Molecular dynamics simulation of dissociation behavior of various crystalline celluloses treated with hot-compressed water

The dissociation behavior of the crystalline cellulose polymorphs I_β, II, III_I, and IV_I (Cell I_β, etc.) at 503 K and 100 bar was studied by molecular dynamics simulation, and the mechanism of the experimental liquefaction during treatment with hot-compressed water was elucidated. The results showed that the mini-crystals of Cell I_β and Cell IV_I exhibited similar resistance to dissociation, which implies the occurrence of crystal transformation from Cell IV_I to Cell I. On the other hand, the mini-crystal of Cell II gradually dissociated into the water environment with the progress of time in the simulation. The water molecules gradually penetrated the Cell II crystal, especially along the (1 (overline{1}) 0) crystal plane. In contrast, the dissolution behavior differed for the surface and the core areas of the mini-crystal of Cell III_I. The cellulose chains on the surface were dissociated into the water environment, whereas the ordered structure of the chains in the core region was maintained for the entire simulation period. The detailed investigation showed that the core part of Cell III_I was transformed into Cell I at an early stage of the simulation: Cell I is resistant to dissociation of the structure even in the hot-compressed water environment. It can be confirmed that the stability of these four crystals under high temperature and pressure conditions follows the order: Cell II < III_I < IV_I ≈ I_β.