**4. Conclusions**

In this research work, we used trehalose as a model molecule and neutron scattering and EPSR simulation as the main tools to study the rapid dissolution mechanism of cellulose in alkali/urea aqueous solution. The three-dimensional atomic structures of trehalose in three different alkali/urea aqueous solutions were thus compared. Alkali, urea and water work cooperatively to dissolve trehalose. A layered hydration shell is crucial. Cations directly interact with the Os of the glucose rings. They first break the inter- and intra-hydrogen bonding. The smaller the ions, the more easily they penetrate into the glucose rings. Urea molecules are too large to approach the glucose ring. As proton donor, their amino group can form a hydrogen bond with the hydroxyl group. The resultant electronegative complexation constitutes the second hydration layer via bridging force. They further stabilize trehalose and prevent it from re-aggregating.

We also found an interesting phenomenon, that is, Li+ ions are more concentrated around O1 and O2, while K<sup>+</sup> ions are more concentrated in the vicinity of O3 and O4, especially near O4. Although the low-temperature aqueous solution of KOH urea could not dissolve cellulose, it could dissolve chitin; at the same time, the low-temperature aqueous solution of LiOH/NaOH urea could dissolve cellulose, but could not dissolve chitin [18]. Based on this experimental evidence, we speculate: in the dissolution of cellulose, breaking the hydrogen bond formed by O1 and O2 is the key factor, while, in the dissolution of chitin, breaking the hydrogen bonds formed by its hydroxyl and amide groups is crucial. These findings are expected to be verified in future experiments.

**Author Contributions:** Conceptualization, H.C. (He Cheng) and C.M.; methodology, H.C. (He Cheng); software, C.M.; validation, T.Z., Z.H. and Y.L.; formal analysis, C.M.; investigation, H.C. (He Cheng); resources, W.Y.; data curation S.G. and H.C. (Huaican Chen); writing—original draft preparation, C.M.; writing—review and editing, H.C (He Cheng). and C.C.H.; visualization, C.M.; supervision, H.C. (He Cheng); project administration, H.C. (He Cheng); funding acquisition, H.C. (He Cheng) All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (NSFC) (grant numbers U1932161, U1830205 and 11805035); the National High Energy Physics Data Center of China (grant number NHEPSDC-OP-2021-001); and the National Key Research and Development Program of China (grant number 2017YFA-0403703).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are openly available in ISIS Neutron and Muon Source Data at http://doi.org/10.5286/ISIS.E.RB1820090 (accessed on 27 November 2018) and http://doi.org/10.5286/ISIS.E.RB1920190 (accessed on 15 December 2019).

**Acknowledgments:** The experiments were conducted using SANDALS at the ISIS Neutron, Muon Source and NOVA of J-PARC and the Multi-Physics Instrument (MPI) of CSNS. The authors thank the staff of the ISIS Disordered Materials Group, Materials and Life Science Experimental Facility of J-PARC and the instrument scientists of SANDALS and NOVA for the support with experiment design, operation and data analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

