Molecular Dynamics Studies On The Structures And Physical Properties Of Nanoconfined Fluids

The structure of the nanoconfined fluid is closely related to many fields, including physics, chemistry, energy, environment, life and earth sciences, etc. Nanoconfined fluid has attracted considerable attention over the years. Due to the restriction of the spatial dimensions, the physical and chemical properties of confined fluid can be dramatically different from their bulk counterpart. The study of the nanoconfined fluid in the molecular level not only helps us to understand the essential characteristics and kinetics law, but also provide the theoretical foundation for the design and production of nanodevices. With the enhancement of computer and the development of computational methods, theoretical calculations have been widely used in the nanoconfined fluid. Because it is easy to obtain the microscopic detail information that cannot be obtained in experiment and the properties in extreme condition, the molecular dynamics (MD) simulation has been used to study the confined fluid. In this thesis, we perform MD simulations to study the structures and physical properties of confined fluid. This thesis contents are stated for four parts as follows.(1) We perform MD studies on the structure and dynamics of methanol-water fluid mixtures surrounding an open-ended carbon nanotube (CNT). The main focus of this study is the trend of selective adsorption with different methanol concentrations in the bulk. Remarkably, for all CNTs considered in this study, the mass fraction (MF) of methanol inside CNTs is always much higher than that in the bulk. In particular, for the (6,6) CNT the selectivity of methanol is extremely high, and the MFs of CH3OH inside the (6,6) CNT are always nearly100%. By analyzing microscopic structures of the fluid mixture around CNTs, we find that the selective adsorption reflects a cooperative effect of the van der Waals (vdW) interaction between CNT and the methyl groups of CH3OH molecules, as well as the hydrogen bonding interaction among water and methanol molecules. This cooperative effect can be extended to other fluid systems, such as ethanol/water and ethanol/methanol mixtures.(Chapter3)(2) Molecular dynamics simulations of monolayer water confined in a nanoslit with two smooth hydrophobic walls (D=0.51-0.65nm) with or without lateral electric field (EL) at T=200K are carried out. Our simulations demonstrate that there are four classes of monolayer crystalline phases including two previously unreported confined monolayer ice phases. The two new monolayer ice phases are the middle-density hexagonal ice (MDHI) composed of hexagonal rings, a kind of ferroelectric with spontaneous polarization, and flat high-density rhombic ice (fHDRI), respectively. The other two monolayer ices previously reported are the low-density4-82ice (LD48I), and puckered high-density rhombic ice (pHDRI), respectively. Based on the results of MD simulations, we also give the D-PL phase diagrams for monolayer ice. From the D-PL phase diagrams, we find that in the absence of electric field, a creek (liquid phase) separates FHDRI and PHDRI from the LD48I and MDHI. With increase of EL, the creek narrow down, i.e. the liquid gradually freezes into MDHI, FHDRI or PHDRI phases, and LD48I is slowly induced into MDHI. When EL reaches0.4V/nm, the creek transforms a small lake. Up to1V/nm, only MDHI, FHDRI and PHDRI phases are observed, while the LD48I and liquid completely disappear. At EL=10V/nm, all the other phases completely transform into FHDRI or PHDRI phases, i.e. only FHDRI and PHDRI phases are observed.(Chapter4)(3) We report findings from MD simulations of the spontaneous nucleation and growth of bilayer methane hydrate. We find that bilayer methane hydrate can spontaneous nucleate and grow rapidly under high lateral pressure (500≤PL≤800MPa) at room temperature (300K). The resulting structure after nucleation and growth is a combination of bilayer octagon (82) and heptagon (72). The cages are face-sharing partial cages and linked by bilayer hexagonal, pentagonal and tetragonal rings, and the configurations of bilayer methane hydrate are amorphous phases.(Chapter5)(4) The permselectivity of H2/O2, H2/N2, H2/CO, and H2/CH4mixtures passing a graphdiyne membrane is studied by molecular dynamics simulations. At pressure from0.047GPa to4GPa, H2can pass the graphdiyne membrane quickly, while all the O2, N2, CO, and CH4molecules are blocked. At pressure of0.047GPa, the hydrogen flow is7mol·m-2·s-1. With increase of the pressure, the flow of H2molecules goes up, and reaches maximum of6x105mol·m-2·s-1at1.5GPa. Compared to other known membranes, graphdiyne can be used for means of hydrogen purification with the best balance of high selectivity and high permeance.(Chapter6)...