Hydrogen bonds-triggered differential extraction efficiencies for bifenthrin by three polymeric ionic liquids with varying anions based on FT-IR spectroscopy

Abstract

Herein, we fabricated three imidazolium-based polymeric ionic liquids (PILs) with different anions (P[VEIM]BF₄, P[VEIM]PF₆ and P[VEIM]Br), and analyzed their differential extraction efficiencies for bifenthrin through H-bonding induced effects. Three PILs all presented an irregular block structure with rough surface and lower specific-surface area (SSA, 11.2–18.7 m² g⁻¹) than carbon-based nanomaterials. They formed hydrogen bonds with free-water molecules in the lattice of PILs, including C_2,4,5–H⋯O–H, Br⋯H–O–H⋯Br, O–H⋯Br, C_2,4,5–H⋯F–P, P–F⋯H–O–H⋯F–P, C_2,4,5–H⋯F–B and B–F⋯H–O–H⋯F–B. After extraction, the O–H stretching-vibration peak was prominently intensified, whereas the C–H bond varied slightly concomitant with reduced B–F and P–F vibration. Theoretically, the C–H vibration should become more intense in the C_4,5–H⋯H₂O and C₂–H⋯H₂O bonds after extraction in contrast to before extraction. These contrary spectral changes demonstrated that the hydrogen bonds between cations in the PILs and free-water molecules were broken after extraction, yielding the H-bonding occurrence between bifenthrin and H–O–H in the lattice. As a time indicator for the free-water binding and releasing process, the highest slope for the plot of I_t/I₀ against time implied that the shortest time was required for P[VEIM]PF₆ to reach an adsorption equilibrium. Overall, the strong hydrophobicity, small SSA and electrostatic-repulsion force for P[VEIM]PF₆ are all not conducive to its efficient adsorption. Beyond our anticipation, P[VEIM]PF₆ provided the highest extraction recovery for bifenthrin up to 92.4% among three PILs. Therefore, these data lead us to posit that the above high efficiency results from the strongest H-bonding effect between P[VEIM]PF₆ and bifenthrin. These findings promote our deep understanding of PILs-triggered differential efficiency through a H-bonding induced effect.