Ionic Liquid Cross-Linked High-Absorbent Polymer Hydrogels: Kinetics of Swelling and Dye Adsorption

Abstract

The use of polymer gels for the removal of toxic chemicals from wastewater is an important area in terms of both academic and industrial research. This work presents a simple approach to the fabrication of chemically cross-linked cationic hydrogel adsorbents using designed ionic liquid-based cross-linkers and their successful use in the removal of organic dyes. Two different ionic liquid cross-linkers, [VIm-4VBC]­[Cl] (ILA)/[DMAEMA-4VBC]­[Cl] (ILB), are synthesized by the simple nucleophilic substitution reaction of 4-vinylbenzyl chloride (4VBC) separately with 1-vinylimidazole (VIm) and 2-(dimethylamino)­ethyl methacrylate (DMAEMA). Cross-linked poly­(acrylamide) (CPAam) and poly­(2-hydroxyethyl methacrylate) (CPHEMA) hydrogels are then prepared from the corresponding monomers and as-synthesized cross-linkers (ILA and ILB) by free radical polymerization in the presence of a redox initiator combining ammonium persulfate (APS) and N,N,N′,N′-tetra­methyl­ethylene­diamine (TEMED). The dried CPAam and CPHEMA xerogels exhibit macroporous morphology and high thermal stability. The hydrogel samples exhibit high swelling behavior, and the diffusion of water molecules into the hydrogels follows pseudo-Fickian kinetics. The cationic cross-linking sites in the hydrogel networks allow preferable binding with anionic dyes, and these dye uptake capacities are determined using different model anionic dyes via UV–vis spectroscopy. The dye adsorption onto these hydrogels follows a pseudo-second-order kinetic model. The adsorption mechanism is also analyzed by employing intraparticle diffusion and Boyd kinetic models. The relationship between the maximum equilibrium adsorption capacity (q m) of the hydrogels for eosin B (EB) dye and the equilibrium EB concentration can be better described by Langmuir and Freundlich isotherm models, and the estimated q m using the Langmuir isotherm can reach more than 100 mg g–1. The cross-linked hydrogels can be easily regenerated and have a recycling efficiency of >80% for up to three consecutive dye adsorption–desorption cycles, which is promising for their use in wastewater treatment.

Poly(Acryloyl-l-Serine): A Reactive Polypeptide to Introduce Zwitterion and Amphiphilicity for Stimuli-Responsiveness and Gelability

“…The use of IL cross-linkers for developing ionic porous polymeric gel materials has been found to be extensively intensified in recent times. 40 The incorporation of the IL moiety increases the hydrophilicity of the formed gels, which enhances their performance in terms of functions and applications. 52 Acrylate moieties of P(AcSer) are also susceptible for free radical polymerization for the generation of porous cross-linked gels.…”

“…FESEM image analysis of the freeze-dried G3 aerogel clearly showed the formation of porous network-like structures with pore diameters in the range of 100–300 nm (Figure S30C). Such kind of larger-sized porous cross-linked structures were generated due to the repulsion among the cationic cross-links of the G3 gel and can be used as an efficient absorbent material. , To examine the swelling behavior of G3, a fixed amount of dried G3 gel sample was allowed to swell separately in DMF and water. The outcome of this experiment showed a remarkable swelling profile of G3 up to 341% in water after a time period of 1 h and attained an equilibrium after 10 h after reaching a maximum swelling capacity of 415%.…”

“…While, in DMF, the % swelling reached to a value of 130 after 24 h of equilibration (Figure S31C). Such high swelling ability of G3 in water was due to the existence of mobile chloride ions associated with cationic cross-links of G3, which creates an osmotic pressure difference between the gel matrix and surrounding water phase, causing expansion of the gel network . However, in DMF, the swelling of the gel matrix is solely decided by the interactions between the gel matrix and the solvent molecules, as observed in the cases of G1 and G2.…”

“…To explore such a possibility for G3, the experiment was carried out using EB and SZ as model anionic dyes and drugs, respectively. EB and SZ showed maximum absorbance at λ max = 520 nm in water and at λ max = 359 nm in PBS (pH = 7.4), respectively. , Linear calibration curves for both the EB and SZ were constructed by plotting the maximum absorbance against concentration, as shown in Figures S33A–D. In a typical procedure, 3 mL of EB (2 × 10 –5 M) or SZ (1 × 10 –4 M) solutions were prepared for the experiment.…”

“…It is expected that cationic hydrogels can adsorb anionic drug/dye molecules within their pores mainly through Columbic interaction. 40 To explore such a possibility for G3, the experiment was carried out using EB and SZ as model anionic dyes and drugs, respectively. EB and SZ showed maximum absorbance at λ max = 520 nm in water and at λ max = 359 nm in PBS (pH = 7.4), respectively.…”