Molecular Dynamics Study On The Dissolution Of Cellulose In Alkali/Urea Aqueous Solution

Cellulose is the most abundant natural polymer on the earth and has attracted many attentions due to its chemical stability,biocompatibility,non-toxicity and no pollution.Dissolution of cellulose is a key step to effectively convert this renewable resource into high value-added products,such as recycled materials(fiber,film,food packaging,sponges,etc.)and cellulose derivatives(esters and ethers).However,due to the complex aggregation structures and high crystallinity,there is a strong interaction between cellulose molecules.The solubility of cellulose in water or common chemical reagents is very low,which greatly limits the application of cellulose.Finding efficient,fast,economic and environmentally friendly cellulose dissolving system has become the key to further development and utilization of cellulose.In recent years,a series of alkali/urea aqueous solutions with quick dissolving power of cellulose have been developed.Due to their low cost,low energy consumption,non-toxicity and non-pollution,they have attracted considerable attentions.The alkali/urea aqueous solution systems have shown significant advantages in dissolution speed,dissolution cost,solvent recycling and environmental protection.However,these solvents still suffer from low-temperature pre-cooling and low solubility,which severely limits the application in industry.In order to further develop a new room-temperature,fast-dissolving system with higher solubility,a comprehensive study of the cellulose dissolution mechanism in alkali/urea aqueous solution is urgently needed.However,the low-temperature rapid dissolution mechanism of cellulose in alkali/urea aqueous solution is still unclear.The roles of different components in the dissolution of cellulose are still controversial.Based on the current experimental analysis methods,it is difficult to reveal the rapid dissolution mechanism of cellulose in alkali/urea aqueous systems.To fully study the stable aggregation configuration of cellulose inclusion complexes and clarify the effects of alkali metal cations,hydroxide ions,urea and water molecules on the dissolution process,the cellulose/alkali/urea aqueous solution was systematically studied by molecular dynamics simulations.The results give a better insight into the interaction between cellulose molecules and solvent components,and promote the design of new cellulose aqueous solution systems.The main research contents and innovation results are summarized as follows:The complete dissolution state of cellulose in NaOH/urea aqueous solution using molecular dynamics simulation was investigated.The aggregated sturture of the stable inclusion complex,the spatial distribution of solvent particles around cellulose,the interaction of solvent components with cellulose molecules and the spatial orientation of water molecules were systematically examined.It is found that the structure of cellulose inclusion complex is not a simple layered distribution.There are obvious anisotropic properties in the structure of cellulose inclusion complexes.The sodium ions and hydroxide ions located at the regions of hydroxyl and hydroxymethyl groups of the cellulose molecule and hydroxide ions formed very strong hydrogen bonds with the hydroxyl groups.The urea molecules occupied the face of the hydrophobic pyranose rings and formed hydrogen bonds with acetalic oxygen atoms of cellulose molecules.The hydrophobic face of the pyranose rings could be well-solvated by the aggregated urea molecules.The strong affinity of urea to cellulose molecules further improves the stability of cellulose inclusion complex.Additinally,water-water angles in cellulose inclusion complex were also discussed in the study.The distribution of water-water angles in cellulose inclusion complex shows that the hydrophobicity of cellulose molecules acts a key role in determining the orientation of water molecules.And a higher low angle population nearby the cellulose chain is more conducive to the formation of hydrogen bonds in the inclusion complex and plays an important role in the stability of the cellulose inclusion complexTo further verify the microscopic information of the cellulose inclusion complexes,a single-chain cellulose simulation,where the main chain of cellulose was freezed,was systematically studied.In the restained molecular dynamics simulation,the effect of alkali metal cations on the cellulose inclusion complexes and the interactions between the cellulose molecule and solvent particles were explored.The diameter of the cellulose inclusion complexe was estimated to be 1.2 nm by using the density distribution function.We examined the aggregated morphology of solvent particles around the cellulose molecule and analyzed the radial distribution of sodium ions,hydroxide ions and urea molecules around different oxygen atoms of cellulose.The simulation results showed that the sodium ions and hydroxide ions mostly located at the regions of hydroxyl groups of cellulose molecules and the urea molecules occupied the faces of the hydrophobic pyranose rings.In the cellulose inclusion complex,the hydroxyl groups of cellulose form many hydrogen bonds with the hydroxide ions,urea and water molecules while the pyranose ring oxygen only form few hydrogen bonds with urea molecules.At the same time,the non-bond interactions between cellulose molecules and solvent molecules were also calculated.The interaction between cellulose molecules and solvent particles is mainly the coulomb interaction.Nearby the hydroxyl groups,the non-bonding interactions between hydroxide ions and hydroxylic oxygen atoms of cellulose molecules were the strongest.On the faces of the pyranose rings of cellulose molecules,urea molecules exhibited the strongest interaction with hydroxylic oxygen atoms.In addition,the influence of alkali metal cations on the stability of cellulose inclusion complexes in KOH/urea and NaOH/urea solutions was discussed using the potential of mean force.The sodium ions have stronger destructive effect on the intermolecular and intramolecular hydrongen bonds than potassium ions.The cellulose inclusion complexes formed by NaOH and urea with cellulose molecules is more stable,which is consistent with the experimental outcomes.Dissolving behavior of a molecular layer of cellulose molecules in NaOH/urea aqueous solution was investigated using the molecular dynamics simulation method.Focusing on the aggregation evolution of the sheet of cellulose,the aggregation evolution of solvent components within cellulose inclusion complexes,the effects of solvent particles on the aggregating behaviors of cellulose molecules,the distribution of solvent particles around cellulose molecules and the effects of NaOH and urea on the cellulose dissolving process were discussed.In the simulation,the molecular sheet of cellulose was dramatically changed,and cellulose molecules went through a dispersion process,equilibrium process and reaggregation process.By monitoring the aggregated amount of solvent particles,we found that there was an obvious increase in the amount of sodium ions around 06 and the amount of hydroxide ions around 03 and 02 in the dispersion process.In the reaggregation process,the amount of sodium ions near 06 and the amount of urea around 01 and 05 significantly decreased.The stable dispersed structures of cellulose,i.e.,single molecule and bimolecular offset stacked configurations,were obtained in the equilibrium period.The offset stacked structure showed a strong stability in subsequent simulation.We also compared the final aggregation structure of the molecular layer of cellulose molecules in pure water,NaOH aqueous solution and NaOH/urea aqueous solution.It was found that the dispersion of cellulose molecules was the result of the combined action of NaOH and urea molecules.In the pure water system,the cellulose molecules aggregated together and the dispersion process was not observed.In the NaOH aqueous solution,the cellulose molecules exhibited a lower degree of dispersion.In the NaOH/urea aqueous solution,the cellulose molecules are better dispersed and the dispersion phenomenon occurred earlier.In addition,we examined the effect of temperature on the dissolution of cellulose and discussed the changes of sodium hydroxide hydrated ions at different temperatures.The cellulose model with 62.5%crystallinity was also used to explore the dissolution of cellulose crystal in NaOH/urea aqueous solution.In this simulation,we studied the aggregated structure of cellulose molecules and the aggregation evolution of solvent particles around cellulose molecules.The possible dissolution process of cellulose crystal was discussed.The simulation results showed that sodium ions,hydroxide ions and urea molecules mainly aggregated on the cellulose(010)crystal plane,especially for the hydroxide ions.We also found that the configuration of cellulose molecules on the edge of the(010)facet changes drastically.Therefore,we speculate that the(010)crystal plane may be the main region where the solvent particles interact with the cellulose crystal,and the dissolution of the cellulose crystal maybe firstly occurred on the edge region of the(010)crystal palne.