Adsorption And Phase Behavior Of Fluids In Confinement Of Nano-cylindrical Materials

Molecules confined within narrow pores, with the size of a lew molecular diameters, can exhibit different physical behaviors from the bulk fluid. To understand these behaviors resulting from the finite-size effect, surface force and varying dimensionality is of significant scientific interest. In addition, understanding these phenomena is necessary for many industrial operations. Computer simulation and the density functional theory (DFT) are used in this work to study the adsorption of simple molecules and their phase behaviors in cylindrical pores of MCM-41 and in single walled carbon nanotubes (SWNTs) arrays.An analytical potential model describing the interaction between the wall of a cylindrical pore and an LJ molecule confined in the pore has been proposed. The model gives good fit to the results from the pseudoatom model and the cylindrical surface model in the limit where the wall thickness reaches zero.GCMC simulations have been carried out to study the adsorption of nitrogen in MCM-41 at 77 K. By comparing the simulated adsorption isotherm and the experimental data reported by He et al, a set of potential parameters for the surface of MCM-41 is obtained. With the set of potential parameters, the adsorption of methane in MCM-41 of different pore sizes at different temperatures has been simulated. And simulated results show that the amounts of methane adsorption in the MCM-41 pore of 3.525 nm can reach 7.86mmol/g and 9.75 mmol/g at 300 K and at 5.4MPa and7.1MPa, respectively.A comparison of the adsorption isotherms from DFT calculations and from GCMC simulations indicates that the two methods agree well in general. The effects of temperature, pore diameter, wall thickness and energy parameter of fluid-solid interactions on adsorption isotherms have been studied by the DFT method in this work.Layering transitions within cylindrical pores at several temperatures and different fluid-solid interactions have been studied by both the GCMC and DFT methods. With GCMC method, the critical temperatures of 0->l layering transitions with twofluid-solid potentials, corresponding to ps and 2 ps (ps =2.0 g/cm3) in the pore of 3.525 nm are all at the reduced temperature T*=0.44~0.5. Layering transitions are found within the hysteresis loops of capillary condensations at low fluid-wall potentials. While the layering transitions of the pore with the stronger fluid-wall interaction get out the hysteresis loops of capillary condensations. Moreover, a small hysteresis loop of 0->l layering transition is found in this case. However, the critical temperatures of 0~> layering transitions determined by the DFT method is higher than 0.5. The diffusion properties of the fluid and the effects of temperature, the fluid-wall potential and number density of fluid particles on the diffusivities nearby layering transitions are investigated by MD method. MD results show that fluid molecules are not evenly distributed on the surface of the wall but aggregate in some areas.GCMC simulations have been carried out to investigate the adsorption and the phase behavior of methane in SWNTs. Simulation results demonstrate that the total amount of adsorptive storage of methane in SWNT square array can reach 22 mmol/g for the tube of 4.077 nm at 6 MPa and 300 K, more than that in a slit pore of the same size. Simulation results also suggest that in the interstice formed by outer surfaces of square arranged carbon tubes of 4.077 nm, the solid-like structure can be recognized at 125 K or below this temperature. DFT calculations also have been performed to study the phase transitions of methane molecules within the tubes of SWNTs.Molecular simulations have been carried out to study ethane adsorption in SWNTs, in which ethane molecules are represented by two-site LJ particles. The structure of SWNTs is found to hardly make any difference regarding the amount of ethane adsorption in tubes of SWNTs. Ethane molecules near the wall both in the tubes of SWNTs and interstices intend to lay along the surface of SWNTs. When the pore diameter of SWNTs is 2.719 nm, th...