Molecular Dynamics Studies On The Competitive Adsorption Between Different Kinds Of Small Molecules Confined Within Carbon Nanotubes

The carbon nanotubes (CNTs) possess well-difined hollow structures and thus serve as desirable materials for molecules, such as hydrogen, water, and small organic molecules including drug molecules, adsorbing into their interiors. In recent years, it has been realized that the CNTs can be used to separate different kinds of molecules. In this thesis, on the basis of molecular dynamics (MD) simulations, we have studied the interesting phenomena of competitive adsorption between different kinds of molecules inside the single-walled carbon nanotubes (SWNTs), using the cases of alcohol/water, and enthan/ethylene systems for illustrations. We find that the dispersion interactions of inner molecules with the SWNT wall play a key role in the competitive adsorption. The implications of our findings for the dehydration of aqueous alcohols and the ethylene/ethane separation are also discussed. This thesis contains two sections:i) Alcohol-induced drying of carbon nanotubes and its implications for alcohol/water separation:Alcohols are important products in chemical industry, but separating them from their aqueous solutions is very difficult due to the hydrophilic nature of alcohols. Based on MD simulations, we observe a striking nanoscale drying phenomenon and suggest an energy-saving and efficient approach toward alcohols/water separation by using SWNTs. We use various common linear alcohols including C1-C61-alcohols and glycerol for demonstration (the phenol is also used as comparison). Our simulations show that when SWNTs are immersed in aqueous alcohols solutions, although the alcohols concentration is low(1M), all kinds of alcohols can induce dehydration (drying) of nanotubes and accumulate inside wide [(13,13)] and narrow [(6,6) or (7,7)] SWNTs. In particular, most kinds of alcohols inside the narrow SWNTs form nearly uniform1D molecular wires. Detailed energetic analyses reveal that the preferential adsorption of alcohols over water inside nanotubes is attributed to the stronger dispersion interactions of alcohols with SWNTs than water. Interestingly, we find that for the wide SWNT, the selectivity for1-alcohols increases with the number of alcohol’s carbon atoms (Ncarbon) and exhibits an exponential law with respect to Ncarbon for C1-C51-alcohols; for narrow SWNTs, the selectivity for1-alcohols is very high for methanol, ethanol, and propanol, and reaches a maximum when Ncarbon=3.The underlying physical mechanisms and the implications of these observations for alcohols/water separation are discussed. Our findings provide the possibility for efficient dehydration of aqueous alcohols (and other hydrophilic organic molecules) by using SWNT bundles/membranes.ii) Competitive adsorption between ethane and ethylene inside SWNTs: Ethylene/ethane separation is a very important process in chemical industry. Traditionally, this process is achieved by cryodistillation which is extremely energy-intensive. The adsorptive separation is an energy-saving and environmentally benign alternative. Herein we employ MD simulations to study the competitive adsorption of equimolar mixture of gaseous ethane and ethylene inside single-walled carbon nanotubes (SWNTs) of different diameters at room temperature. We find that for narrow SWNTs, i.e.,(6,6) and (7,7) SWNTs, the selectivities towards ethane, fselec, can reach values of3.1and3.7, respectively. Such high selectivities are contrary to the opinion of many researchers that the adsorptive separation of ethylene/ethane mixture by means of dispersion interaction is difficult due to the same carbon number of ethane and ethylene. The key for our observation is that the role of dispersion interaction of ethane’s additional two hydrogen atoms with the SWNT becomes significant under extreme confinement. Interestingly, the (8,8) SWNT prefers ethylene to ethane with fselec=0.6. For wider SWNTs,fselec converges to～1. The mechanisms behind these observations, as well as the kinetics of single-file nanopore filling and kinetics of confined gas molecules are discussed. Our findings suggest that efficient ethane/ethylene separation can be achieved by using bundles/membranes of SWNTs with appropriate diameters.We believe that the current study is helpful to better understand the adsorptive and dynamical properties of small molecules confined within carbon nanotubes, and provides a usefully theoretical insight into the experimental realization of adsorptive separation of different small molecules using carbon nanotubes.