Computational Molecular Simulations Of Surface And Interface Systems

Surfaces and interfaces are involved in energy, chemical engineering, materials and biology. A lot of reaction and synthesis occur over the surfaces and interfaces, which usually exhibit anomalous behaviors.MD simulations are carried out to study the interfacial profiles of alkali metal cations and halogen anions in different electrolyte solutions at different temperatures and concentrations. It is found that the profiles of ions in the solution closely relate with their hydration strength, for ions with stronger hydration ability, they will solvate more water molecules to form a stable hydration shell structure which makes them prefer staying in the bulk; The hydration ability of Li~+ is stronger than that of I~- in the LiI solution, so larger I~- will aggregate in the interfacial region while smaller Li~+ will reside in the interior bulk. As for different anions, the weaker the hydration ability, the higher the profiles of anions at interfaces. The hydration ability of anions is descending in the order of Cl~- > Br~- > I~-, so the increasing enhanced concentration of anions in the order of Cl~- < Br~- < I~- is obtained at the interface. Meanwhile, the aggregation of anions at the interface is affected by co~-existed cations, the weaker the hydration abilities of co~-existed cations, the lower the profiles of anions in the interfacial regions, we also find the higher the temperature and the larger the concentration, the lower the profiles of anions at the interface. Besides, we get that cations will also aggregate in the interfacial region when the co~-existed anion is F~-, which has provided a new insight into the interfacial profiles, the enhanced concentrations of cations at the interface are Na~+ < K~+ < Rb~+ < Cs~+.To achieve a melting point around room temperature is important for applications of ionic liquids. In this dissertation, molecular dynamics simulations are carried out to investigate the melting point of ionic liquid [emim]Br by direct heating, hysteresis, void~-nucleation, sandwich, and microcanonical ensemble approaches, to find a suitable method to predict the melting point. The melting points obtained from the five different methods are 547±8 K, 429±8 K, 370±6 K, 390±4 K, 365±3 K respectively, which are approximately 55.4%, 21.9%, 5.1%, 10.8%, 3.7% higher than the experimental value of 352 K. Also, the advantages and disadvantages of each method are discussed.We use molecular dynamics simulations to obtain the melting temperature and mechanism of pentane and hexane monolayers adsorbed on the surface of graphite. Our results agree well with experiment data, The simulated melting temperature is 150~155 K for pentane monolayer and 180~185 K for hexane monolayer, while the experimental values are 150 K, 175 K respectively; the discrepancies are only ~1.67% and ~4.29% respectively. Although, a detailed analysis reveals that the two monolayers can have different melting behaviors and mechanisms. The reason lies in the different lengths of the chain alkanes, for pentane and shorter chain alkanes monolayers, tilting is the main reason for melting; while for hexane and longer chain alkanes monolayers, second layer (layer promotion) accounts for that. A correct interaction plays the decisive role in obtaining the right melting temperature and corresponding mechanism. This work gives new insight into the understanding of melting mechanism of 2D system.To achieve the controlled synthesis of colloidal nanomaterials with selected shapes and sizes is an important goal for a variety of applications that can exploit their unique properties. In this dissertation, we focus on the binding interaction of 2-Pyrrolidone (2P) adsorbed on the Ag(100) and Ag(111) surfaces by density functional theory, to elucidate the role of PVP (Polyvinylpyrrolidone) in facilitating nanostructure formation. Three different methods (PBE, DFT-D2, and DFT-ZK) are used to predict the interaction of 2P with Ag surfaces. In the PBE method, van der Waals interactions are ignored; while in the DFT-D2 method, the interactions are over-estimated, which makes these two methods fail in predicting the interaction of 2P with Ag surface. In the DFT-ZK method, the van der Waals interactions are adjusted with more accurate parameters, so this method describes a clear binding interactions of 2P with different Ag facets. We find that 2P prefers to lie down on the surface rather than to stand up and 2P binds more strongly to the Ag(100) surface than to the Ag(111) surface, which would explain the formation of nanostructures with different shapes.