Role of sonication pre-treatment and cation valence in nano-cellulose suspensions sol-gel transition

TL;DR

This study explores how sonication pre-treatment and cation valence influence the gelation process and properties of carboxylated cellulose nanocrystal suspensions, revealing the importance of ion type, concentration, and sequence in gel formation.

Contribution

It demonstrates that sonication induces rapid gelation and that cation valence and addition sequence critically affect the gel's structural and rheological properties, providing insights for tailored material design.

Findings

Sonication accelerates gelation and affects gel stiffness.

Cation valence and sequence of addition influence gel structure.

Water confinement varies with ion type and addition order.

Abstract

Sol-gel transition of carboxylated cellulose nanocrystals is investigated using rheology, SAXS, NMR and optical spectroscopies to unveil the distinctive roles of ultrasounds treatment and ions addition. Besides cellulose fibers fragmentation, sonication treatment induces fast gelling of the solution. Gelation is induced independently on the addition of cations, while the final rheological properties are highly influenced by the type, the concentration as well as on the sequence of the operations since salts must be added before sonication to produce stiff gels. Cations with various charge and dimension have been associated to ultrasounds to induce gelation and the gel elastic modulus increase proportionally with the charge over the ion size ratio. SAXS analysis of the Na+ hydrogel and Ca2+ hydrogel to which the ion was added after sonication shows the presence of structurally ordered domains where water is confined as indicated by 1H-NMR investigation of the dynamic of water exchange in the hydrogels. Conversely, separated phases containing essentially free water, characterize the hydrogels obtained by sonication after Ca2+ addition, confirming that this ion induces irreversible fiber aggregation. The rheological properties of the hydrogels depend on the duration of the ultrasound treatment and it enables the design of materials programmed with tailored energy dissipation response.