life science.28−30 However, there are two major impediments for practical applications of CNTs: their insolubility in a common solvent such as water, resulting from the strong tendency toward aggregation due to strong van der Waals attractions between the individual CNT, and the random orientation of CNTs. Therefore, the dispersion and achievement of uniform alignment have attracted the interest of researchers. In consideration of the long-range orientational order along a special direction of LLCs, it seems feasible to induce the alignment of dispersed CNTs by LLCs. Recently, studies focused on using LLCs to disperse and align CNTs have been reported.31−35 The CNTs are dispersed in surfactant and then incorporated in the preformed LLC matrix as fillers. In this work, we synthesized two substituent isomers, viz., 1dodecyl-3-methylimidazolium m- and p-hydroxybenzoate (mC12mimHB and p-C12mimHB), and systematically investigated their aggregation behavior in water and EAN. Compared to the previous report on the aggregation behavior of 1-dodecyl-3methylimidazolium salicylate (C12mimSal), the aim of the present work is to study the position of substituents and solvent effects, namely, hydroxyl and carboxyl, on the self-assembly behavior of SAILs. In addition, multiwalled CNTs (MWCNTs) could be uniformly dispersed in the H1 phase formed by the C12mimHB/H2O system, which does not destroy the structure of the H1 phase but influences its structure effectively. We expect that this work not only can enrich our knowledge of the aggregation behavior of SAILs but also can highlight the design of new self-assembled systems for dispersing CNTs.
controlled by a thermostatic water bath with an uncertainty of ±0.1 °C. 2.4.2. Polarized Optical Microscopy Observation. A polarized light microscope (XPF-800C, Tianxing, Shanghai, China) with a digital camera (TK-9301EC, JVC, Japan) was used to acquire the optical image of LLCs. 2.4.3. Small-Angle X-ray Scattering Measurements. The structural characterizations of LLCs were performed in an Anton Paar SAX Sess mc2 system with Ni-filtered Cu Kα radiation (0.154 nm) operating at 2 kW (50 kV and 40 mA). The LLC samples were placed in a stainless steel tank and sealed with a transparent film. The distance between the sample and detector was 264.5 mm. All samples were held in a vacuum steel holder to provide thermal contact to the computer-controlled Peltier heating system (Hecus MBraun, Austria). 2.4.4. Rheological Measurements. The rheological properties were measured on a Haake RS75 rheometer with a cone−plate sensor (Ti, diameter 35 mm, cone angle 1°, distance 52 μm). Frequency sweep measurements were performed at a constant stress that was determined from the strain−sweep measurements. 2.5. Dispersion of MWCNTs. The stock solutions with different concentrations of MWCNTs dispersed with the H1 phase were prepared by adding MWCNTs and m- or p-C12mimHB to H2O. In these systems, the weight ratio between the MWCNTs and SAILs was kept constant at 1:1. The solutions were sonicated for about 60 min (50 W, 40 kHz) in an ultrasonicator, followed by centrifugation at 905.5g for 20 min to remove nondispersed MWCNTs. In this instance, the precise quantity of MWCNTs dispersed in aqueous solution is not known. However, in contrast to the initial MWCNTs added, the quantity of precipitates is so small that it is negligible for the study. Thus, the calculation of the concentration of MWNTs in weight percent is based on the assumption that all MWCNTs are in the solution. 2.6. Synthesis of MWCNTs/LLC Composites. MWCNTs of different concentrations were added to the preformed LLCs, followed by the addition of a certain amount of C12mimHB so that the total concentration of C12mimHB was maintained at 60 wt % in all samples. This C12mimHB concentration was chosen to avoid phase separation. After an appropriate quantity of dispersed MWCNTs was added to the LLCs, the samples were heated to 70 °C in order to achieve the homogeneous phase. This procedure was followed by sonication for 1 h (50 W, 40 kHz) and centrifugation for 20 min at 905.5g to remove air bubbles. Before measurements, all samples were equilibrated in a constant-temperature cabinet at 25 °C for at least 4 weeks.
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION 3.1. Micellization of SAILs in EAN. Surface tension measurements were carried out to characterize the surface activity of C12mimSal, m-C12mimHB, and p-C12mimHB in EAN. Figure 1 represents the variations of the surface tension (γ) versus concentration of SAILs in EAN at 25 °C. It is
2.1. Materials. 1-Methylimidazole (99%) was obtained from Acros Organics and distilled prior to use. 1-Bromododecane (98%), mhydroxybenzoic (99%), and p-hydroxybenzoic (99%) were all obtained from J&K Scientific Co., Ltd. and used as received. MWCNTs were purchased from Shenzhen Nanotech Port Co., Ltd. of China and used as received (purity >95 wt %, prepared by the chemical vapor deposition method, with a length of 5−15 μm, amorphous carbon
