Transport Behavior Of Water In Nanoconfined Space

Transport behavior of water in the nanoconfined space is closely related to many key issues in the fields of physics, chemistry, chemical engineering, environment and life sciences et al. It is essential to understand the transport mechanism of water in the nanoconfined space for the design and preparation of the micro-nano device. Therefore, transport behavior of water becomes research spot in many fields in recent years. On the one hand, due to the nano-space constraint, the physical and chemical properties of water are greatly different from that of the bulk water. The size, configuration, and material of the nanoconfined space have significant effect on the water dynamics. Exploring the nano-scale structure and transport behavior of water helps people to understand the essential characteristics of water transport in the confined space and the kinetics law. On the other hand, the nanotechnology based on the understanding of the structure and transport properties of water in the confined space will play key role in the nano-device application. It is can be used not only to fabricate nano-fluid device, but also to assemble bio-sensors, micro-chemical reactor, and to be a micro-flow meter to control water transport and drug release. If we can study and master the transport mechanism and micro-transfer law of water in the nanoconfined space deeply in the molecular level, which is of great significance for understanding the transport of the confined flow in the some nanochannel systems, and simultaneously, providing a solid theoretical basis for the design and production of nano-devices (for example, nano-fluid heat pipe,devices for seawater purification) in the molecular scale. The classical theory is often suitable to describe the macroscopic phenomena. In the nano-channels, due to the space constraint, the transport properties of water in the confined space is of great difference from that of the bulk water, and that is very difficult to direct determination by experimental methods. In this dissertation, we focus on the water and nanotube and study the transport behavior of water molecules in the confined space deeply. We analyze the transport mechanism of water in the carbon nanotube (CNT) driven by external force, predict the mode of water transport in the nonlinear CNTs, and explore the effect law of different materials of the nanotubes on the water transport behavior. The dissertation content is stated for four parts as follows.(1)It is studied that the transport behavior of water molecules are driven by the methane molecules in the single diameter CNT, and the effects of the number of the methane molecules and the size of the CNT on the transport behavior of the water molecules in the tube are investigated. It is found that the water molecules confined in the CNT move with the diffusion of the methane molecules due to the attractive interaction. The move direction of water molecules is decided by the result of competition between the methane-water attractive force and the CNT-water attractive force. When the methane-water attractive force is stronger than CNT-water attractive force, the water molecules could move with the methane molecules and be drawn out successfully. The greater the number of methane molecules, the greater the attraction between the methane molecules and water molecules, the transport velocity of water molecules increases in the CNT with the number of methane molecules. On the contrary, if the number of methane molecules keep constant, a increase in the diameter of the CNT is not in favor of water transport in the tube, the transport velocity of water molecules in the CNT decreases with the number of methane molecules. The continued diffusion of methane molecules results in the decrease of the attraction between the methane molecules and water molecules, which makes displacement of water molecules in the tube appear the "retraction" phenomenon.(2) Taking the (5/7) topological defects in multi-diameter end-to-end joint CNTs, it is studied the transport behavior of water molecules in the CNTs with different diameters and found that the junction region and the number of the joints have significant effect on the water transport in the nanotube. CNTs with different diameters will provide methane diffusion driving force, the water molecules will follow methane molecules to move in the tube under the water-methane interaction. When the water molecules reach the junction region, the junction region would hinder the transport of water molecules in the tube. If attractive force from the methane molecules overcomes the potential barrier caused by the diameter difference, methane molecules could drag water molecules to another compartment, otherwise, they would be jammed in the junction region. For the bottle-like CNT with one junction region, the smaller the size of the tube, the greater the transport barrier for water molecules across the junction region. The transport velocity of water molecules driven by methane molecules increases with the tube size. For the terrace-like CNTs with more junctions, methane molecules just start to diffuse in the tube within the first 100 ps, which results in the transport behavior of water molecules in the terrace-like CNTs similar to that in the bottle-like CNTs. The transport velocity of water molecules driven by the methane molecules increases with the terrace-like tube size. After 100 ps, the transport velocity of water molecules driven by the methane molecules decreases with the increase of the terrace-like tube size. Because diffusion of methane molecules is relatively slow inside the smaller size of terrace-like CNT, resulting in the longer time for diffusion of methane molecules to reach equilibrium, which provides a more durable driving force for the movement of water molecules in the tube and water molecules move for longer time in the tube. The displacement of water molecules transport in the smaller terrace-like CNT is longer than that in the bigger ones. It is worth noting that translocation displacement of water molecules in terrace-like CNTs is obviously longer than that in bottle-like CNTs. This is because more of the junction regions will provide methane with a continued diffusion force. Methane molecules can pull water molecules from one junction region to another one under the diffusion of the methane molecules.(3) Building models of Y-/T-type SWCNT to study the transport behavior of water molecules in the nonlinear CNT. Because the nonlinear CNT is symmetric, the attraction between each branch and the water molecule confined in the junction region is equal, and therefore the first water molecule enters one of the two branches randomly and fairly. Moreover, the smaller the angle 6 between the branches, the larger potential barrier for the water molecule to enter the branch of R1 or R2, and therefore the more difficult for the water molecule to enter the branches. After the first water molecule entering the branch R1, it takes the water molecules longer time to split into two flows with the increase of the angle between the two branches, which results in the greater difference of the number in the two branches and the stronger asymmetry of the water transport. The water molecules intermittently move between the two branches under the chemical potential difference. Namely, after several water molecules entering the single branch R1, they would not move forward but fluctuate in the equilibrium position. Until the same number of the water molecules enters the single branch R2, the water molecules can move in the branch R1. When the branch R1 is full of water molecules, they will wait for the branch R2 filled with water molecules, and then water molecules overflow from the two outlets of the branches. However, the water molecules will conduct symmetrical transport in a symmetrical nonlinear tube under pressure.(4) Compare the contact angle of the water droplet formed on the graphite sheets, silicon carbide (SiC) graphite-like sheets, and silicon (Si) graphite-like sheets, and the contact angle of water confined in the single-walled carbon nanotube (SWCNT), single-walled silicon carbide nanotube (SWSiCNT), and single-walled silicon nanotube (SWSiNT), respectively, and then different materials hydrophilic/ hydrophobic strength is obtained. It is observed that the water transport in the same size of SWCNT, SWSiCNT, and SWSiNT, respectively. It is found that for the same material, the hydrophobic tube becames stronger hydrophobic with the curvature of the tube decreasing, and the hydrophilic tube becames stronger hydrophilic with the curvature of the tube decreasing. The sequence of the hydrophobicity for three different materials of the sheets and the tubes is uniform. The sequence of the hydrophobicity for three tubes, from the strongest to the weakest, is SWCNT> SWSiNT> SWSiCNT. The sequence of the single water chain spontaneously entering and passing through the three nanotubes is SWCNT, SWSiCNT, SWSiNT, which is not related to the sequence of the hydrophobicity of three tubes. The water cluster moves in three tubes with an initial rate. The sequence of the water cluster leaving the three tubes is SWCNT, SWSiNT, SWSiCNT, which is accordant with the hydrophobicity sequence of the three tubes. Therefore, the stronger hydrophobic tube is more beneficial to the water transport through the tube in a "ballistic" way.It is of important theoretical significance of the results in this study for deeply understanding water transport in nanoconfined space under the external driving force, effectively predicting the transport type of water in the nonlinear CNTs, fully realizing the influence rules of different nanotube materials on the transport behavior of water, and further developing nano-molecular scale devices.