Exploration of Low-Dimensional Gas Clathrate Hydrates

水分子由两个氢原子和一个中心氧原子组成，呈回飞镖形状。当许多水分子凝聚成液态水或固体冰时，这种简单的原子排列导致了独特的分子级结构和复杂的性质，这些结构和性质只取决于水分子中原子的排列，而与水分子所处的容器无关。至少对于大型容器来说是这样。当容器的尺寸缩小到纳米（大约比一张纸薄10万倍）时，液体的结构也受到单个分子与容器侧面的相互作用的影响。纳米尺度的约束可以产生与散装水截然不同的结构和性质。在化学学部化学结构动力学与机理（CSDM-A）项目资助下，内布拉斯加州林肯大学（UNL）的Joseph S. Francisco、Zeng Xiao Cheng、Chin Li bbb和Jaeil Bai教授利用计算机模拟和实验室实验相结合的方法，研究了水和水/甲烷混合物（称为笼形水合物）在纳米缝和分离的碳纳米管中的物理性质。从这项工作中获得的基本见解有助于我们理解广泛的社会问题，包括土壤冻胀（在冻结条件下土壤向上膨胀）和从页岩中提取天然气。此外，该项目还在计算机辅助化学研究和先进实验装置设计方面培养学生和博士后。测量是在能源部国家实验室进行的。该项目侧重于对高度受限水和冰的物理性质和相变、低维多晶和多晶转变的分子洞察，以及对低维冰和笼形水合物新相的预测。更具体地说，UNL团队正在追求三个目标：在宽度为8 - 12埃的纳米狭缝（或双层到三层冰形式）中寻找新的多晶和多晶水和天然气水合物相；研究低维二氧化碳（CO2）和一氧化碳（CO）水合物的形成机理；以储氢为重点，研究纳米尺度约束下分子氢水合物的形成机理。分子动力学（MD）模拟，包括最先进的复制交换MD模拟，用于计算受限流体的自由能表面。

Water molecules consist of two hydrogen atoms bound to a central oxygen in a boomerang shape. When many water molecules condense into liquid water, or solid ice, this simple placement of atoms results in a unique molecular-level structure and complex properties that depend only on the arrangement of atoms in the water molecule, and not the container in which its placed. At least this is true for large containers. As the size of the container is reduced to nanometers (about 100,000 times thinner than sheet of paper), the structure of the liquid is also affected by the interaction of the individual molecules with the sides of the container. Nanoscale confinement can result in structures and properties that differ dramatically from bulk water. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) Program of the Chemistry Division, Professors Joseph S. Francisco, Xiao Cheng Zeng, Chin Li Cheung, and Jaeil Bai of the University of Nebraska-Lincoln (UNL) are using computer simulations combined with laboratory experiments to explore the physical properties of water and water/methane mixtures (known as clathrate hydrates) when they are confined to nano-slits and isolated carbon nanotubes. The fundamental insights gained from this work contribute to our understanding of a broad range of societal problems, including frost heaving in soil (an upwards swelling of soil during freezing conditions) and the extraction of natural gas from shale rock. In addition, the project is training students and postdoctoral associates in computer-aided chemical research and in the design of advanced experimental devices. Measurements are carried out in a Department of Energy National Laboratory. The project focuses on molecular insights into physical properties and phase transitions of highly confined water and ice, low-dimensional polymorphous and polyamorphous transitions, as well as the prediction of new phases of low-dimensional ice and clathrate hydrates. More specifically, the UNL team is pursuing three objectives: seek new polymorphous and polyamorphous phases of water and gas hydrate in nano-slits with widths of 8 - 12 Angstroms (or the bilayer to trilayer forms of ice); investigate the mechanism of low-dimensional carbon dioxide (CO2) and carbon monoxide (CO) hydrate formation; study the molecular hydrogen hydrate formation mechanism in nanoscale confinement focusing on hydrogen storage. Molecular dynamics (MD) simulations, including the state-of-the-art, replica-exchange MD simulations for computing the free-energy surface of confined fluids, is used.