Effect of the counter-ion on nanocellulose hydrogels and their superabsorbent structure and properties

Carboxylated nanocellulose gels and superabsorbents (SAPs) can be engineered by ion exchange of TEMPO treated cellulose fibers with different cations prior to shearing, thus creating a nanofibrous network ionically cross-linked. The structure and properties of these materials are highly influenced by the type counter-ion used as it controls both the degree of fibrillation and crosslinking.

Functionalised nanocellulose SAPs were made using TEMPO-mediated oxidation followed by ion-exchange before fibrillation into a hydrogel and freeze-drying. Seven different cations were tested: 4 of valency 1 (H, Na, K, NH₄), and 3 of valency 2 (Ca, Mg, and Zn). The effect of the counter-ion on the gelation mechanism and the superabsorbent performance was evaluated. The SAP absorption capacity in deionised water was related to the superabsorbent structure and morphology.

The gel stability of nanocellulose superabsorbents is governed by the counter-ion type and valency. The viscoelastic properties of all nanocellulose hydrogels are controlled by its elastic regime, that is storage modulus (G') > loss modulus (G″). The type of cation dictates the rheology of these gels by altering the fibrillation efficiency due to the extent of ionic cross-links occurring before and after fibrillation. The driving force for gelation in monovalent gels is due to the coupling of nanofibrils by physical interactions, creating an electrostatic stabilisation of the ionised COO^- groups at high shear forces. Cation - carboxylate interactions dominate the gelation in divalent gels by supressing the repulsive charges generated by the COO^- and also creating interfibril connections via ionic-crosslinks, as confirmed by the zeta potentials. The superabsorption performance is dominated by the counter-ion and is in the order of: NH₄⁺ > K⁺ > Na⁺ > Mg²⁺ > Zn²⁺ > Ca²⁺. NH₄⁺-SAPs present the slowest kinetics and the highest absorption capacity. Its high pore area, which extends the number of accessible carboxyl groups that participates in hydrogen bonding with water, is responsible for this behaviour. Nanocellulose SAPs are attractive renewable materials, suited for many applications, including as nutrient cation carriers in agriculture.