水及鹽類水溶液之熱力學性質、鍵結行為及系統結構之探討:分子動力學模擬的研究

Abstract

本論文之目的，是找出一個適合模擬高分子溶液(Polymer solution)的水分子模型(Water model)並且以此模型建立一水溶液(包含離子)模擬系統。相較於其他水分子模型，Fexibile three center (F3C) model已經用於大尺度的巨分子(Macromolecules)溶液的模擬，並且相當符合實驗結果。由於其勢能的短距離截斷(Short range truncation)的補償計算相當好，因此它適合於需要高計算速度的模擬系統。在統計性質上，如水溶液的結構、動態與熱力學性質在不同溫度、壓力以及系統尺度，都與實驗結果相當符合。本研究在相同模擬系統下，統計性質也與文獻相當符合。 研究分為三部分：帶電的水溶液(Charged aqueous solutions)的結構與動態性質；改變溫度、加入不同的離子種類與數量對奈米液滴的影響；給予帶電水溶液的液滴不同的合併速度，對於液滴合併過程的影響。目前對於帶電荷的奈米液滴的結構性質、碰撞以及合併過程的瞭解相當少。研究結果顯示，水合作用(Hydration)對水溶液性質影響甚劇，主要是因為庫倫靜電力影響水分子的空間排列與方位，直接影響氫鍵的形成。但是，由於Cl...H鍵結相當的強，因此能加強水分子在溶液中的結構性質。而模擬帶電水溶液的奈米液滴(液滴半徑約1nm)在真空系統中，是模擬液滴擁有不同種類與數量的離子，如Sodium ions、Chloride ions或兩種離子等量存在。純水溶液或是僅含有Sodium ions的液滴皆無法在330-360K的真空環境下，維持奈米液滴的形狀。給予兩液滴額外的速度在特定方向，可觀察兩液滴合併的過程。結果顯示，給予不同的速度，5 m/s、50 m/s、150 m/s與200 m/s，兩液滴會從僅僅靠近乃至於完全合併。在150 m/s時，兩液滴間會產生數個水分子所形成的架橋結構(Bridge structure)。而液滴的表面積與體積是相當重要的參考依據，本研究藉由Molecular modeling software：Tinker，以Solvent accessible surface的演算法計算表面積與體積，得到了單一個液滴、兩液滴合併以及與合併液滴是相同狀態的液滴，後二者的液滴的表面積對體積比率，確定此合併液滴是一穩定結構。

This study investigates on obtaining a water model suited to long simulation time of macromolecules in solution and constructing a simulation system of aqueous solutions. Comparison between other models revealed that flexible three-center model has been already used in many large-scale simulation and it’s provided with experimental data. Because the model works well with short-range truncation suited to high-speed computation. It’s tested by comparing the structural, dynamic and thermodynamic properties of water in aqueous solution, at several temperature and density, with the other models and experimental data. Our program also was tested by calculation such properties and fitted these literature very well. Therefore, the aims of research divide into three parts: First is tested for many water properties comparing with literature；Second investigates on the structural and dynamic properties of brine solutions；the other simulates charged aqueous nanodroplets for different condition, which were different temperatures, the number of ions, the type of ion and the size of droplets, in vacuum and these nanodroplets were given some additional velocity ranging from 1 m/s to 200 m/s to observe two nanodroplets bumped into or merged each other or merged . Studies show that ionic solvation shell effects strongly on the water structure in aqueous solution, like Cl- anion makes water more slow meaning ionic solvation shells are rigid. Computation of Bond time correlation functions shows that Cl--water pair can hold longer than water-water pair. The rigidity can play an important role in charged aqueous nanodroplets. At several conditions, the nanodroplet including Cl- ions were stable. Giving two nanodroplets a velocity in a direction to overcome the surface energy of the droplet made a formation of bridge structure and giving more kinetic energy performed the merged process. To calculate the surface area and volume of a nanodroplet, there are the merged nanodroplet and a nanodroplet had the same number water molecules and ions of the merged nanodroplet, we use the molecular modeling software TINKER implemented the algorithms of solvent accessible surface. The result of computation can prove the merged nanodroplet is the stable structure.