Molecular Simulation On Interface Between Ionic Liquids And Solid:Dissolution Of Cellulose And Dispesion Of Carbon Nanotubes

Ionic liquids (ILs) are a novel solvents composed with cations and anions, which inspired great interest due to many benefits including wide electrochemical window, high conductivity, extremely low vapor pressure, designable properties and thou-sands of potential candidates. It is regarded as an alternative of volatile organic solvent in various applications such as gas absorption, thermal conduct, lubrication, preparation and dispersion of materials, cellulose pretreatment and catalysis. The unique properties of ILs depend on the interactions between cations, anions and other solvents or solutes, especially the interplay between long-range electrostatic interaction and various hydrogen bondings. Molecular simulation is a valuable tool to investigate the systems directly in atomistic scale. In this work, we focus on the interfaces between ILs and solids, which is just a few molecules thick, but is crucial for any real processes. Two kinds of materials, i.e., cellulose and single wall carbon nanotube (SWNT), are chosen to study due to their potential applications. The results in this work can be potentially used in solvent design of ILs in cellulose dissolution and SWNT dispersion.In chapter2, the cellulose surface is built in virtue of available crystal data. Molecular dynamics (MD) simulations are performed for the interfaces between cel-lulose and ILs of [Bmim]Cl,[Emim]Cl,[Omim]Cl and [Bmim][BF4]. The microscopic structure and interactions between ILs and cellulose are analyzed to elucidate the mechanism of dissolution. We found that the Pair Energy Distribution (PED) is a good indicator to reflect the dissolving power of ILs. The PED between anions and cellulose is substantially larger than that between cations and cellulose. The average interaction of chloride-cellulose reaches-33.4kcal/mol combined with Bmim cation. The sequence of PEDs by simulations for different ILs is in good agreement with that of cellulose solubilities in experiments. Various kinds of hydrogen bonds (HB) are found between cellulose and anions, in a value of0.58for each glucose unit averagely.It is well known that the viscosity of IL is too high to efficiently dissolve cellu- lose. It can be overcome by adding some small molecule solvents. In chapter3, two kinds of solvents of water and DMSO are added in simulation to investigate their impact on IL-cellulose interface, which is found very different. An evident peak in weaker interaction appears in PED by adding water, resulting a decrease of average energy of about8kcal/mol. In contrast, DMSO has little effect on the PED. The interaction is even strengthened when the mole fraction of DMSO reaches0.5. In atomistic scale, strong HBs are formed between chloride and water, which weakens their interactions with cellulose. DMSO molecules are in a region between the anion layer and cation layer. Thus, the interactions between cations and anions is weak-ened in the presence of DMSO, which could enhance the anion-cellulose interactions. By the simulations of this work, DMSO can be used as co-solvent.In chapter4, the cylindrical interface between IL and the infinite long SWNT is simulated to reveal the mechanism of SWNT dispersion in ILs. It is found both the anions and cations form three high density layers. In addition, the orientation of cations is also highly ordered in these layers, tending to be parallel to the surface of SWNT. The simulation results coincide well with the observations in experiments of multi-layer and solid-like structure of ILs near the solid surface.The dispersion of SWNT can be directly linked in terms of free energy. In chapter5, we calculated potential of mean force (PMF) of SWNT in ILs and other solvents by using Weighted Histogram Analysis Method (WHAM) in MD simula-tions. The attraction interactions among SWNT are very strong even in ILs. Thus, the dispersion must be accomplished by external energy input such as heavily stir-ring and ultrasound. The solvent affects the energy barrier in PMF to avoid the aggregation of SWNT. The value of barrier in IL is about three times larger than that in water, indicating IL is more efficient in dispersion than water.