Gum Arabic-Grafted Poly(NIPAM-co-Acrylic Acid) Thermo-Responsive Hydrogel for Selective Adsorption of Methylene Blue: Experimental and DFT Insights

Abstract

A dual thermo- and pH-responsive hydrogel, gum arabic-grafted poly(N-isopropylacrylamide-co-acrylic acid), hereafter GAg poly(NIPAM-co-AAc), was prepared by free-radical graft copolymerization using ammonium persulfate as initiator and N,N′-methylenebisacrylamide as crosslinker, and evaluated for the selective uptake of methylene blue (MB) from aqueous solution. FTIR confirmed grafting of the vinyl monomers onto the polysaccharide backbone, XRD pointed to an essentially amorphous network, and FESEM revealed a porous, interconnected morphology with nanoscale wall features (40–80 nm) that favor dye diffusion. The best removal (≈97%) was reached at pH 8 with 0.05 g of hydrogel and an equilibrium time of 90 min. Kinetic data obeyed the pseudo-second-order model (R² > 0.99), while intraparticle diffusion participated in, but did not solely control, the overall rate. Equilibrium data followed the Langmuir isotherm with a maximum monolayer capacity of 312.5 mg g⁻¹ at 318 K, and crossing the LCST of the PNIPAM segments (≈34 °C) provided a convenient adsorption– regeneration switch. Thermodynamic analysis indicated a spontaneous, endothermic and entropy-driven process (ΔH° = +24.6 kJ mol⁻¹; ΔS° = +0.118 kJ mol⁻¹ K⁻¹). DFT/TD-DFT calculations at the B3LYP/6-311G(d,p) level in water reproduced the visible band of MB and located the positive electrostatic potential over the heteroaromatic core, whereas 50 ns MD simulations showed that electrostatic attraction to carboxylate groups, assisted by hydrogen bonding and π-stacking, dominates the binding. The hydrogel retained more than 90% of its capacity after five consecutive cycles.

Highlights
 A dual-responsive GA-g-poly(NIPAM-co-AAc) hydrogel was built by graft copolymerization.
 Langmuir monolayer capacity toward methylene blue reached 312.5 mg g⁻¹ at 318 K.
 Uptake followed pseudo-second-order kinetics and was spontaneous, endothermic and entropy-driven.
 Switching across the LCST (≈34 °C) enabled selective capture and almost eluent-free regeneration.
 DFT/TD-DFT descriptors and 50 ns MD runs rationalized the electrostatic/H-bonding mechanism.