Observed Mechanism for the Breakup of Small Bundles of Cellulose Iα and Iβ in Ionic Liquids from Molecular Dynamics Simulations

Abstract

Explicit, all-atom molecular dynamics simulations are used to study the breakup of small bundles of cellulose Iα and Iβ in the ionic liquids [BMIM]Cl, [EMIM]Ac, and [DMIM]DMP. In all cases, significant breakup of the bundles is observed with the initial breakup following a common underlying mechanism. Anions bind strongly to the hydroxyl groups of the exterior strands of the bundle, forming negatively charged complexes. Binding also weakens the intrastrand hydrogen bonds present in the cellulose strands, providing greater strand flexibility. Cations then intercalate between the individual strands, likely due to charge imbalances, providing the bulk to push the individual moieties apart and initiating the separation. The peeling of an individual strand from the main bundle is observed in [EMIM]Ac with an analysis of its hydrogen bonds with other strands showing that the chain detaches glucan by glucan from the main bundle in discrete, rapid events. Further analysis shows that the intrastrand hydrogen bonds of each glucan tend to break for a sustained period of time before the interstrand hydrogen bonds break and strand detachment occurs. Examination of similar nonpeeling strands shows that, without this intrastrand hydrogen bond breakage, the structural rigidity of the individual unit can hinder its peeling despite interstrand hydrogen bond breakage.

Diffusion of 1-Ethyl-3-methyl-imidazolium Acetate in Glucose, Cellobiose, and Cellulose Solutions

“…Most experimental and modeling results show that ILs solvate carbohydrates through the formation of hydrogen bonds between the IL anion and hydroxyl groups of the sugar solutes. 16,18,24,25 …”

Cellulose dissolution: insights on the contributions of solvent-induced decrystallization and chain disentanglement

“…Ratio of boundary layer thickness to the fiber radius g , d/R 0.05 0.05 a Given in the experimental work (Jiang et al 2012) b Calculated in this work using the Stokes-Einstein equation (Ghasemi et al 2016;Rabideau et al 2013) c Calculated from the extrapolation of the reported diffusivity data (Jiang et al 2011) of semi-dilute solution of cellulose in [bmim]Cl (Ghasemi et al 2016) d Considered based on the difference between diffusive transport processes in amorphous and crystalline states (Ghasemi et al 2016;Hancock and Zografi 1997) e Calculated in this work using the thermodynamics of swollen polymer networks (Ghasemi et al 2016;Horinaka et al 2011) f Considered to be the first instance at which the concentration of dissolved cellulose chains was experimentally detected (Jiang et al 2012;Cai et al 2012) g Assumed on the basis of the solution conditions and geometry of the system (Ghasemi et al 2016;Narasimhan and Peppas 1996) the disentanglement rate of cellulose is obtained to be 2.5 9 10 -10 m/s, however, in the first hour of the process, the fiber diameter changes very little compared to the significant swelling observed at 70°C (Fig. 3b).…”

“…After contacting the cellulose fiber with solvent, the solvent gradually penetrates into the fiber, at first mostly into amorphous domains, resulting in intercrystalline swelling (Klemm et al 1998). If the strong inter-and intra-molecular forces between cellulose chains can be overcome by the solventcellulose interactions, the crystalline network of cellulose is gradually destroyed and the possibility of conformational chain movement is enhanced (Rabideau et al 2013). As a result of increasing chain movements, due to a weakening of the inactions between cellulose chains, the solid cellulose fiber is transformed to a swollen gel-like medium (Lu et al 2011).…”

Using Molecular Simulation to Study Biocatalysis in Ionic Liquids

“…Similar to the analysis methods for studying enzymes in ILs, many studies of substrates in ILs have focused on determining the solvent's structural and spatial distribution around the substrate to, for example, glean mechanistic aspects of cellulose solvation and dissolution in ILs (Cho, Gross, & Chu, 2011;Gross, Bell, & Chu, 2011;Gupta, Hu, & Jiang, 2011;Mostofian, Smith, & Cheng, 2011;Youngs, Hardacre, & Holbrey, 2007;Youngs et al, 2011). Thermodynamic analysis of the cellulose solvation process in ILs has revealed an entropic and energetic driving force for cellulose dissolution in ILs Gross, Bell, & Chu, 2012), and more recently published papers have identified key structural and thermodynamic differences of bundles of cellulose vs individual chains in ILs (Li et al, 2015;Rabideau, Agarwal, & Ismail, 2013;Rabideau & Ismail, 2015;Zhao, Liu, Wang, & Zhang, 2013). New and interesting methods of analysis include analyzing the induced changes in the conformational free energy landscapes of sugar rings by ILs (Jarin & Pfaendtner, 2014), and performing dynamical calculations of mean lifetimes of different binding states of IL anions with hydroxyl groups on cellulose (Rabideau & Ismail, 2015).…”