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采用简化模型探索液态水的异常性质
Alternative TitleExploring Anomalies of liquid water with simplified models
涂育松
Subtype博士
Thesis Advisor方海平
2009-05-12
Degree Grantor中国科学院上海应用物理研究所
Place of Conferral上海应用物理研究所
Keyword分子动力学 Monte Carlo 方法 液态水 液态水异常行为 强弱氢键 局域结构 液液相变
Abstract水是简单的,却又极其复杂;水是重要的(一切生命活动不可缺少的),但因其太普遍,又最容易被忽视。它的简单是因为它极其简单的分子组成,只包含了一个氧原子和两个氢原子。迄今为止,世上还没有人真正完全理解它,这个覆盖了三分之二个地球,在人体内占80%的物质依然保持着神秘的面纱,越是仔细观察,积累的问题却越多,新技术越是深入地探索液态水的分子构形,却显示出越多的谜团 [Nature 452:291-292,~2008]。 目前,对水的研究仍停留在它的基本层面上。比如一些问题,液态水是否存在两种局域结构?水的氢键相互作用以何种模式使得单成分的液体可以形成两种不同的局域结构?更进一步,水的局域结构和液态水的异常行为的关系又如何呢?当前,科学家们仍在就这些问题进行剧烈的争辩。特别是,2004年,基于 x 射线的放射光谱和吸收光谱的研究,Nilsson 等人提出80%的水分子只有一个O-H基团形成强氢键,另一个仅形成弱氢键或不形成氢键,而剩余的20%的水分子则按照四面体的排布方式形成四个强氢键,这个新氢键模式对传统水的物理图像中的氢键模式发起了挑战,之后,他们的观点是遭遇了大量的质疑而形成了关于水的氢键结构的剧烈争辩。 在本论文中,为了理解水的异常行为的物理本质,我们发展了一个水的相当简化的物理模型。这个物理模型考虑到关于水的氢键的两个最基本的假设,一个是水分子倾向于以正四面体的方式用氢键按排四个近邻水分子,另一个是,水分子之间存在两种带有不同强度的氢键作用。但是,我们的模型与Nilsson提出的模型是不同的,Nilsson提出的模型中每个水分子通常带有两个强和两个弱的氢键,而在我们的模型中,强弱氢键形成的概率是随机的。 通过Monte Carlo模拟计算,我们发现,单一氢键模型可以再现常温下水的主要特征,包括液态水的密度异常和等温可压缩性的极值异常。在引入强弱氢键之后,模型液体很好描述了液态水的可能存在的奇异性,比如,热容的极值随着温度降低而迅速增加的行为与液态水的实验趋势是一致的,进一步通过相图分析,两个氢键的模型的结果与液液相变理论对液态水的预测结果也是一致的,这个模型刻画了在整个相关液态的水的相图中的主要特征。在带有两种氢键的模型中,我们分析氢键数目与温度的变化关系,发现模型液体存在两种水局域结构,一种是强键占优势的低密度局域结构,另一种是弱键占主导的高密度局域结构,在这两种结构之间的激烈竞争引发液态水的热力学异常现象。因此,我们的结果给出了个清晰的,重要的结论:液态水的在低温区可能存在奇异性的物理根源来自于强弱氢键之间的交换。 我们的模型应该是再现水的异常行为的最简化的Off-Lattice三维水模型,我们也是第一个用Off-Lattice模型得出氢键的取向性带来的密度异常的封闭区域,发现氢键的取向性与强弱二元性是再现水的异常行为两个必不可少的物理因素。我们这些结论对于理解液态水中的氢键结构是十分有帮助的。 此外,由水分子的氢键的取向性是水分子中最重要的特征之一,水分子在纳米受限的条件下会表现出极为特殊的性质,一维的水分子单链就是其中之一。所以,本论文就这一点,也做了进一步探究,发现由单电子诱导的在纳米通道中的信号可以通过受限在一个两分叉的支管中水分子而被传递和转向成两个或多个稳定的信号,观察的现象可能对将来的在分子尺度上的电子设备和生物系统中有着重要的应用。考虑到水是生命之源,水在生物分子中的结构和功能中扮着非常重要的作用,我们这些发现有助于理解生物分子信号传递本质。
Other AbstractWater is a rather simple molecule, with an oxygen atom and two hydrogen atoms. It is the most important molecule for life to survive. So far, no one really understands water, although it is quite embarrassing to admit it, but the stuff that covers two-thirds of our planet is still a mystery. The worse thing is that the more we look, the more the problems accumulate: new techniques probing deeper into the molecular architecture of liquid water are throwing up more puzzles [Nature 452:291-292, 2008]. At present, there are still hot debates on some fundamental issues of liquid water. For example, what kinds of hydrogen bond structure exist? Are there two types of local structures in such a one-component liquid as water? Is there a liquid-liquid transition critical point in supercooled liquid water? Especially, in 2004, based on X-ray absorption spectroscopy, Nilsson et al proposed that 80% of the molecules of liquid water have just one strong Hydrogen-bonded O-H group, challenging the conventional water picture. Later, this proposal has been questioned and there have been hot debates on the hydrogen bond structure of water. In this thesis, we developed a simplified physical model of liquid water, in order to explore the nature of water anomalies. In the model, we have two basic hypotheses. i) In liquid water, each water molecule is thought to be hydrogen bonded to approximately four other water molecules in a tetrahedral arrangement. ii) There are two types of hydrogen bonds with different strengths. However, different from Nilsson's model where each water molecule usually has two strong hydrogen bonds and two weak hydrogen bonds, here the probabilities for each hydrogen bond to be strong and weak are at random. Via Monte Carlo simulations, we found, the model with one type of Hydrogen bond can be used to reproduce the main character of liquid water at normal temperature, including the anomalies of the density and the isothermal compressibility. By introducing the strong and weak hydrogen bonds, the model liquid can describe a possible singularity of liquid water at supercooled temperature. For example, on cooling, the rapid increase of heat capacity in model liquid is consistent with the experimental one in liquid water. In its relevant phase diagram, the results are consistent with the prediction of liquid-liquid transition theory for liquid water. We found two kinds of local liquid structure, by analyzing the number of hydrogen bonds in response to the temperature change. One is a low-density-like structure where strong hydrogen bonds predominate, and the other is a high-density-like structure where weak hydrogen bonds predominate. The competition between these two types of local structures brings about the thermal anomalous behaviors of liquid water. Therefore, the possible singularity of liquid water at low temperature may stem from the exchange between strong and weak hydrogen bonds. Our model might be the most simplified off-lattice water-like model in three dimensions to reproduce water anomalies. We first use a off-lattice model to achieve the closed region of density anomaly caused by the hydrogen bonding orientation, and find that the orientation and the strong-weak duality of Hydrogen-bonding is indispensable to reproduce the anomalous behaviors of liquid water. Our study is helpful in understanding the hydrogen bond structure in liquid water.
Pages160
Language中文
Document Type学位论文
Identifierhttp://ir.sinap.ac.cn/handle/331007/7307
Collection中科院上海应用物理研究所2004-2010年
Recommended Citation
GB/T 7714
涂育松. 采用简化模型探索液态水的异常性质[D]. 上海应用物理研究所. 中国科学院上海应用物理研究所,2009.
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