石油一直以來都是人類相當依賴的資源,但石油在地球上的蘊含量是有限的,
而在石油的產物中,又以燃料佔相當多的比例,因此近幾年各國紛紛努力研究替代之燃料以減少對石油的依賴,其中又以生質燃料最受到矚目.生質燃料是將含糖分的農作物釀製成酒精,再加入汽油中以減少汽油的用量.然而要將生質燃料加入石油中,生質燃料中的含水量,其重量百分率必須少於1.3%,否則將無法溶於汽油中.其中利用分子篩薄膜分離水與酒精也是一個可行的方法,因此在本文中,我們選擇金管為組成分子篩的基本結構,為了能夠得知金管薄膜分子篩對於水與酒精分子之效果,因此對於了解水與酒精分子在金管內部穩定狀況(steady state)之行為是十分重要的.除此之外,由於不同的重量分率(水/酒精=25/75,50/50,75/25)以及不同金管半徑(13, 22, 31.1 Å),對於兩分子之行為現象都有很相當明顯的影響. 因此我們利用分子動力學模擬水與酒精分子在金管內部的行為,並藉由徑向密度分度的結果,我們可以將金管內部分成三區,由管壁至管中心分別為接觸區,過渡區以及塊材區.在接觸區中,水分子都會傾向於吸附在管壁上,而形成殼層狀的高密度結構,也因此原因,在水分子越靠近管壁之區域水的密度越大,而酒精的密度則相反,而在25/75的比例下,塊材區中酒精的重量分率最高可達到99%,已經達成要可以加入汽油的條件.除此之外,我們還有探討水與酒精分子之氫鍵分佈狀況以及兩者的擴散係數,由於每一區的密度分佈皆不同,也直接的影響到其氫鍵分佈狀況.在擴散係數的探討中,由於水分子被在接觸區被金管壁所吸引,因此水分子在接觸區的擴散係數最小,越接近管中心其數值也隨之增加,而酒精分子也有相同的趨勢.

In this dissertation, the molecular behaviors of water/ethanol mixtures of different weight fractions inside Au nanotubes of different radii at steady state were investigated by molecular dynamics simulation. Five weight fractions of water/ethanol (0/100, 100/0, 25/75, 50/50, and 75/25) and three radii of Au nanotubes (13, 22, and 31.1 Å) were considered in order to understand the effects of Au nanotube size and water/ethanol fraction on the structural and dynamical behaviors of the water and ethanol molecules.
The density profiles show two shell-like formations inside the Au nanotubes because water molecules prefer to adsorb on the wall of Au nanotube. According to the density distribution, the space inside Au nanotubes can be divided into three regions, those of contact, transition, and bulk regions, in order from the interior wall surface to the nanotube center. The bulk region has a lower local weight fraction compared to the system water/ethanol weight fraction. In addition, the local water/ethanol weight fraction in the contact region is higher than that of the system. When the system water/ethanol weight fraction becomes higher, the local water/ethanol weight fraction also becomes higher.
In 25/75, 50/50, and 75/25 weight fraction mixtures, the number of H-bonds per water and per ethanol are different from those of pure 100% water and 100% ethanol in the Au nanotube due to the nanoconfinement effect. Moreover, the distribution of number of H-bonds in regions where there is only one material will be similar to the distribution in the corresponding region of the pure material, whether 100% water or ethanol. In all regions, the probability to form different H-bonds is affected significantly by the local weight fraction of water/ethanol.
Three radii of Au nanotubes (13, 22, and 31.1 Å) were considered in order to understand the effects of Au nanotube size and water/ethanol fraction on the structural and dynamical behaviors of the water and ethanol molecules. In the transition and bulk regions, diffusion coefficients for water and ethanol molecules become higher due to the weak interaction with Au atoms. The values of diffusion coefficients for water molecules in the contact regions are much lower than for those in other regions and are similar for different water/ethanol weight fractions due to the strong interaction with Au atoms. When the radius of the Au nanotube becomes larger, the values of local weight fraction inside the larger radius Au nanotube become higher than those inside small radius Au nanotubes because the ratio of water number to the nanotube inner surface area becomes higher. In addition, water inside a larger radius Au nanotube has a shorter water-water hydrogen bond lifetime (H-bond) in the contact region because the smaller curvature causes weaker interaction with Au atoms.

Contents i
List of Figures ii
List of Tables v
List of Symbols vi
Abstract x
摘要 xii
Chapter 1 Introduction 1
1-1 Introduction of molecular sieve membrane 3
1-1-1 Inorganic membrane 3
1-1-2 Polymeric membrane 5
1-1-3 Nanotube membrane 6
1-1-3-1 Au nanotubes and their applications 6
1-1-3-2 Review of molecular modeling for molecules inside nanotubes at a steady state 8
1-2 Nanoconfinement effect on molecules 10
1-2-1 Review of molecular modeling for nanoconfinement effect 11
1-3 Motivation 13
1-4 Outline of this dissertation 15
Chapter 2 Molecular Dynamics simulation 16
2-1 Introduction 16
2-2 Empirical force field 18
2-2-1 Introduction 18
2-2-2 ENCAD 20
2-2-3 Tight-binding force field 22
2-2-4 Spohr potential 23
2-2-5 Dreiding force field 25
2-3 Equations of motion 26
2-4 Ensembles 27
2-5 Constant temperature dynamics 28
Chapter 3 Numerical Methodology 30
3-1 Periodic boundary condition (PBC) 30
3-2 Neighbor list for non-bonded interaction 31
3-3 Flow chart of molecular dynamics simulation 33
Chapter 4 Result and Discussion 34
4-1 Simulation details 34
4-2 Effect of weight fraction for water/ethanol mixtures inside Au nanotubes 36
4-2-1 Density distribution 36
4-2-2 Hydrogen bond (H-bond) 42
4-3 Effect of Au nanotube size on water/ethanol mixtures 52
4-3-1 Density distribution 52
4-3-2 Hydrogen bond and lifetime 58
4-3-3 Diffusion coefficient 64
4-3-4. Predicting water/ethanol behavior inside a large radius of Au nanotube 66
Chapter 5 Conclusions and Future Works 67
5-1 Conclusions 67
5-1-1 Effect of weight fraction for water/ethanol mixtures 67
5-1-2 Effect of Au nanotube size on water/ethanol mixtures 68
5-2 Future Works 69
References 70

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