Elucidating the structural changes of cellulose molecules and dynamics of Na ions during the crystal transition from cellulose I to II in low temperature and low concentration NaOH solution

Abstract

Low-concentration alkali treatments at low temperatures facilitate the crystal transition of cellulose I to II. However, the transition mechanism remains unclear. Hence, in this study, we traced the transition using in situ solid-state ¹³C CP/MAS NMR, WAXS, and ²³Na NMR relaxation measurements. In situ solid-state ¹³C CP/MAS NMR and WAXS measurements revealed that soaking cellulose in NaOH at low temperatures disrupts the intramolecular hydrogen bonds and lowers the crystallinity of cellulose. The dynamics of Na ions (NaOH) play a crucial role in causing these phenomena. ²³Na NMR relaxation measurements indicated that the Na-ion correlation time becomes longer during the crystal transition. This transition requires the penetration of Na ions (NaOH) into the cellulose crystal and a reduction in Na-ion mobility, which occurs at low temperatures or high NaOH concentrations. The interactions between cellulose and NaOH disrupt intramolecular hydrogen bonds, inducing a conformational change in the cellulose molecules into a more stable arrangement. This weakens the hydrophobic interactions of cellulose, and facilitates the penetration of NaOH and water into the crystal, leading to the formation of alkali cellulose. Our findings suggest that a strategy to control NaOH dynamics could lead to the discovery of a novel preparation method for cellulose II.

Keywords: Alkali cellulose; Cellulose structure; Intrachain hydrogen bonding; Low-temperature alkali treatment; Mercerization; Na-ion dynamics.