Interaction of cellulose with amine oxide solvents

Cellulose I, mainly as ramie or as Avicel microcrystalline cellulose, has been monitored by optical microscopy and by C-13 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-beta-d-glucopyranose show no evidence from C-13 chemical shifts for different modes of binding with different solvents. However, N-methylmorpholine N-oxide binds more strongly to methyl-beta-d-glucopyranose in DMSO than does N-ethylmorpholine N-oxide, whereas N-ethylmorpholine N-oxide binds better to H2O. Also, C-13 T-1 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.