All-atom molecular simulation study of cellulose acetate: amorphous structure and the dissolution of small molecule

All-atom analysis was conducted for cellulose acetate (CA) using molecular dynamics simulation. The intermolecular interactions were elucidated at the amorphous state with degrees of acetyl substitution (DS) of 2, 2.5, and 3, and the energetics of dissolution was treated for H₂O, CO₂, and CH₄. It was observed for the CA amorphous that DS strongly affects the hydrogen bonding among the hydroxy groups of CA and that the short-range packing of pyranose rings becomes tighter with acetylation. The free energy of dissolution was computed by the energy-representation method of solvation. The dissolution into CA was more favorable in the order of H₂O > CO₂ > CH₄, and the DS dependence of the dissolution free energy was evident only for H₂O between DS = 2 and 2.5. The roles of the intermolecular interaction components were further addressed. It was seen that the electrostatic component brings the DS dependence of the dissolution free energy for H₂O as well as the difference in the affinity to CA between CO₂ and CH₄. The van der Waals component was still dominant for the nonpolar CO₂ and CH₄, and the summed contribution to it from the acetyl and main-chain groups of CA was weakly dependent on DS. The connection of the dissolution energetics with the underlying structures is also discussed.