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A Molecular Dynamics Study On Micro-characteristics Of Lithium Bromide Aqueous Solution

Posted on:2011-05-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:B B ZhuFull Text:PDF
GTID:1102330332963254Subject:Marine Engineering
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Absorption chillers using the working pair water-lithium bromide (LiBr) have obtained broad market prospects.In absorption refrigeration system, the water vapor absorption into LiBr aqueous solution in absorption vessel is the key stage of systematic function. Improving the absorption speed is thought to be important in advancing system efficiency and reducing the exchange area. Adding a little amount of surfactant such as 2-ethyl-l-hexanol into LiBr aqueous solution can enhance the heat and mass transfer greatly. Due to its great application value, this effect has been the subject of numerous investigations in recent years, while the working mechanism is not fully understood. The present dissertation mainly focuses on molecular dynamics simulation methods as well as experimental research to study the absorption enhancement mechanism of surfactant in microscopic view, willing to enrich and ameliorate existing theories.Molecular dynamics simulation methods are carried out to investigate the temperature effect on the microstructure of liquid-vapor interface of water. Simulation results show that the density of bulk liquid decreases with the increase of temperature; however, interface thickness increases with the increase of temperature. The local structure of water molecules is not affected by the presence of interface. Water molecules are observed to show preferred orientational order at the liquid-vapor interface:water permanent dipoles prefer to lie parallel to the interface; however, with the increase of temperature the interface has little influence on the orientationl order of water molecules. In bulk liquid region, the number of hydrogen bonds per water molecule is roughly constant, and decreases when temperature increasing. As water molecules approach the interface, the number of hydrogen bonds per water molecule decreases monotonically.Molecular dynamics simulation methods are used to study the microstructure of the liquid-vapor interface of lithium bromide aqueous solution with various concentration at different temperature. The simulation results demonstrate that liquid density increases with the increasing concentration of the electrolyte solution, meanwhile, interface thickness decreases gradually; however, the temperature has contradict effect on liquid density and interface thickness. The presence of interface cannot affect the orientation order and local structure of water surrounding by ions. The orientation order of water molecules neighbored by ions becomes not clear with the increase of temperature. The concentration of electrolyte solution has little effect on the orientation profile of water surrounding by ions. The variation of temperature or concentration has little influence on the local structure of water molecules around ions.Molecular dynamics simulation methods are adopted to study the effect of the amounts of alcohols on the microstructure of the liquid-vapor interface of the mixture of water and alcohols. The simulation results indicate that alcohol molecules tend to adsorb at the interface and show their dominant orientation with the hydrophobic hydrocarbyl pointing into the vapor phase and hydrophilic hydroxyl pointing into the liquid phase, while the hydroxyl groups forming a hydrogen bonding network with water which makes the alcohol molecules seek more hydrophilic interactions with water molecules. Alcohols with short hydrocarbon chains have intensive solubility. When the concentration of n-octanol,2-ethyl-l-hexanol or n-hexanol is slightly lower, the alcohols can form monolayer at the interface, and graduate into bilayers with the increase of the amounts of alcohols. The interface thickness increases with the increase of the amounts of alcohols.Molecular dynamics simulation is introduced to study the impact of the types, the amounts of alcohols and the concentration of LiBr aqueous solution on microscopic structure of the liquid-vapor interface of the mixture of alcohols and electrolyte solution. The computed results reveal that n-alcohol molecules tend to adsorb at the interface with preferred orientation, meanwhile, the tendency of this kind of preferred orientation becomes distinct with the increase of the amounts of n-alcohol molecules. The interface thickness increases with the increase of the amounts of n-alcohol molecules or the length of hydrocarbon chains. The hydrocarbon chains of n-alcohol molecules are inclined to be close to stay upright near the interface while the amounts of n-alcohol molecules is more, meanwhile, this orientational ordering becomes significant with the increase of the amounts of n-alcohols. The direct interactions between hydroxyl hydrogen of n-alcohols and anion exist, and moreover, there are much stronger electrostatic interactions between oxygen of n-alcohols and cation. The dissolvability of n-butanol or 2-ethyl-l-hexanol related to LiBr aqueous solution decreases with the increase of the concentration of LiBr aqueous solution, which is consistent with the salting-out effect theory.The dynamic process of water vapor absorption into electrolyte solution with or without alcohol surfactants is explored by molecular dynamics simulation under non equilibrium conditions. Taking the model proposed by Daiguji et al., this dissertation puts alcohols on the two interfaces of LiBr aqueous solution in absorption side. The simulation results suggest that in comparison to lithium bromide aqueous solution without surfactants, the electrolyte solution with surfactants can absorb more water molecules distinctly for 100 ps, which conforms to the experimental tendency.The experimental and molecular dynamics methods are used to study the surface tension. Tensiometer is used to measure the surface tension of water and LiBr aqueous solution at room temperature. The experimental value of surface tension is close to the value from literatures. N-hexanol, n-octanol,2-ethyl-l-hexanol and 2-octanol are added into LiBr aqueous solution by two ways:alone or two of them. The experimental results signify that the surface tension of LiBr aqueous solution with compound surfactants is near to the one which makes surface tension smaller. The computed value of surface tension of water and LiBr aqueous solution is smaller than the experimental value. But the variation tendency with the temperature or concentration of LiBr aqueous solution accords with the experimental results. The simulation results show that n-butanol, n-hexanol or 2-ethyl-l-hexanol can reduce the surface tension of LiBr aqueous solution, meanwhile, the surface tension decreases with the increase of the amounts of n-octanol, which meets the experimental results.
Keywords/Search Tags:Surfactant, Lithium bromide aqueous solution, Liquid-vapor interface, Molecular dynamics simulation, Surface tension
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