A hydrogel formed by cetylpyrrolidinium bromide and sodium salicylate

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. Through the rheological characterization of the hydrogels, both the NaSal concentration and pH have significant effects on the formation and properties of the gels. ¹H NMR, FT-IR, and SEM have also been employed to investigate the molecular arrangement and morphology of the gels. Hydrophobic and electrostatic interactions are considered as the main 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 ([C_nmim]⁺). 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.