High solubility of cellulose in slow-cooling alkaline systems and interacting modes of alkali and urea at the molecular level

The dissolution of microcrystalline cellulose (MCC) in a urea-NaOH system is beneficial for its mechanical processing, but the solubility has been low. The MCC solubility was greatly improved to 14 wt% under a slow-cooling condition with a cooling rate of − 0.3°C/min. The cooling curve or thermal history played a crucial role in the dissolution process. An exotherm (31.9 ± 1.6 J/g MCC) was detected by DSC only under the slow-cooling condition, and the cryogenic dissolution of MCC is attributed to the exothermic interaction between MCC and solvent. More importantly, the low cooling rate promoted the dissolution of MCC by providing enough time for the diffusion of OH − and urea into MCC granules at higher temperatures. The Raman spectral data showed that the intramolecularly and intermolecularly hydrogen bonds in cellulose were cleavaged by NaOH and urea, respectively. XPS and solid-state 13 C NMR results showed that hydrogen bonds were generated after dissolution, and a dual-hydrogen-bond binding mode between urea and cellulose was confirmed by DFT calculations. The increase of entropy dominated the spontaneity of cryogenic dissolution of MCC, and the decrease of enthalpy played a minor role. The high solubility of MCC in the slow-cooling process and the dissolution mechanism are beneficial for the studies on cellulose modification and mechanical processing.