Colloids and Surfaces A: Physicochemical and Engineering Aspects
A hydrogel formed by cetylpyrrolidinium bromide and sodium salicylate
Graphical abstract
A novel hydrogel constructed by N-methyl-N-cetylpyrrolidinium bromide (C16MPBr) and sodium salicylate (NaSal) is investigated using 1H NMR, FT-IR, SEM, and rheological measurements.
Highlights
► A hydrogel formed by C16MPBr and NaSal is investigated. ► The gel formation process undergoes a transition from wormlike micelles to gels. ► Both salt concentration and pH have effects on the properties of the gels. ► Hydrophobic and electrostatic interactions are driving forces to form the gels.
Introduction
Amphiphilic molecules can generally self-assemble to form diverse well-organized structures in micrometer or even nanometer length scale, such as micelles [1], fibers [2], vesicles [3] and lamellar phases [4]. As a typical self-assembled system, surfactant solutions have been intensively studied in the recent years. When the concentration of surfactant is beyond the critical micelle concentration (CMC), surfactant molecules can self-assemble to form globular micelles. With the change of the temperature, pressure, ionic strength, pH, salt concentration and other experimental conditions [4], [5], [6], [7], the initial globular micelles can grow in length and then self-assemble into various microstructures, like “threadlike micelles” and “wormlike micelles”. These wormlike micellar solutions often exhibit prominent viscoelastic behaviors [8]. Under appropriate conditions, the wormlike micelles can intertwine to form a three-dimensional network, which directly leads to the formation of hydrogel. In a hydrogel system, water content is extremely high, almost in the range of 95–99 wt%, while the solute content is relatively low [9]. The properties of the gels can be tuned by the external conditions: pH, temperatures, anions, and sound waves [10], [11], [12].
Ionic liquids (ILs) have aroused intensive investigations in the recent years due to their unique physical and chemical properties, which can be designed just by altering the cations and anions [13], [14], [15], [16]. Self-assemblies, like wormlike micelles, microemulsions, and lyotropic liquid crystals [15], [16], [17], formed in ILs have been studied. Reports about the gels formed in ILs mainly focused on the ion gel electrolytes composed of the gelators and short-chain ILs with hydrophobic chains of four or six carbon atoms. Hanabusa's group [18] found two special gelators which can gel a large number of ILs, such as imidazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium, and ammonium salts. He et al. [19] firstly employed a kind of triblock copolymers to gel 1-butyl-3-methylimidazolium hexafluorophosphate with as low as 5 wt%. Kadokawa's group [20] reported the formation of gels formed by chitin and cellulose in 1-alkyl-3-methylimidazolium type ILs. Recently, Ribot et al. [21] have designed a reverse thermoresponsive ion gel composed of a quaternary ammonium oligo(propyleneoxide)-based IL and water. This gel system possessed enhanced conductivity and tunable gelation point in a wide temperature range.
Investigations about gels formed by the long-chain ILs in water are relatively few and mostly concentrated on 1-alkyl-3-methylimidazolium salts ([Cnmim]+). Huang et al. [22] designed a thermo-responsive hydrogel using C16mimBr and NaSal. They found that the hydrogel could transform from a wormlike micelle state to a gel state according to different temperatures. Zhao et al. [23] constructed the hydrogels by mixing C16mimCl and SDS in aqueous solutions.
In the present work, a novel hydrogel is constructed by N-methyl-N-cetylpyrrolidinium bromide (C16MPBr) and NaSal (Scheme 1). The gel formation process goes through a transition from wormlike micelles to hydrogels. Both salt concentration and pH have significant effects on the properties of the gels.
Section snippets
Chemicals
C16MPBr was synthesized and purified according to the methods reported previously [24]. NaSal was an AR grade product of Tianjin Kermel Chemical Reagent Company. Sodium hydroxide and hydrochloric acid were purchased from Beijing Chemical Reagent Company. All of them were used without purification. Water was triply distilled.
Characterization
Rheological measurements. The rheological measurements were performed on a Haake Rheostress 6000 rheometer with a Rotor C35/1 system. The temperature was kept at 20.0 ± 0.1 °C.
The effect of NaSal concentration on the structure of hydrogel
Fig. 1(a) shows the zero-shear viscosity of the samples with the variation of NaSal concentration in the 100 mM C16MPBr solution. The viscosities firstly increase significantly with the NaSal concentration, which may be due to the formation of the three-dimensional network structures. With the addition of NaSal, the initial formed spherical micelles may grow in length and form the long, flexible wormlike micelles. The appearance of the wormlike micelles leads to the increase of the viscosities.
Conclusions
In summary, a novel hydrogel can be constructed by a kind of long chain pyrrolidinium ILs and the added salt. Both salt concentration and external pH values have significant effects on the gel structures. The hydrophobic interactions and electrostatic interactions are regarded as the main driving forces for the formation of the gels. We expect this work can helps us to better understand the formation of the gels and the phase behaviors of ILs.
Acknowledgements
The authors are grateful to the National Natural Science Foundation of China (No. 50972080) and National Basic Research Program (2009CB930101).
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