Physical, chemical, and structural properties of subcritical water-treated cellulose derived from Sargassum horneri

To overcome the limitations of conventional nanocellulose - specifically cellulose nanocrystals and cellulose nanofibers - this study developed subcritical water-treated cellulose (SWTC) using subcritical water treatment (SWT). In this work, SWT was applied to a solid residue obtained through conventional alkali and bleaching pretreatments to produce nanocellulose and the structural, physical, and chemical properties of SWT-C under various SWT conditions were evaluated. The yield of SWT-C decreased with increasing temperature and time, ranging from 9.54 ± 1.19% to 13.15 ± 1.27%. Entangled nanofibrillar networks were observed in SWTC, with fiber diameters decreasing under harsher conditions. XRD analysis demonstrated that SWT enhanced crystallinity to 69.43 ± 2.61% (at 180 °C for 20 min). This value was higher than that of both the initial raw material (Sargassum horneri, 43.17 ± 1.07%) and the extracted micron-scale cellulose (57.25 ± 3.67%). FTIR spectra demonstrated enhanced peaks at 1430 and 898 cm^-1, indicating removal of amorphous regions and improved molecular alignment. Colorimetric changes, attributed to 5-(Hydroxymethyl)furfural and furfural formation, included decreased L* and increased a* value as treatment conditions became harsher. Thermal analysis showed degradation and onset temperatures of 352.95 °C and 241.34 °C, respectively. Surface conductivity and polydispersity index (PDI) remained below 0.4 under optimal aqueous conditions: 160 °C for 60 min (0.1 wt%) and 160 °C for 40 min (0.2 wt%). These findings demonstrate that SWT significantly affects the physicochemical properties of SWT-C and highlight its potential as a sustainable material for food-related applications.