密度與空間侷限對水在石墨奈米夾層內結構與動力學之影響

在此論文中,我們使用TIP4P/2005水模型去模擬水分子在石墨夾層,溫度300K的結構以及動力學,研究分為夾層寬度因素(confinement effect)與密度因素(density effect),夾層寬度分為20Å,15Å,10Å,7Å,寬度因素就是利用不同夾層寬度大小分析石墨板對水的影響,而在密度因素分析中,夾層寬度大小定在7Å,並且改變其密度,分析密度對水的影響。結果顯示,在夾層因素中,會使得水分子形成不同層數的水層,20Å,15Å,和10Å分別為六層，四層，三層, 由外到内命名為外層, 次層, 和内層，因為層數的關係, 内層只有在20Å系統。在密度因素中,密度較高時,會同一層形成平面方形結構。水分子可以由solid-angle distribution density分成solid-like與liquid-like三維結構,為了瞭解水的流動能力我們計算水分子的擴散係數(diffusion coefficient), 而在計算氫鍵時, 發現兩種平面結構與同層氫鍵與跨層氫鍵數量有關。當平面方形結構行成時,氫鍵數量會減少,當氫鍵減少時,水分子轉動的束縛也就減少。在頻譜分析中,隨著寬度以及密度變化,會出現藍移現象(blue shift)。

關鍵字

二維系統；水模型；石墨夾層

並列摘要

In this thesis, we use MD simulations for studying TIP4P/2005 water confined between parallel graphite plates at 300K. The study includes the confinement and density effects, in which the confined systems in the former case have a silt width varying from 20Å, 15Å, to 10Å, and the systems in the latter case have different number densities of molecule in a slit with a width of 7Å. Our results show that in the confinement effect, water molecules in the slit of 20, 15, and 10Å form six, four and three layers, respectively. The layers insider a slit are referred as the outer, the next, and the inner, from a graphite plate toward its center, where the inner layer only occurs in the slit of 20Å. In the density effect, as the confined system has a higher number density, water molecules arrange in square structure. According to the solid-angle distribution density, we classify confined water into two phases, liquid-like and solid-like. Further, in term of diffusion coefficient, we can examine the mobility of soild-like water and that of liquid-like water, and they are highly related to the confinement effect. As water forms square structure, molecules have less hydrogen bonds, and due to the reduction in hydrogen bond, there is less constraint on molecular rotation. In spectrum analysis, the smaller slit width and the higher molecular density, the power spectrum of translational motions in confined water has a blue shift.