Abstract
Problem Definition and Purposes: Biobased hydrogels have attracted considerable attention in medical, agricultural, pharmaceutical, and environmental applications due to their unique properties such as biocompatibility, biodegradability, and high water absorption capacity. One of the main challenges in producing cellulose-based hydrogels is the difficulty of dissolving cellulose, arising from its highly crystalline structure and strong intermolecular hydrogen bonding. Alkaline systems such as sodium hydroxide/urea have been introduced as non-derivatizing solvents capable of effectively dissolving cellulose. However, the role of the coagulation step in determining the final properties of cellulose hydrogels has not been thoroughly investigated. The objective of this study was to evaluate the effect of the type of coagulant (water and sulfuric acid) on the structure and physicochemical properties of cellulose hydrogels prepared via alkaline dissolution.
Materials and Methods: High-purity cotton linter fibers were subjected to chemical pretreatments and subsequently dissolved in an aqueous sodium hydroxide/urea system (7:12 weight ratio) at low temperature. Cellulose solutions with different concentrations were prepared and converted into hydrogels by injection into two coagulation media: deionized water and 5 wt% sulfuric acid solution. The resulting hydrogels were characterized using swelling tests, Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FE-SEM), and nitrogen adsorption analysis based on the multipoint Brunauer–Emmett–Teller (BET) method.
Results: The cotton linter fibers used in this study exhibited an α-cellulose content of 99.23% and a degree of polymerization of 4840, indicating high purity and excellent quality of the raw material. Swelling tests conducted at different cellulose concentrations revealed that a concentration of 3 wt% resulted in the highest swelling ratio for both types of hydrogels; therefore, this concentration was selected as the optimum for further analyses. At this optimal concentration, the swelling ratio of the cellulose hydrogel coagulated in water was approximately 70% lower than that of the hydrogel coagulated in sulfuric acid, highlighting the significant influence of the coagulant on water absorption capacity and hydrogel structure. FT-IR results confirmed a reduction in the intensity of peaks associated with hydroxyl groups and glycosidic bonds, attributed to cellulose dissolution and structural regeneration. FE-SEM images showed that the water-coagulated cellulose hydrogel (CHW) possessed a loose, layered, and non-uniform surface morphology, whereas the sulfuric-acid-coagulated hydrogel (CHA) exhibited a more compact, homogeneous, and integrated structure. BET analysis indicated that both hydrogels belonged to the mesoporous material category. Nevertheless, the CHA sample demonstrated superior structural properties, with a specific surface area of 304.66 m²/g, a total pore volume of 0.250664 cm³/g, and an average pore diameter of 6.30 nm, which were significantly higher than those of the CHW sample.
Conclusion: The coagulation step plays a decisive role in determining the final quality of cellulose hydrogels. The use of sulfuric acid as a coagulating agent significantly enhances porosity, specific surface area, and swelling behavior, rendering the resulting hydrogels more suitable for biomedical and adsorption-related applications. These findings underscore the importance of careful selection of the coagulation agent in the processing and production of biobased materials.