Small-angle neutron scattering from cellulose solutions in phosphoric acid at different water content

Cellulose from biomass is an abundant and renewable alternative source for chemicals and fuels, yet its utilization by chemical or biological process requires pre-treatment in order to release the macromolecules from their tightly packed crystal structure. Phosphoric acid (PA) has been known for many years to be an efficient solvent for crystalline cellulose. It is also established that a certain quantity of water content in PA is required for efficient pretreatment. This study uses small-angle neutron scattering (SANS) measurements to evaluate cellulose dissolution in deuterated phosphoric acid (dPA), at different wt% dPA between 78 and 97 % (different D₂O content). The SANS method is useful for this purpose due to the availability of deuterated dPA, its contrast in scattering length density towards cellulose, and its low incoherent scattering cross-section. The results indicate that most of the cellulose in 2 wt% solution is dissolved in PA as individual chains, at acid content of 81–94 wt% PA. Structural differences of the dissolved cellulose in PA of the various water compositions in this range are insignificant. At 78 % dPA the cellulose crystal still seem to be disrupted, yet the structure can be modeled as mass-surface fractals of small fibrils with irregular surface, possibly due to dissolved chain segments, which are aggregated as mass fractals of rods. At 97 % dPA evidence for a small content of undissolved fibrils is noted.