Interaction of cellulose with amine oxide solvents

Abstract

Cellulose I, mainly as ramie or as Avicel microcrystalline cellulose, has been monitored by optical microscopy and by 13C CPMAS NMR, over the course of its dissolution in hot N-methylmorpholine N-oxide solvent. Its interaction with the near-solvent N-ethylmorpholine N-oxide and related non-solvents has also been investigated. NMR shows that N-methylmorpholine N-oxide partly converts crystalline cellulose I into amorphous solid cellulose. The changes in chemical shift imply increased flexibility at the glycosidic bonds. In contrast, N-ethylmorpholine N-oxide converts cellulose I to cellulose IIII, without dissolution. Microscopy shows that the ramie fibres swell laterally, and at least some also shorten longitudinally, during dissolution. Model studies using methyl-β-d-glucopyranose show no evidence from 13C chemical shifts for different modes of binding with different solvents. However, N-methylmorpholine N-oxide binds more strongly to methyl-β-d-glucopyranose in DMSO than does N-ethylmorpholine N-oxide, whereas N-ethylmorpholine N-oxide binds better to H2O. Also, 13C T1 values for aqueous cellobioside show increasing rotational freedom of the –CH2OH sidechains as N-methylmorpholine N-oxide is added. Together, these observations imply the initial penetration of solvents and near-solvents between the molecular cellulose sheets. Subsequently, N-methylmorpholine N-oxide breaks H-bonds, particularly to O-6, just sufficiently to loosen individual chains and then dissolve the sheets.