Deep eutectic solvent-assisted pretreatment of Ulva seaweed cellulose and in silico characterization

Ulva lactuca, a rapidly growing seaweed, is a sustainable biomass resource rich in polysaccharides such as cellulose. Conventional cellulose extraction methods often rely on harsh chemicals. Here, we investigated hydrophilic and hydrophobic deep eutectic solvents (DESs) as green alternatives for pretreating seaweed-derived amorphous cellulose. DESs were selected from computational predictions of eutectic point formation and classified by their partition coefficients and phase behavior in aqueous media. The selected hydrophilic DESs were betaine:urea, choline chloride (ChCl):urea, and ChCl:citric acid, and the hydrophobic DESs contained thymol and decanoic acid, octanoic acid, formic acid, or methanesulfonic acid. Pretreatment of the DESs with ChCl:urea and thymol:decanoic acid facilitated formation of seaweed cellulose microfibers (SCMF) with average diameters of 53.51 ± 14.00 and 153.60 ± 55.45 nm, respectively. High yields (75 %–84 %) were obtained because these systems did not result in extensive cellulose dissolution. Conversely, DESs containing organic acids, such as thymol:methanesulfonic acid and ChCl:citric acid, helped remove non-cellulosic impurities and amorphous regions, which increased the purity and crystallinity (crystallinity indexes: 48.50 and 27.30, respectively). The SCMF were readily dispersed in water and could adsorb methylene blue (up to 554.76 mg/g). Molecular dynamics simulations corroborated the experimental findings. Radial distribution function, coordination number, and hydrogen bond analyses revealed that DESs disrupted the cellulose's hydrogen-bond network and influenced fiber morphology. Hydrophobic DESs preserved the fiber integrity, while hydrophilic DESs promoted chain rearrangement. This present work demonstrates that DES pretreatment can produce SCMF with desired structural attributes and offers a sustainable and versatile approach for biorefinery applications.